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Title: Scaffolding
Author: Thatcher, A. G. H.
Language: English
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original, except for apparent typographical errors which have been
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SCAFFOLDING
PRINTED BY
SPOTTISWOODE AND CO. LTD., NEW-STREET SQUARE
LONDON
[Illustration: PLATE I.
_Photo by Thatcher_] [_28 Cobourg Road, S.E._
DERRICK STAGING.]
SCAFFOLDING
A TREATISE ON
THE DESIGN _&_ ERECTION OF SCAFFOLDS,
GANTRIES, AND STAGINGS,
With an Account of the Appliances used in
connection therewith
FOR THE USE OF CONTRACTORS, BUILDERS,
CLERKS OF WORKS, ETC.
With a Chapter on the Legal Aspect of the Question.
By A. G. H. THATCHER,
BUILDING SURVEYOR
SECOND EDITION, REVISED
WITH 146 DIAGRAMS AND 6 FULL-PAGE PLATES.
LONDON:
B. T. BATSFORD, 94 HIGH HOLBORN
1907.
PREFACE
Scaffolding up to quite recent years has been considered by builders
and others concerned, with the exception of the actual workmen, to be a
matter of small importance and consequently unworthy of study. Recent
legislation, however (the Workmen’s Compensation Act, 1897, and the
Factory and Workshop Act, 1901, and preceding dates), has brought it
into greater prominence, with the result that more attention has lately
been given to it. Special study of the subject has, however, remained
very difficult owing to the lack of accessible information.
The author, in the course of considerable experience in the building
trade, has had opportunities of examining a large number of scaffolds
throughout the country, affording him exceptional facilities for
thoroughly studying the subject, and he has been led to prepare this
treatise in the hope that it may prove useful to those engaged both
in the design and erection of scaffolds for building purposes. While
special attention has been given to practical details, the theory has
not been neglected, but has been dealt with by the use of terms well
understood in the building trade. The various formulæ given have been
simplified as far as possible, and it is hoped that in these days of
technical education they will not be beyond the scope of the reader.
The illustrations have generally been drawn to scale, but for the sake
of clearness, details are given to a larger scale where necessary.
The practice of allowing workmen to erect scaffolds without the aid
of expert supervision, as is generally the case, is to be strongly
deprecated. The architect, builder, or clerk of works, should in all
cases be responsible for their erection—the risk of defective or unsafe
work being thereby minimised, and an economy often effected in both
labour and material.
The author desires to acknowledge his indebtedness to Mr. G.
Thatcher, of H.M. Office of Works and Hampton Court Palace, for
valuable information contributed by him, and to Mr. J. Clark, of
the Factory Department of the Home Office, for his very careful
reading of the proofs; while his best thanks are due to the following
manufacturers:—Mr. C. Batchelor, Messrs. Bullivant & Co., Ltd., Messrs.
Butters Bros., Mr. J. Fishburn, Messrs. Frost & Co., and Mr. E. Palmer,
who have furnished him with particulars of their various specialities.
A. G. H. T.
LONDON: _February 1904_.
NOTE TO THE SECOND EDITION
Recent alterations in and additions to the legal enactments affecting
scaffolding and the persons employed in its erection have rendered
necessary a second edition of this work. The author has accordingly
revised the information relating to the law of the subject, and has
embodied the important series of suggestions for the guidance of those
engaged in building operations, published in the Annual Report of the
Chief Inspector of Factories, which, if followed out, will undoubtedly
be the means of reducing the number of fatal and other accidents
occurring at such works.
A. G. H. T.
_October 1907._
CONTENTS
PAGE
PREFACE v
CHAPTER I
_SCAFFOLDING_
Scaffolding—Definition of—Northern system—Derrick
stagings—Working platforms—South country system—Gantries,
commonly called travellers—Travelling gantry—Gantries
which serve as a base for lighter forms of
scaffolding—Stagings—Pole scaffolds—Bricklayers’
scaffolds—Masons’ scaffolds—Connections 1-29
CHAPTER II
_SCAFFOLDS FOR SPECIAL PURPOSES_
Needle scaffolding—Scaffolds for chimney shafts,
towers and steeples—Domes and arches—Swinging
scaffolds—Painters’ boats—Boatswain’s boat—Ladder
scaffolds—Supported painter’s boat 30-48
CHAPTER III
_SHORING AND UNDERPINNING_
Shoring—Flying shores—Raking shores—Underpinning 49-61
CHAPTER IV
_TIMBER_
Classification and structure—Defects in the living
tree—Felling—Conversion—Seasoning—Description—Selection
—Decay—Preservation—Durability—Use of scaffolding timber 62-75
CHAPTER V
_CORDAGE AND KNOTS_
Cordage—Strength of—Knots 76-89
CHAPTER VI
_SCAFFOLDING ACCESSORIES AND THEIR USE_
Ladders—Trestles—Cripples—Buckets and
skips—Baskets—Barrows—Stone bogies—Hand
barrows—Hods—Timber trucks—Sack trucks—Crates—Ashlar
shears—Stone clips and slings—Stone lewises—Stone
cramps—Wire and chain scaffold lashings—Tightening
screws—Rollers—Levers—Dog irons—Bolts—Straps—Wire
ropes—Chains—Slater’s truss—Duck runs—Mortar
boards—Wedges—Nails—Spikes—Scaffolder’s hatchet 90-114
CHAPTER VII
_THE TRANSPORT OF MATERIAL_
Crane engines—The crane—Pulleys—Differential
pulleys—Winch—Jibs—Shears—Gin—Rails—Sack
trucks—Attachment of material—Ironwork—Timber
—Bricks—Slates—Stone 115-130
CHAPTER VIII
_THE STABILITY OF A SCAFFOLD_
Stability—Wind pressures—Force of weight or gravity—To
find the centre of gravity of a body—Two or more
bodies—Of a dependent scaffold and the effect of loads
upon it—Of a gantry—Of a Scotch derrick 131-142
CHAPTER IX
_THE STRENGTH OF A SCAFFOLD_
Failure of beams—Pillars and struts—Ties and
traces—Dead loads—Live loads—Breaking weight—Safe
load—Constants—Beams subject to a transverse
strain—Posts and struts subject to compression—Braces
and ties subject to a tensional strain 143-153
CHAPTER X
_THE PREVENTION OF ACCIDENTS_
Short ladders—Sagging of ladders—Guard rails—Well
holes—Edge boards—Traps—Runs—Supports to centering—Damp
cordage—Sound material—Competent control—Fan guards—Due
care by workmen 154-160
CHAPTER XI
_LEGAL MATTERS AFFECTING SCAFFOLDING_
Scaffolding bye-laws—Burgh Police (Scotland) Act—Factory
and Workshop Act, 1901—Abstract of same, issued from
the Home Office—Notice of Accident Act—Report on
Building Accidents by a Home Office Inspector—Workmen’s
Compensation Act, 1906—Application of Act and definitions
161-178C
APPENDIX: Weight of Material 179
INDEX 183
LIST OF ILLUSTRATIONS
_PLATES_
I. Derrick staging _Frontispiece_
II. External chimney scaffold _Facing p._ 33
III. Knots 79
IV. Knots 81
V. Knots 85
VI. Knots 87
_ILLUSTRATIONS IN TEXT_
FIG. PAGE
1. Elevation of staging for derrick crane 4
2. Plan of king leg 6
3. Showing shoring to central standard 6
4. Plan of derrick platform partially covered 7
5. Showing method of fixing ladders 8
6. Showing derrick staging mounted on travelling bogie 10
7. Elevation of square derrick staging 11
8 and 8A. Elevation of gantry for traveller 13
9. Footing blocks for struts 15
10. Elevation of travelling gantry 15
11. Gantry or elevated platform over footpaths 17
12. Example of stagings 18
13. Elevation of pole scaffold 20
14. Method of fixing rising standard 21
15. Method of connecting ledgers 22
16. Method of connecting ledgers 22
17. Method of connecting ledgers 23
18. Method of connecting ledgers 23
19. Method of supporting putlogs where windows occur 24
20. Method of supporting putlogs where windows occur 24
21. Shores and ties for dependent scaffolds 25
22. Methods of laying boards 26
23. Methods of laying boards 26
24. Masons’ scaffold: end elevation 27
25. Landing stages 28
26. Needle scaffold 31
27. Internal chimney 33
28. Method of climbing chimneys, steeples, &c. 35
29. Method of climbing chimneys, steeples, &c. 36
30. Brackets for climbing chimneys, steeples, &c. 37
31. Methods of slinging painters’ boats on chimneys, &c. 38
32. Chimney scaffolds for repairs, &c. 39
33. Details of chimney scaffolds for repairs, &c. 39
34. Details of chimney scaffolds for repairs, &c. 40
35. Details of chimney scaffolds for repairs, &c. 40
36. Details of chimney scaffolds for repairs, &c. 41
37. Method of supporting standards within high towers 41
38. Scaffold designed for repairing roofs or arches
where roadway has to be kept open 42
39. Painters’ boats or cradles 43
40. Method of slinging cradle to move horizontally 44
41. Improved painter’s boat 44
42. Boatswain’s boat 45
43. Elevation of ladder scaffolding 46
44. Details of ladder scaffolding 47
45. Details of ladder scaffolding 47
46. Details of ladder scaffolding 47
47. Examples of flying shores 51
48. Examples of single raking shore 52
49. Examples of system of raking shores 53
50. Details of raking shores 54
51. Details of raking shores 55
52. Details of raking shores 56
53. Example of underpinning 59
54. Example of window strutting 60
55. Example of cup shakes in timber 63
56. Example of radial shakes in timber 63
57. Method of cutting stiffest beam from log 66
58. Method of cutting strongest beam from log 66
59. Method of cutting planks from log 67
60. Method of stacking timber for seasoning 68
61. Examples of weather shakes in balks 68
62. Method of strapping ends of boards 74
63. Details of ladder construction 91
64. Details of ladder construction 91
65. Example of painters’ trestles 92
66. Example of cripples 93
67. Example of cripples 93
68. Example of tipping pail 94
69. Details of improved safety baskets 95
70. Details of improved safety baskets 95
71. Details of improved safety baskets 96
72. Details of improved safety baskets 96
73. Details of improved safety baskets 96
74. Example of navvy’s barrow 98
75. Example of stone bogie 98
76. Example of hand barrows 98
77. Example of labourer’s hod 98
78. Example of timber trucks 98
79. Example of sack trucks 98
80. Example of brick crates 99
81. Example of ashlar shears 100
82. Example of ashlar shears 100
83. Example of stone clips 101
84. Method of slinging stone 101
85. Method of slinging stone 101
86. Method of hoisting by means of the lewis 102
87. Method of hoisting by means of the lewis 102
88. Method of hoisting by means of the lewis 102
89. Method of hoisting by means of the lewis 102
90. Example of the stone cramp 103
91. Wire rope lashing 104
92. Chain and bracket lashing 105
93. Coupling links 105
94. Rollers 106
95. Levers 106
96. Dog irons 107
97. Methods of fixing dog irons 107
98. Methods of fixing dog irons 107
99. Methods of fixing dog irons 107
100. Iron bolts 108
101. Iron straps 109
102. Slaters’ truss 112
103. Duck runs 112
104. Scaffolder’s hatchet 113
105. Manual building crane 117
106. Example of pulley wheel 118
107. Example of groove for pulley wheel 119
108. Example of groove for pulley wheel 119
109. Example of groove for pulley wheel 119
110. Example of groove for pulley wheel 119
111. Example of groove for pulley wheel 119
112. Example of sprocket wheel 119
113. Example of single movable pulley 120
114. Example of blocks and tackle 120
115 and 116. Example of differential pulley 122
117. The winch 124
118. The winch in use with a double rope 125
119. Example of a jib 125
120. Example of a jib for fixing purposes 126
121. Example of a mason’s jib 127
122. Example of shear legs 127
123. Example of shear legs to carry jib 128
124. Example of movable shear legs 128
125. Example of a gin 129
126. Method of slinging girders 129
127. Diagram showing method of finding the centre of
gravity of a rectangular body 136
128. Diagram showing method of finding the centre of
gravity between two combined weights 137
129. Diagram showing method of finding the centre of
gravity between three or more combined bodies 138
130. Method of finding the centre of gravity of a
rectangular surface 139
131. Method of finding the centre of gravity of a
triangular surface 141
132. Diagram showing the method of calculating the
breaking weights of beams 147
133. Diagram showing the method of calculating the
breaking weights of beams 147
134. Diagram showing the method of calculating the
breaking weights of beams 148
135. Diagram showing the method of calculating the
breaking weights of beams 148
136. Diagram showing the method of calculating the
breaking weights of beams 148
137. Diagram showing the method of calculating the
breaking weights of beams 148
138. Diagram suggestion for preventing accidents
on short ladders 154
139. Method of staying ladder to prevent sagging 155
140. Method of staying ladder to prevent sagging 155
141. Illustrating dangerously laid boards 156
142. Illustrating the danger of a trap 156
143. Illustrating method of supporting centering 157
144. Illustrating method of supporting centering 158
145. Fan guards 158
146. Illustrating cause of fatal accident 159
SCAFFOLDING
CHAPTER I
_SCAFFOLDING_
Scaffolding is the art of arranging and combining pieces of timber in
order to enable workmen to proceed with their work, and from which, if
required, to lift and carry the material necessary for their purpose.
Many definitions of a scaffold have been given by authorities on
building construction; some of the best known are as follows:—
Mitchell (C. F.): 'Temporary erections constructed to support a number
of platforms at different heights, raised for the convenience of
workmen to enable them to get at their work and to raise the necessary
material for the same.’
Tredgold (Hurst): 'A scaffold as used in building is a temporary
structure supporting a platform by means of which the workmen and their
materials are brought within reach of their work.’
Rivington: 'Scaffolds are temporary erections of timber supporting
platforms close to the work, on which the workmen stand and deposit
their materials.’
Banister F. Fletcher, in 'Carpentry and Joinery’: 'A scaffold is a
temporary structure placed alongside a building to facilitate its
erection by supporting workmen and raising materials during the
construction, or for the repair of buildings.’
Recent cases tried under the Workmen’s Compensation Act have given a
wider meaning to the word, and the following definition is perhaps the
most comprehensive at the present time:
A scaffold, as used in building, is a temporary arrangement of timbers
combined and supported in various ways to enable the workmen to proceed
with their work, and where required, to afford facilities for the
lifting and carrying of the materials.
The two principal methods of scaffolding are known respectively as the
North and South country systems. The northern, as indicated by the
name, was at one time in use only in Scotland and the north of England,
but its many advantages, more especially for the transport of material,
have now caused it to become general throughout the country.
The second method is essentially the South country system, and is of
greater use when power is not necessary for the construction of the
building.
A combination of both methods is commonly seen, and found useful in
practice.
In scaffolding, the vertical timbers are known as standards or
uprights. The horizontal timbers between the standards are known as
ledgers when of cylindrical section, but as transoms and runners when
of rectangular section. Braces, shores, struts and ties of any section
are pieces used to stiffen the structure. The putlogs, or joists as
they are called when of greater length, carry the boards which form the
working platform.
=The Northern System.=—This scaffolding can be divided into two
parts. First, the derrick staging from which the transporting power
acts; and, second, the platforms, which bring the workmen within reach
of their work.
=Derrick Stagings.=—These stagings, also known as Scotch derricks
and 'Scotchmen,’ are erected to carry the power required, usually a
steam crane.
They consist of three or four timber towers or legs supporting a
platform upon which the crane stands. The number of legs depends upon
the area over which the power is required to act.
When one crane is to be erected, three legs are sufficient to carry the
platform.
If the building is a large one, several such stagings may be
constructed; but in some cases two cranes are required where the size
of the building will not allow of two stagings. In these cases the
platform is square and supported at each angle by a leg. The cranes are
then fixed diametrically opposite each other.
In determining the position of the legs they must be placed where the
effective range of the crane is most required, and also where they will
cause the least possible obstruction to the progress of the building.
The position of the tower that carries the crane, and which is known
as the principal or king leg, is first fixed. The secondary or queen
legs are set out from it in the form of an isosceles triangle. The
distance between the king and queen legs depends upon the length of
the sleepers. These run from below the engine to the lower ends of the
guys, and average from 25 to 30 feet in length.
The legs, especially the king legs, if intended to rise from the earth,
must have a foundation of two thicknesses of 3-inch timbering laid
crosswise. This is unnecessary if there is a concrete or other solid
foundation.
Apart from the necessity for any foundation, the standards should rise
from a framework of balk timbers of about 12 in. by 12 in., laid on the
ground, and halved at their intersection (fig. 1).
[Illustration: FIG. 1.—ELEVATION OF STAGING FOR DERRICK CRANE]
In the case of the queen legs an extra balk is placed under the
framework, as shown in fig. 1.
The legs are from 6 to 10 feet square on plan, the king legs being the
larger. Each leg comprises four standards, either of whole timbers or
battens bolted together. The standards for the king legs should measure
not less than 9 in. by 9 in.; if of battens, then three pieces 9 in. by
3 in. should be used.
For the queen legs, balks 7 in. by 7 in., or three pieces 7 in. by
2-1/2 in., are sufficient sectional area. Where battens are used they
are given a lap equal to one-third of their full length, and are bolted
together by 3/4-inch wrought-iron bolts.
Whole timbers are used when they can be obtained in one piece of
sufficient length.
As the standards rise they are divided into bays by transoms. These are
made out of about 9-in. by 3-in. deals, placed from about 6 to 10 feet
apart.
The bays are triangulated by cross-braces 7 in. by 2-1/2 in.; both of
these are usually placed on the outside of the bay, but one or both
may be placed on the inside. The first method is the better, as the
braces, by butting against the transoms, give an increased resistance
to movement.
The king leg, having to carry the weight of the engine, requires
greater strength. This is gained by running an extra standard up the
centre of the leg. If it is whole timber, balks about 14 in. by 14 in.
are required; if built up, four deals 16 in. by 4 in. are used. It
should stand upon an extra balk laid with the horizontal framing at the
bottom of the leg, and should rest on a solid foundation.
To prevent any tendency to flexure this extra standard should be
strutted from all four of the outer standards behind each transom (see
fig. 2).
Another method is to shore the central standard from the foundation, as
in fig. 3.
The legs in this manner can be made to support a platform up to 120
feet in height.
The required height having been reached, the legs are connected by
trussed beams in the following manner: Two balk timbers of about 12 in.
by 8 in. are laid immediately above each other between the king leg
and each queen leg, resting on the two top transoms, as shown in fig.
1. They are from 6 to 9 feet apart, the top bay being sometimes made
slightly lower than the others.
The lower balks are connected to the centre standard of the king leg by
wrought-iron straps.
[Illustration: FIG. 2.—PLAN OF KING LEG]
[Illustration: A, Central Standard. B, Shorings.
FIG. 3.—SHOWING SHORING TO CENTRAL STANDARD]
The top balks project from 6 to 10 feet beyond the king leg, and
are halved at their point of intersection. The projecting ends are
connected to each other by pieces 8 in. by 6 in., and again to the
return balk by similar pieces (see fig. 4). They are also supported by
struts from the central standard, as shown in fig. 1. The upper and
lower balks are connected by iron bolts about 10 feet apart, and each
bay thus formed is cross-braced in the same manner as the legs.
The iron bolts are covered by pieces of the same scantling as the
braces.
In the single derricks the queen legs can be connected by a trussed
beam similarly formed, or by a single balk carried across and laid on
the top transom.
[Illustration: FIG. 4.—PLAN OF TOP PLATFORM PARTIALLY COVERED]
If the span is considerable, struts can be carried from the queen legs
towards the centre of the underside of the balk to prevent sagging.
On the trussed beams thus formed, joists of 9 in. by 3 in. or ordinary
poles are laid about 3 feet apart.
They are laid parallel to one another, and in a direction at right
angles to the truss or single beam forming the back support of the
platform.
The centre joists are continued to the ends of the balks which project
beyond the king leg.
The advantage of having continued the top balks can now be seen, as it
gives greater area to the platform immediately round the engine.
The boards 9 in. by 1-1/2 in. are laid at right angles to the joists.
Another way of forming the platform is to cover only partially the
surface between the legs. In this case two additional joists, 6 in.
by 6 in., are thrown across the king leg (see fig. 4), the boards not
extending beyond their length.
When this is done, the workmen reach the platform from the
communicating ladder which usually passes up a queen leg, by means of a
run two boards wide. It is better to lay the larger platform, as, apart
from the question of safety to the men, it serves as a storage for coal
for the engine, the weight of which tends to keep the erection steady.
Double boards should be laid under coals or other heavy stores.
To reach the platform, ladders are fixed in different ways. They can
run up inside, or be fixed to the outside of the queen legs. In either
case they are nearly or entirely upright. A better method is shown in
fig. 5, and should be carried out wherever possible.
[Illustration: FIG. 5.—SHOWING METHOD OF FIXING LADDERS]
The derrick sleepers, two in number, are of balk timber, and lie across
the platform from beneath the engine bed to which they are connected,
to the centre of the queen legs.
The guys or stays, also of balk timbers, besides being connected to the
mast, are attached to the sleepers over the queen legs (see fig. 1).
To counteract the overturning force exerted by the jib and the material
lifted, the guys are chained down to the timber balk at the bottom of
the queen legs (fig. 1).
This balk supports a platform which is loaded with bricks or stones
more than equal to double the weight that will be lifted. The chain,
which works loose with the vibration of the scaffold, is tightened by
means of a screw coupler fixed in its length. The arrangement is as
follows:—Two lengths of heavy chain with large links at each end are
required. One length is carried round the sleeper and then taken down
the centre of the leg. The other length is taken round the balk which
is placed underneath the staging, and carried up through the load, when
the tightening screw can be applied and the correct tension brought up.
To prevent lateral motion the legs are cross-braced by poles or deals
between each leg as shown on frontispiece. The poles are tied to the
legs just beneath the platform and connected at their meeting point.
When crossing they should be at right angles to each other.
Deals 9 in. by 3 in. can take the place of the poles if required, bolts
in this case being used instead of tyings.
At the building of the new Post Office, Leeds, 1893, a different method
of raising the platform for the crane was adopted. The legs, instead
of being framed, consisted of a single balk of timber strutted on
each side from the ground level, the sleepers and guys being firmly
attached to the standards themselves.
When erecting long ranges of buildings it may be more convenient to
have the derrick mounted upon a travelling bogie than to dismantle the
structure in order to re-erect at another point.
Fig. 6 illustrates the system, the travelling power being usually
manual. The arrangement is suitable for small derricks, and is employed
where the crane is erected outside the building.
[Illustration: FIG. 6.—SHOWING STAGING MOUNTED ON TRAVELLING
BOGIE]
Another method of using travelling cranes is to erect a platform as
shown in fig. 7.
The standards, which may be of balk timber or built up, as previously
shown, are about 10 feet apart longitudinally and 20 to 30 feet
transversely. They stand upon sills of the same section where the
foundation is not solid. On the head of the standards, the runners are
laid connecting all the standards in the same row.
Head pieces may be fitted between the standards and runners; this
serves to distribute pressure. All the connections are securely made by
dog irons, bolts, and straps. The stability depends entirely upon the
bracing, and this, it is important to note, should be between each bay
longitudinally, and at least every second bay transversely.
[Illustration: Front Elevation
End Elevation
FIG. 7.—ELEVATION OF DERRICK STAGING]
Timbers placed as A in fig. 7 give rigidity to the standards by
preventing flexure, and are necessary when the lengths of the uprights
exceed 30 times their least diameter.
The deals used for braces are bolted to the standards; for poles, tying
is resorted to.
=Working Platforms.=—The working platforms used in conjunction
with overhead or overhand work depend upon the requirements of the
building.
By over head or hand work is meant that the material upon which the
mechanic is to be employed reaches him from over head or hand.
When no outside scaffolding is needed, the platforms are laid upon
the floor joints in the interior of the building, being raised upon
trestles as the work proceeds, and until the next floor is reached.
Light forms of scaffolds, as the ordinary masons’ and bricklayers’ pole
scaffolds, are now frequently used as working platforms in connection
with the Scotch system.
=The South Country System.=—This system is divided into two
classes according to the strength required. For the first, square
timbers are used; for the second, poles are employed. The scaffolds
built of square timbers are known as gantries and stagings, and the
pole erections are termed bricklayers’, and masons’ or independent
scaffolds.
=Gantries.=—The term gantry was originally given to erections
constructed with a view to the easy carriage of heavy material, but of
late it has also come to mean a structure arranged to support lighter
forms of scaffolding over footpaths which have to be kept open for
public use.
[Illustration: FIG. 8.—ELEVATION OF GANTRY FOR TRAVELLER]
[Illustration: FIG. 8A.—END ELEVATION OF GANTRY SHOWN IN FIG.
8]
1st. Gantries for transport of material, commonly called travellers.
Figs. 8 and 8a show the general construction.
The distance between the outer rows of standards and the wall depends
upon circumstances. If possible, the space should be allowed for a
cart-way, as the material can thus be brought quite close to the work
before being lifted. If, owing to adjacent footpaths or any other
reason, this cannot be done, the uprights should be placed close to the
wall on either side, the material being lifted at the end of the gantry
or other convenient spot, over which the lifting gear can be brought.
The standards of square timber for the gantry are from 6 in. to 12
in. square, and are erected upon sleepers, or, as they are sometimes
termed, sills laid in the same direction as the run of the scaffold.
One row of standards is placed on each side of the wall. The standards
are placed 8 to 10 feet apart. On the top of the standards runners are
fixed connecting each standard in the same row. Sills, standards, and
runners should be of the same sectional area. The runners are strutted
on their underside, from the standards by pieces of, at least, half the
sectional area of the supported timbers. If the struts are of equal
size to the runners, double the weight can be carried.
The cleats from which the struts rise, are simply spiked to the
standards, but if designed to carry excessive weights they are slightly
housed in. As the space between each row of standards has to be kept
open for the building, no cross bracing can be allowed except at the
ends. Strutting is therefore resorted to in order to give stability.
The struts, one to each standard, are bolted to the upright near the
top, and again to a foot block driven into the ground. Other methods of
fastening down the bottom ends of the struts are shown in fig. 9; the
use of each depends upon the nature of the soil.
Struts are also fixed at the ends to prevent lateral movement. Head
pieces, or corbels, as they are sometimes termed, are occasionally
inserted between the standards and runners, and serve to distribute
pressure.
Straining pieces spiked on the underside of the runners, for the struts
to pitch against, are used when the standards are considerably apart.
[Illustration: FIG. 9.—FOOTING BLOCKS FOR STRUTS]
Rails upon which the travelling engine or traveller can move are laid
on top of the runners, and are turned up at the ends of the platform to
serve as buffers to the engine platform.
[Illustration: End Elevation
Side Elevation
FIG. 10.—ELEVATION OF TRAVELLING GANTRY]
The engine platform consists of two trussed beams of timber about 3
feet apart, connected at their ends with short pieces of the same
scantling, and fitted with grooved wheels to move upon the rails.
Rails are also laid upon each beam and serve for the traversing motion
of the crab. Movement of the traveller is obtained from the crab,
which is worked either by manual or steam power, and acts through a
system of shafting and geared wheels. Movement in three directions is
necessary from the crab: vertically for lifting, and horizontally in
two directions, transversely and longitudinally. Travellers are made up
to 50 feet wide and any required length.
Another method of building travellers is shown in fig. 10.
In this case, the rails upon which the traveller moves in a
longitudinal direction are fixed on sleepers on the ground level,
and the standards and runners of the first example are not required.
In their place is constructed a triangulated system of balk timber
framing. The platform is fixed to the head pieces, and is braced as
shown. Less timber is used in their construction, but owing to the
greater weight a steam winch is required to impart motion.
=Gantries which serve as a base for lighter forms of
scaffolding.=—These erections are in reality elevated platforms,
and allow of a clear way for a footpath where required. They are
constructed of two frames, placed apart according to the width of the
path over which the platform stands (fig. 11).
The method of erection, so far as the side frames are concerned, is
the same as for the first example of travelling gantries. Stability
is, however, gained by cross-bracing as shown in figure, thus making
strutting unnecessary. The platform can be laid by placing short boards
9 in. by 3 in. across the runners when the platform is narrow. It is
more usual, however, to place joists 10 in. by 2 in. across, and on
these to lay the boards longitudinally. The joists average 2 to 3
feet apart, the braces are about 2 in. by 7 in. On the outside of the
scaffold, parallel to the sills, balk timbers are placed forming a
'fender’ to prevent the vehicular traffic from injuring or disturbing
the erection.
[Illustration: Front Elevation
End Elevation
FIG 11.—GANTRY OR ELEVATED PLATFORM OVER FOOTPATHS]
=Stagings.=—Stagings are erected in a manner similar to travelling
gantries, but are carried more than one storey high (fig. 12). It is a
form of scaffolding rarely seen, more especially since the introduction
of the Scotch derrick system. The timbers are erected to the height of
the first runner in the same manner as the frames in fig. 11. In order
to carry the scaffold higher, horizontal pieces are laid across the
scaffold, over the standards, and are made to project 9 or 10 feet on
each side of the runners.
On these beams, uprights, as in the first tier, are raised, being
connected in like manner, longitudinally by transoms. The rising tiers
of standards are strutted by timbers A A, rising from the
projecting portion of the beam called the footing piece, which serves
in the same manner as a footing block. The footing piece is supported by
struts, B B, rising from the lower standards. The struts B
B are in two pieces, being bolted to the sides of the footing
pieces and uprights. This allows the shores A A to pass
between them.
[Illustration: Front Elevation
Cross Section
FIG. 12.—EXAMPLE OF STAGINGS]
Strutting within the bays formed by the standards is carried out on
each tier with the exception of the top, where braces are fixed, as
shown at C.
On the top runners rails are laid for a traveller.
In constructing the foregoing square timber erection, note should be
taken of the following points:—
That the uprights of the upper tiers should stand immediately over
those of the lower tiers, in order to prevent cross strains on the
runners.
That the timbers should fit as evenly as possible, as thereby the whole
erection is rendered more stable.
That joints between the runners should occur immediately over the
standards.
The several parts of this structure, if for temporary purposes, can
be connected by dog irons; if for a more permanent use, by bolts and
straps.
POLE SCAFFOLDS
=Bricklayers’ Scaffolds.=—A bricklayer’s scaffold consists of a
series of upright poles or standards, to which are lashed horizontal
poles, termed ledgers. The ledgers and the wall of the building carry
the putlogs, on which boards are laid to support the workman, his
material, and tools (fig. 13).
The standards are first erected, and may stand singly or in pairs. In
a repairing job, unless of great height, and where there is no great
weight of material, single poles are sufficient.
Where double poles are required, the first pair are erected of
different lengths.
The short pole is termed a puncheon. The difference of length allows of
a lap in connecting the succeeding poles.
The lap should equal half of the full-length pole. The standards are
placed 6 to 8 feet apart, and from 4 to 6 feet away from the building.
[Illustration: FIG. 13.—ELEVATION OF POLE SCAFFOLD]
The butt-ends are embedded about 2 feet in the ground, which affords
some resistance to overturning. If they cannot be embedded, they
should be placed on end in barrels filled with earth tightly rammed.
As the building rises additional poles are added, being lashed to the
standards already erected.
If the standard is a single pole, the second pole, having a lap of 10
or 15 feet, stands upon a putlog placed close to the first pole for
that purpose (fig. 14).
The inner end of the putlog is securely fastened down to the scaffold
or inserted into the building.
If the standard is double, the rising pole is placed upon the top end
of the puncheon, and afterwards others are placed on end upon the
lowest free end of the standards already fixed.
[Illustration: FIG. 14.—METHOD OF FIXING RISING STANDARD]
As the standards rise, they are spliced or 'married’ together with band
ties.
At a height of 5 feet, this distance being the greatest at which a man
can work with ease, a ledger is tied across the standards to form a
support for the working platform.
Where a single pole is insufficient in length to form a continuous
ledger, two are joined in one of three ways.
[Illustration: FIG. 15]
In the first they are lapped over each other as fig. 15. This method
gives a strong connection, but prevents the putlogs being laid evenly.
[Illustration: FIG. 16]
The second way provides that the ledgers shall lap horizontally side by
side. This allows of evenness of line for the putlogs, but is not so
strong (fig. 16).
In both of these methods the lap should cover two standards, and not as
shown in fig. 17.
The third manner of connection (fig. 18) is the best. The ledgers butt
end to end. Underneath, a short pole is placed crossing two standards.
The tying at the standard embraces the double ledger. A band tie is run
round the supporting pole and the ends of the ledgers where they butt.
Great strength is obtained in this way and the putlogs can be evenly
laid.
Additional ledgers are fixed as the work proceeds.
[Illustration: FIG. 17]
On the ledgers, and at right angles to them, putlogs are laid, resting
outwardly on the ledgers and inwardly on the wall, where header bricks
have been left out for their reception.
[Illustration: FIG. 18]
The putlogs, which are placed about 3 or 4 feet apart, should be tied
to the ledgers and fastened by wedges into the wall. This is not often
done, but at least one putlog to every tying between standard and
ledger should be so treated.
Where the putlogs cannot be carried by the wall owing to an aperture in
the building, such as a window, they are supported by bearers fixed as
shown in figs. 19 and 20.
[Illustration: FIG. 19]
[Illustration: FIG. 20]
By wedging the inner end of the putlog into the wall, some stability
is given to the scaffold, but the connection cannot be considered
satisfactory, as the putlog would draw under very little strain.
Greater stability can be gained if the outer frame of the scaffold is
supported by one of the three methods given as follows.
[Illustration: FIG. 21.—SHORES AND TIES FOR DEPENDENT
SCAFFOLDS]
A shore or tie can be fixed between the erection and the ground as
shown in fig. 21, or, if there are openings in the wall, supports can
be fixed as ties shown in the same diagram.
The ties or struts should be placed to every third or fourth standard
at about 25 feet from the ground, and their fastenings made good.
Additional ties should be carried within the building at a greater
height where possible. The stability of the scaffold under loads and
cross wind pressure, depends greatly upon the ties or shores, and their
fastenings should be well made and kept in good order. The historical
instance of the mechanic who, to escape a shower of rain, stood upon
the inner board of the platform, and by leaning against the building
pushed the scaffold over, should have no opportunity of recurrence.
[Illustration: FIG. 22]
To stiffen the scaffold longitudinally braces are tied on the outside
of the scaffold in the form of a St. Andrew’s cross (see fig. 13).
They start from the lower end of one standard and rise obliquely across
the scaffold to near the top, or some distance up a standard in the
same run. Tied at their crossing-point, connections are made to all the
main timbers of the scaffold with which they come in contact. Braces
are fixed in all exposed situations, and generally where the scaffold
is more than one pole (30 feet) in height.
[Illustration: FIG. 23]
The only exception to effective bracing being carried out is where the
building, being of irregular form, creates many breaks and returns in
the scaffolding. It is obvious that where a scaffold butts against or
breaks with a return wall, the tendency to lateral motion is lessened.
The boards, which are placed longitudinally across the putlogs, can be
laid to lap or butt at their ends. When lapping, one putlog only is
required to carry the ends of two series of boards (fig. 22).
When butting, two putlogs are required placed about 4 inches apart
(fig. 23).
The second method is the better, as the boards are not so likely to
lose their place or to trip the workmen. If heavy work is in progress
the boards are laid double. As the building rises, the boards are
carried up to each successive platform, but each tied putlog is left in
its place.
[Illustration: FIG. 24.—MASONS’ SCAFFOLDS: END ELEVATION]
=Masons’ Scaffolds.=—Masons’, or independent scaffolds differ from
the bricklayers’ in that they have to be self-contained. Owing to the
different material of which the building is erected, the putlogs cannot
rest upon the wall. If openings were left for them, as in brickwork,
the wall would be permanently disfigured, more especially when ashlar
fronted.
In order to gain the necessary support two parallel frames of standards
and ledgers are erected along the line of wall to be built (fig. 24).
They are from 4 to 5 feet apart, the inner frame being as close to
the wall as possible. As a heavier material has to be dealt with, the
standards are placed closer together, say from 4 to 5 feet.
[Illustration: FIG. 25.—LANDING STAGES]
The ledgers and braces are placed as before, the putlogs now resting on
ledgers at each end, and not on the wall at the innermost end, as in
the bricklayer’s scaffold.
To prevent cross movement of the scaffold, an additional method of
bracing is available in this system. An inner and outer standard are
connected by short braces across each bay, as shown in fig. 24.
This method of cross-bracing can be continued to the top of the
scaffold, and the braces should be put in longitudinally, about 20 feet
apart.
The platforms laid on all pole-scaffolds are from 4 to 5 feet wide. It
is usually necessary, on anything but the smallest jobs, to keep this
width free for the workman and his material.
In order, therefore, to provide a platform on which the material can be
landed, it is convenient to erect, on the outside of the scaffold, an
additional platform from 5 to 10 feet square (fig. 25).
It is constructed of standards, ledgers, and braces, in like manner as
the scaffold to which it is attached.
The face-boards, as shown in this figure, should be fixed wherever
material is being hoisted, to prevent any projection of the load
catching under a ledger and upsetting.
=Connections.=—The members of pole scaffolds are connected by
cordage. The names of the various knots are given in Chapter V.
The arranging of the various timbers used in erecting scaffolds is a
dangerous occupation, and one requiring skill and considerable nerve on
the part of the workmen. In the majority of cases, the timbers on the
ground level are placed in position by manual labour only, shear legs
being used to facilitate matters. When the scaffold rises, advantage is
taken of any rigid member on which pulley wheels can be hung, and by
this means the succeeding poles, &c. are raised, manual dexterity and
strength being responsible for their final position.
CHAPTER II
_SCAFFOLDS FOR SPECIAL PURPOSES_
When applying the given methods for scaffolding, difficulties arise
owing to the varying designs of the buildings under construction or
repair.
It is impossible to deal with these cases in detail; they must be left
to the scaffolder, who, while holding closely to the principles, by
the exercise of ingenuity will make combinations and variations of the
various systems to suit the special requirements demanded in each case.
There are, however, certain types of scaffolding which occur with some
regularity, and these will now be dealt with.
=Needle Scaffolding.=—Needle scaffolding is necessary where it is
impossible or too expensive to carry the scaffold from the ground level
or other solid base. It is used both for repairing and new erections.
The needles from which the scaffold takes its name are timbers (usually
poles or balks) placed horizontally through and at right angles, or
nearly so, to the wall of the building. The projections support a
platform upon which an ordinary pole scaffold is erected (fig. 26).
Windows, or other openings in the wall, are utilised where possible for
the poles to pass through. In other cases holes have to be made in the
walls, cut as nearly as can be to the size of the needles in use.
The needles must be of sufficient scantling to carry the weight of the
scaffold and attendant loads. The stability of the structure depends
upon the means taken to fasten down the inner end of the needle.
The usual plan is to tie it down to a convenient joist or other rigid
member of the building itself, but the method shown on the diagram is
better, as resistance to movement is gained both from above and below.
Struts from the building below the needles to their outer end, give
greater strength to the beam.
When erecting needle scaffolding around buildings of small area, say of
a tower or chimney shaft, the needles can be laid across the building
in one length, piercing the wall on opposite sides. In these cases, if
the needles are wedged in, the weight of the building and the scaffold
itself on the opposite ends of the needles, is sufficient to maintain
equilibrium.
[Illustration: FIG. 26.—NEEDLE SCAFFOLD]
The platform is formed of 9-in. by 3-in. deals, and on this is erected
whatever scaffolding may be necessary.
=Scaffolds for Chimney Shafts, Towers, and Steeples.=—The erection
of chimney shafts can be carried on entirely by the aid of internal
scaffolding. As the work rises putlogs are laid across the shaft, the
ends being well built into the wall. On the putlogs the platform is
laid, being carried up as the work proceeds. The putlogs may be left in
for the time, and struck on completion. The platform is fitted in its
centre with a hinged flap door through which the material is hoisted as
required.
There is some objection to this method of scaffolding where the wall
is more than 1 foot 10-1/2 inches thick (which is the greatest depth
of brickwork over which a man can reach and do finished work), for
the mechanics, in order to reach the outside joints, have to kneel on
the freshly laid material, which is detrimental to good workmanship.
For this reason the system of carrying up an ordinary pole scaffold
externally until the height is reached where the wall is reduced to 1
foot 10-1/2 inches in thickness, is to be preferred.
The walls of a chimney shaft decrease in thickness 4-1/2 inches at a
time, forming an internal set-back of that width at every 20 feet in
height.
This set-back is of advantage to internal scaffolding when the full
height of the brickwork is reached, and the cap has to be fixed. The
cap or coping, when of stone or iron, does not admit of the insertion
of putlogs. To overcome the difficulty, four or more standards are
erected at equal distances, and standing upon the top set-back (fig.
27).
The standards project sufficiently to carry the pulley wheel well above
the total height of the chimney, in order to give head room and to
assist the workman in fixing the coping.
To stiffen the standards, short ledgers are tied across as shown in
fig. 27.
[Illustration: PLATE II.
[_Photo by W. Cottrell_ _Hightown, Manchester._]
EXTERNAL CHIMNEY SCAFFOLD.
Erected for the Willesden Electric Lighting Works, under the
supervision of E. Willis, Esq., A.M.I.C.E., etc.]
When the chimney is to be erected by external scaffolding the
ordinary mason’s or bricklayer’s scaffold is used. Owing to the small
area of the erection the outside frames of the scaffold have a quick
return. This makes it practically impossible for the scaffold to
fail by breaking away from the building under the influence of the
loads it may carry. Shoring or tying is therefore not so important.
Wind pressures have, however, a greater effect, especially when the
direction is not at right angles to one of the faces of the scaffold.
If in that direction, the tied putlogs would offer resistance. Braces
are therefore imperative, and they should be fixed at right angles
to each other, each pair thus bracing a portion of the height of the
scaffold equal to its width. (See plate 2.)
For the repair of chimney shafts without scaffolding from the ground
level, means have to be taken to bring, first the mechanic, and
afterwards his material, within reach of the work.
[Illustration: FIG. 27]
The preliminary process of kite-flying is now rarely seen, except for
square-topped chimneys, and even in these cases the delay that may
arise while waiting for a suitable steady wind is a drawback to its
practice. The kites used are about 10 feet long and 8 feet wide. They
are held at four points by cords which continue for a distance of about
16 feet, and then unite into one. Near this point on the single rope
another cord is attached, which serves to manipulate the kite into
position.
Stronger ropes or chains are then pulled over the shaft, after which a
workman ascends, and the necessary pulley wheels and timbers to form a
regular means of ascent are sent up after him.
A light line carried up in the interior of the shaft by a hot-air
balloon is another means of communication.
The most certain and safest method of ascent is to raise on the
exterior of the shaft a series of light ladders, which are lashed to
each other and firmly fixed to the chimney as they ascend.
The ladders have parallel sides, and are used up to 22 feet in length.
One method of fixing is as follows:—
A ladder is placed against the shaft on its soundest side. It rests
at its top end against a block of wood slightly longer than the width
of the ladder, and which keeps it from 7 to 9 inches away from the
wall. This space allows room for the workmen’s feet when climbing. The
ladder is then fixed by two hooks of round steel driven into the wall,
one on each side immediately under the blocks, the hooks turning in
and clipping the sides of the ladder (fig. 28). The hooks, which have
straight shanks of 7/8-inch diameter with wedge-shaped points, are
driven well home, as the stability of the erection depends upon their
holding firmly.
Above the top end of the ladder a steel hook is driven into the wall
on which a pulley block can be hung, or instead, a pin with a ring
in its head can be so fixed. A rope from the ground level is passed
through this block or ring, and reaches downward again for connection
to the ladder next required. The connection is made by lashing the
rope to the top rung and tying the end to the seventh or eighth rung
from the bottom; this causes the ladder to rise perpendicularly. The
steeplejack who is standing on the already fixed ladder cuts the top
lashing as the hoisted ladder reaches him, and guides it into its place
as it rises. When the rung to which the rope is tied reaches the pulley
block, the ladders should overlap about 5 feet. They are at once lashed
together at the sides, not round the rungs.
[Illustration: FIG. 28]
The workmen can now climb higher, driving in hooks round the sides,
and under the rungs of the ladder alternately, lashings being made at
each point. A wooden block is placed under the top end of the last
ladder and fixed as before. The hoisting rope, which has been kept taut
meanwhile, is now loosened and the process repeated.
The ladders rise in this manner until the coping of the shaft is
reached. Here, owing to the projection of the cap which throws the
ladders out of line, it is impossible to lash the top ladder to the
lower. To overcome the difficulty, the wall is drilled in two places
immediately over the topmost fixed ladder, and expansion bolts are
fitted therein. To these bolts the lower end of the top ladder is tied.
The hoisting rope is then tightened sufficiently to hold the ladder,
and by this means the workmen are enabled to reach the top of the shaft.
A variation of this method of climbing is to replace the wooden blocks
by iron dogs with 9-inch spikes, which should be driven well into the
wall. Short ladders of about 10 feet in length are then used, these
being lashed to the dogs as they rise.
Another method of fixing the ladders is shown in fig. 29.
[Illustration: FIG. 29]
In this case eye-bolts are driven horizontally into the wall in pairs,
rather wider apart than the width of the ladders.
Iron rods hook into these and are fastened to the ladder sides by thumb
screws.
The ladders rise above each other and are connected by 3-inch sockets.
When fixed, they stand about 18 inches from the wall. This is an
advantage, as it enables the workmen to climb on the inside of the
ladders, thus lessening the strain on the eye-bolts, and the ladder can
more easily pass a projecting chimney cap.
On the other hand, the whole weight of the ladders rests upon the
bottom length, so that if through any cause it gave way, for instance
under accidental concussion, the entire length would most certainly
collapse.
This danger could be avoided if the ladders were supported on brackets
as fig. 30. No reliance should be placed upon the thumb screws, as
they may work loose under vibration. Danger from this source would be
avoided if the slot in which the ladder peg moved was made as shown in
fig. 30.
[Illustration: FIG. 30]
The necessary repairs can be carried out by means of boats, cradles, or
scaffolding.
Cradles and boats are swung from balk timbers laid across the top of
the shaft, or from hooks where the design of the chimney permits, as
shown in fig. 31.
The common method of fixing light scaffolds round a chimney or steeple
is shown in fig. 32. They are most easily fixed to square or other
flat-sided erections. The scaffolder having by means of ladders or
boats reached the desired height, fixes a putlog by means of holdfasts
to one of the walls. Another putlog is then fixed on the opposite side
of the building at the same level. The two are next bolted together by
1-inch iron bolts of the required length. The bolts are kept as near to
the wall as possible. The process is repeated again about 6 feet higher
on the building. The boards for the platforms are next laid. The first
are placed at right angles to the putlogs and project sufficiently to
carry the boards which are laid parallel to the putlogs. To prevent the
boards rising when weight is applied at one side of the scaffold, iron
plates bolted together (fig. 33) are fixed at the corners, and clips
(fig. 34) connect them to the putlogs.
[Illustration: FIG. 31] [Illustration: FIG. 32]
[Illustration: FIG. 33]
The stability of these scaffolds depends upon fixing at least two sets
of putlogs, connected by means of stays as shown in fig. 32. Bracing
is unnecessary if the putlogs and bolts tightly grip the building.
When these scaffolds are used on circular chimneys, chucks have to be
fitted on the inside of the putlogs to prevent them being drawn by the
bolts to a curve. The chucks (fig. 35) can be fastened to the putlogs
before they are fixed, if the curve of the building is accurately
known. When this is not the case, the putlogs are fixed by a holdfast
at their centre. The chucks are then placed in position, and clamped to
the putlogs as shown in fig. 36.
Additional holdfasts are then driven into the wall immediately under
the chucks, so that the putlogs are kept level.
[Illustration: FIG. 34]
[Illustration: FIG. 35]
The putlogs are fixed on edge, and when not exceeding 16 feet in length
are 7 in. by 3 in. Above that length they are 9 in. by 3 in. The stays
should be 4 in. by 2 in., and connected to the putlogs by 5/8-inch
iron bolts. The platform is usually of three boards 11 in. by 2 in.
[Illustration: FIG. 36]
[Illustration: FIG. 37]
Hollow towers are erected or repaired in the same manner as chimney
shafts, except that climbing ladders are not often required. External
or internal scaffolds may be erected. Towers being usually of larger
area than chimney shafts, the putlogs for internal scaffolding are
often of short poles from 6 to 8 inches diameter. Even these may
require extra support. This is gained by carrying standards from the
ground level or other solid foundation and tying to the putlogs. If of
great height the standards may be unable to carry their own weight. For
the cases where danger might be apprehended from this cause, fig. 37
shows a system of framing, which, being supported by the set-back in
the thickness of the wall, will carry the upper standards.
Steeples are generally built by the aid of external scaffolds, which,
as in the case of chimney shafts, should be well braced. The lower
portion may also be repaired in this way, the standards rising from the
ground level, or, if so designed, from the top of the tower. A series
of needles could be arranged for the higher portions.
[Illustration: FIG. 38]
=Domes and arches.=—The scaffolding for domes and arches consists
of a series of standards standing upon the area covered by the
building, and connected by ledgers and braces in directions at right
angles to each other. The platform is laid on the top ledgers.
When the building is of large span square timbers are often used, balks
for standards and runners, and half timbers for struts and braces.
Fig. 38 shows a design for repairing roofs and arches where a roadway
has to be kept below.
=Swinging scaffolds. Painters’ boats or cradles.=—Painters’ boats
are useful scaffolds for the repair of buildings, more especially where
the work is light. Fig. 39 shows the general construction. They are
suspended from jibs, fixed usually on the roof for outside work, and by
means of blocks and falls they can be moved in a vertical direction by
the workmen when in the boat.
[Illustration: FIG. 39]
The boats are fitted with guard boards and rails, and their safety,
providing the jibs are well fixed by balancing weights, is in their
favour. They are not self-supporting, and there is a distinct danger of
their running down if the sustaining ropes are not securely fastened
off. The wind causes them to sway considerably, and their use is
confined chiefly to façade work. An improved cradle is now in general
use, which is slung by head blocks from a wire cable running between
two jibs (see fig. 40). By the aid of guy lines movement in this case
can be also obtained horizontally, which removes the necessity of
shifting the jibs or employing a greater number of boats as in the
older method.
[Illustration: FIG. 40]
[Illustration: FIG. 41]
Another cradle as shown in fig. 41 has advantages which cannot be
ignored. It has steel cables with a breaking weight of 15 cwt. instead
of fibre ropes, and the cradle is raised and lowered by means of
gearing and a drum fixed in the gear case A. It is self-supporting,
and therefore safer than the cradle mentioned above. The lower ends of
the cable are fastened to the drum, and the gearing gives sufficient
mechanical advantage for one man to raise the scaffold by turning the
handle B. The uprights and rails are of angle steel or barrel and will
take apart and fold.
[Illustration: FIG. 42]
The boatswain’s boat (see fig. 42) is useful under some circumstances,
especially for making examinations of buildings for possible damage.
It is dangerous and awkward to work from, and is also acted upon
considerably by the wind.
The boat is slung from a single needle. The workman has no control over
its movement, as he has to be raised or lowered as required by men
having charge of the other end of the fall.
[Illustration: FIG. 43]
[Illustration: FIG. 44]
[Illustration: FIG. 45]
[Illustration: FIG. 46]
=Ladder scaffolds.=—A light scaffold of ladders braced, and
connected by rails, which also serve the purpose of guard rails, is
shown in fig. 43. The ladders, which have parallel sides, are placed
about 2 feet away from the building. The boards forming the platform
can be laid on the ladder rungs, or if necessary on brackets as shown
in fig. 44. The ladders are prevented from falling away from the
building by ties which are connected to the ladder as shown in fig. 45,
and fastened to the window openings by extension rods as shown in fig.
46. The same figure illustrates the method of tying in the scaffold
when the ladders are not opposite to the windows, the rail A
being connected to at least two ladders. The braces and guard rails are
bored for thumb screws at one end, the other being slotted so that they
can be adjusted as required. This form of scaffold is only suitable for
repairing purposes, and no weight of material can be stored upon it.
A light repairing scaffold lately put on the market has a platform
which is supported and not suspended, but otherwise affords about the
same scope to the workmen as the painters’ boats. It consists of one
pole and a platform, the latter being levered up and down the pole as
required by a man standing on the platform itself. The whole apparatus
can be moved by one man standing at the bottom. It is an arrangement
comparatively new to the English trade, but is in considerable use in
Denmark, Germany, and Sweden.
CHAPTER III
_SHORING AND UNDERPINNING_
Shoring is the term given to a method of temporarily supporting
buildings by a framing of timber acting against the walls of the
structure. If the frame consists of more than one shore, it is called a
system; if of two or more systems, it becomes a series.
The forces that tend to render a building unstable are due primarily
to gravity, but owing to the various resistances set up by the tying
together of the building, the force does not always exert itself
vertically downwards.
This instability may arise from various causes, the most common being
the unequal settlement of materials in new buildings, the pulling
down of adjoining buildings, structural alterations and defects, and
alterations or disturbances of the adjacent ground which affect the
foundations. The pulling down of an adjoining building would, by
removing the corresponding resistance, allow the weight of the internal
structure of the building to set up forces which at first would act
in a horizontal direction outwards. Structural defects, such as an
insufficiently tied roof truss, would have the same effect. Structural
alterations, such as the removal of the lower portion of a wall in
order to insert a shop front would allow a force due to gravity to act
vertically downwards.
To resist these forces, three different methods of shoring are in
general use, and they are known as flying or horizontal shores, raking
shores, and underpinning.
_Flying Shores._—Where the thrusts acting upon the wall are in
a horizontal direction, flying or raking shores are used to give
temporary support. The most direct resistance is gained by the
first-named, the flying or horizontal shore. There are, however, limits
to its application, as, owing to the difficulty of obtaining sound
timber of more than 50 to 60 feet in length, a solid body is necessary
within that distance, from which the required purchase can be obtained.
It is a method of shoring generally used where one house in a row is to
be taken down, the timbers being erected as demolition proceeds, and
taken down again as the new work takes its place.
Fig. 47 shows a half-elevation of two general systems of construction.
The framing, as at A, may be used alone where the wall to be
supported is of moderate height and the opening narrow, but larger
frames should be combined, as at B.
The framework C is for wide openings and walls of considerable
height.
The wall plates, 9 in. by 2 in. or 9 in. by 3 in., are first fixed
vertically on the walls by wall hooks. Then, in a line with the floors,
rectangular holes 4 in. by 3 in. are cut in the centre of wall plates.
Into these holes, and at least 4-1/2 inches into the brickwork, needles
(also known as tossles and joggles) of the same size are fitted,
leaving a projection out from the wall plate of 5 in. or 6 in.,
sufficient to carry the shore of about 7 in. by 7 in.
The shore, prior to being fixed, has nailed on its top and under sides
straining pieces 2 inches thick, and of the same width as the shore. To
tighten, oak folding wedges are driven at one end between the shore and
wall plate.
[Illustration: FIG. 47]
To stiffen the shore, and to further equalise the given resistance
over the defective wall, raking struts are fixed between the straining
pieces, and cleats are nailed above and below the shore. These raking
struts are tightened by driving wedges between their ends and the
straining pieces.
The cleats previous to, and in addition to being nailed, should be
slightly mortised into the wall plate. This lessens the likelihood of
the nails drawing under the pressure.
=A Raking Shore= consists of a triangulated system of timber
framing, and is used to support defective walls where the resistance to
the threatened rupture has to be derived from the ground surrounding
the building.
In its simplest form a raking shore is a balk of timber of varying
scantlings, but as a rule of square section, inclined from the ground
to the defective wall. The angle of inclination is taken from the
horizon, and should vary between 60 and 75 degrees. In settling this
the space available at the foot of the wall has to be taken into
consideration, especially in urban districts where the wall abuts on
the footpath.
[Illustration: FIG. 48]
Fig. 48 shows a raking shore in its simplest form, but usually two or
more shores are used (see fig. 49).
The following table from Mr. Stock’s book[1] shows the general rule and
also the scantlings to be used:
For walls from 15 to 30 feet high 2 shores are necessary in each system
For walls from 30 to 40 feet high 3 shores are necessary in each system
For walls from 40 feet high and upwards 4 shores are necessary in each
system
[Illustration: FIG. 49.—EXAMPLE OF RAKING SHORE]
Taking the angle of the shore at from 60 to 75 degrees:
For walls from 15 to 20 feet high 5 in. by 5 in. may be the scantling
for each shore
For walls from 20 to 30 feet high 6 in. by 6 in. may be the scantling
for each shore
For walls from 30 to 35 feet high 7 in. by 7 in. may be the scantling
for each shore
For walls from 35 to 40 feet high 8 in. by 8 in. may be the scantling
for each shore
For walls from 40 to 50 feet high 9 in. by 9 in. may be the scantling
for each shore
For walls from 50 feet and upwards 12 in. by 9 in. may be the scantling
for each shore
In the greatest length, the beams are 12 in. by 9 in. to give increased
rigidity, which prevents any likelihood of sagging.
The wall plate is the first timber put into position. It is placed
vertically down the face of the wall, and held in its position by wall
hooks. Note should then be taken of the position of floors. If the
floor joists run at right angles to the wall, the shore should abut on
the wall in such position that it points directly below the wall plate
carrying the floor joist. If the joists run parallel to the wall, the
shore should act directly on a point representing the meeting of lines
drawn down the centre of the wall and across the centre of the floor
(see fig. 50).
[Illustration: FIG. 50]
To enable the shore to fulfil this condition, the needle (of 4 in. by 3
in.) should be let through the plate 4-1/2 inches into the wall below
the point in question. To strengthen the needle cleats are nailed, and
slightly let into the plate immediately above.
The footing, or sole piece, has next to be laid. It consists of a
timber 11 in. by 3 in., and long enough to take the bottom ends of
the required number of shores. Attention should be paid to the ground
in which it is to be bedded, and if this is at all soft, additional
timbers should be laid under, and at right angles with it, to give
greater bearing.
The sole piece should not be laid at right angles to the shore, but
its face should form, with the outside line of the top shore, an angle
somewhat wider, say of 93 degrees. The advantage of this will be seen
presently.
The shore itself has now to be prepared. Its top end should be grooved
sufficiently (fig. 51) to receive the needle. This will prevent lateral
motion when under pressure.
[Illustration: FIG. 51]
The bottom end should be slightly slotted, in order to receive the end
of a crowbar (see fig. 52).
It is now placed in position, and gently tightened up by the leverage
of a crowbar acting in the slot, and using the sole piece as a fulcrum.
The advantage of the sole piece not being at right angles to the shore
can now be seen, as if it were so laid no tightening could be gained
by the leverage. This system is an improvement upon the tightening up
by wedges, as the structure is not jarred in any manner. If the frame
is to have more than one shore, they are erected in the same manner,
the bottom shore being the first put up, the others succeeding in their
turn. When in position the shores are dogged to the sole piece and a
cleat is nailed down on the outer side of the system. The bottom ends
are then bound together by hoop iron just above the ground level. To
prevent the shores sagging, struts are fixed as shown on fig. 49.
[Illustration: FIG. 52]
Besides preventing the sagging these struts serve the purpose of
keeping the shores in position. They may be fixed as nearly at right
angles to the shores as possible, or at right angles to the wall; in
any case they should reach to the wall plate at a point just below the
needle. The struts should be nailed to the shores and wall plate. If
the latter is wider than the shores, it should be cut to receive the
struts.
It sometimes occurs that the timbers are of insufficient length to
reach from the sole piece to wall plate. To overcome this difficulty,
a short timber is laid on the sole piece against and parallel to the
next middle raker, and on this short timber a rider shore stands
reaching to its position on the wall plate (see fig. 49).
When this is done the top middle raker should be stiffer to resist the
increased cross strain. Stiffness is gained by increasing the depth. A
rider shore is tightened by oak folding wedges driven between the foot
of the shore and the short timber which supports it.
Note must be taken that the outer raker is not carried too near the top
of the building, or else the upward thrust of the shores, which always
exists with raking shores, might force the bond or joints.
Fir is the best wood for shoring owing to the ease with which it can
be obtained in good length. Another advantage is its straightness of
fibre; although, as it is more easily crushed by pressure across the
grain, it does not answer so well as oak for wedges, sole pieces, &c.
In erecting flying or raking shores, notice should be taken of the
following points.
The systems should be placed from 12 to 15 feet apart if on a wall
without openings, otherwise on the piers between the openings.
In very defective walls it is an advantage to use lighter scantlings,
the systems being placed closer together. Heavy timbers handled
carelessly may precipitate the collapse which it is the intention to
avoid.
Wedge driving and tightening should be done as gently as possible. It
should be remembered that support only is to be given, and not new
thrusts set up, which may result in more harm than good.
=Underpinning.=—Underpinning is necessary to carry the upper
part of a wall, while the lower part is removed; for instance, the
insertion of a shop front, or the repairing of a foundation. It is only
kept in position until a permanent resistance to the load is effected.
Underpinning is, as a rule, unnecessary when the opening to be made
is of less width than five feet. This method of shoring is a simple
operation, but yet requires great care in its execution.
The first thing to be done is to remove from the wall all its attendant
loads. This is accomplished by strutting from the foundation floor
upwards from floor to floor until the roof is reached (see fig. 53).
Header and sole plates 9 in. by 2 in. are put in at right angles to the
joists in order to give bearing to the struts.
The portion of the wall to be taken down having been marked out, small
openings are made, slightly above the proposed removal, at from 5 to 7
feet apart, and through these, at right angles to the face of the wall
itself, steel joists or balk timbers 13 in. by 13 in., called needles,
are placed. These are supported at each end by vertical timbers 13 in.
by 13 in., called dead shores, which again rest upon sleepers.
The sleepers serve as a bed to the dead shores to which they are
dogged, and by distributing the weight over a larger area, they prevent
the dead shores sinking under the pressure. The dead shores, if well
braced, may be of smaller scantling.
Where it is impossible to arrange for the dead shores to be in one
length, the lower pieces are first fixed. They must be of uniform
length, and across their top end a transom is carried to support the
upper pieces, the bottom ends of which must stand directly over the top
ends of the lower pieces (see fig. 53).
Having placed all the timbers in position, and before the tightening up
takes place, the windows or other openings in the wall are strutted
to prevent any twisting which may take place. This is done as shown on
fig. 54, but small windows do not require the centering.
[Illustration: FIG. 53.—EXAMPLE OF UNDERPINNING]
The tightening up is caused by the driving home of oak folding
wedges placed in the joints between the needles and the dead shores.
This position is better than between the shore and sleeper, as any
inequality of driving here would have the tendency to throw the shore
out of the perpendicular. For a similar reason the wedges should be
driven in the same line as the run of the needle, as cross driving, if
unequal, would cause the needle to present an inclined surface to the
wall to be carried.
[Illustration: FIG. 54]
In carrying out these operations note should be taken of the following
points:—
1. That the dead shores should not stand over cellars or such places.
It is better to continue the needle to such a length that solid ground
is reached, and the needle can then be strutted from the dead shore.
2. That extra needles should be placed under chimney breasts, should
there be any in the wall which is to be supported. The same applies
where corbels, piers, &c. occur.
3. Sleepers and shores should be so placed that they do not interfere
with the proper construction of the new foundations or portions of the
building.
4. The inside shores should be carried freely through the floors until
a solid foundation is reached.
5. The removal of the shores after the alterations have been made is
one requiring great care. It should be remembered, that while the work
is new it cannot offer its greatest resistance to its intended load.
Time should therefore be given for the work to well set, and then
the timbers, eased gradually by the wedges being loosened, should be
finally taken out.
6. The raking shores, if used in conjunction with underpinning, should
be left to the last.
CHAPTER IV
_TIMBER_
=Classification and Structure.=—A short study of the
classification and structure of wood will be useful, as it will enable
the scaffolder to use material free from the inherent defects of its
growth.
The trees used by the scaffolder are known as the exogens or
outward-growing trees.
The cross section of an exogenous tree shows, upon examination, that
the wood can be divided into several parts. Near the centre will
be seen the pith or medulla, from which radiate what are known as
the medullary rays. These in the pine woods are often found full of
resinous matter.
Next will be noted the annual rings forming concentric circles round
the pith. They are so called because in temperate climates a new ring
is added every year by the rising and falling of the sap. As the tree
ages, the first-laid rings harden and become what is known as duramen
or heartwood. The later rings are known as alburnum or sapwood. The
distinction between the two is in most trees easily recognised, the
sapwood being lighter and softer than the heartwood, which is the
stronger and more lasting. The bark forms the outer covering of the
tree.
=Defects in the living tree.=—Shakes or splits in the interior of
the wood are the most common defects in the living tree, and are known
as _star_ or _radial_ and _cup_ or _ring shakes_.
The cause of these defects is imperfectly understood. They are rarely
found in small trees, say those of under 10 inches diameter. Stevenson,
in his book on wood, puts forward the following reason, which, up to
now, has not been refuted by any practical writer. 'In the spring, when
the sap rises, the sapwood expands under its influence and describes
a larger circle than in winter. The heartwood, being dead to this
influence, resists, and the two eventually part company, a cup or ring
shake being the result’ (see fig. 55, where the cup shake is shown in
its commonest form).
[Illustration: FIG. 55]
[Illustration: FIG. 56]
The star or radial shake is a variant of the same defect. In this case
the cohesion between the sapwood and the heartwood is greater than the
expansive forces can overcome, the result being that the heartwood
breaks up into sections as shown in fig. 56.
The star shake may have two or more arms. More than one cup shake, and
sometimes both cup and star shakes are found in a single tree. The
radial shake is probably the most common.
The branches suffer in this respect in like manner as the trunk, the
same shakes being noticeable throughout
The development of these defects is the forerunner of further decay in
the tree, giving, as they do, special facilities for the introduction
of various fungi, more especially that form of disease known as the
rot. Wet rot is found in the living tree and occurs where the timber
has become saturated by rain.
Other authorities believe that these defects are caused by severe
frost, and their idea receives support from the fact that in timber
from warmer climates this fault is less often seen. This may be so for
the reason that it would not pay traders to ship inferior timber from a
great distance.
_Loose hearts_: in the less resinous woods, as those from the
White Sea, the pith or medulla gradually dries and detaches itself,
becoming what is known as a loose heart. In the more resinous woods,
such as the Baltic and Memel, this defect is rare.
_Rind galls_ are caused by the imperfect lopping of the branches,
and show as curved swellings under the bark.
_Upsets_ are the result of a well-defined injury to the fibres
caused by crushing during the growth. This defect is most noticeable in
hard woods.
It will be of no practical use to follow the living tree further in its
steady progress towards decay, but take it when in its prime, and study
the processes which fit it for the purpose of scaffolding.
=Felling.=—The best time to fell timber, according to Tredgold,
is mid-winter, as the vegetative powers of the tree are then at rest,
the result being that the sapwood is harder and more durable; the
fermentable matters which tend to decay having been used up in the
yearly vegetation. Evelyn, in his 'Silva,’ states that: 'To make
excellent boards and planks it is the advice of some, that you should
bark your trees in a fit season, and so let them stand naked a full
year before felling.’ It is questionable if this is true of all trees,
but it is often done in the case of the oak. The consensus of opinion
is that trees should be felled in the winter, during the months of
December, January and February, or if in summer, during July. Winter
felling is probably the better, as the timber, drying more slowly,
seasons better.
The spruce from Norway and the northern fir are generally cut when
between 70 and 100 years old. When required for poles, spruce is cut
earlier, it having the advantage of being equally durable at all ages.
Ash, larch and elm are cut when between 50 and 100 years old.
=Conversion.=—By this is meant cutting the log to form balks,
planks, deals, &c. It is generally carried out before shipment. A log
is the trunk (sometimes called the stem or bole) of a tree with the
branches cut off.
A balk is a log squared. Masts are the straight trunks of trees with a
circumference of more than 24 inches. When of less circumference they
are called poles.
According to size, timbers are classed as follow:—
Balks 12 in. by 12 inches to 18 in. by 18 inches
Whole timber 9 in. by 9 inches to 15 in. by 15 inches
Half timber 9 in. by 4½ inches to 18 in. by 9 inches
Quartering 2 in. by 2 inches to 6 in. by 6 inches
Planks 11 in. to 18 inches by 3 in. to 6 inches
Deals 9 inches by 2 in. to 4½ inches
Battens 4½ in. to 7 inches by ¾ in. to 3 inches
When of equal sides they are termed die square. In conversion the pith
should be avoided, as it is liable to dry rot.
When the logs are to be converted to whole timbers for use in that
size, consideration has to be given as to whether the stiffest or the
strongest balk is required. The stiffest beam is that which gives most
resistance to deflection or bending. The strongest beam is that which
resists the greatest breaking strain. The determination of either can
be made by graphic methods.
[Illustration: FIG. 57]
To cut the stiffest rectangular beam out of a log, divide the diameter
of the cross section into four parts (see fig. 57). From each outside
point, A and B, at right angles to and on different
sides of the diameter, draw a line to the outer edge of the log. The
four points thus gained on the circumferential edge, C,
D, E, F, if joined together, will give the
stiffest possible rectangular beam that can be gained from the log.
[Illustration: FIG. 58]
To cut the strongest beam:—In this case the diameter is divided into
three parts. From the two points A and B thus marked
again carry the lines to the outer edge as before. Join the four points
C, D, E, F, together, and the
outline of the strongest possible rectangular beam will result (see
fig. 58).
Boards—a term which embraces planks, deals, and battens—should be cut
out of the log in such a manner that the annual rings run parallel to
the width of the board. This method of conversion allows the knots,
which are a source of weakness, to pass directly through the board, as
A, fig. 59, and not run transversely across as other sections
(B) allow.
[Illustration: FIG. 59]
=Seasoning.=—Poles and scantlings, after conversion, are next
prepared for seasoning.
The drying yard, where the timbers are seasoned, should give protection
from the sun and high winds, although a free current of air should be
allowed. It should be remembered that rapid drying tends to warping and
twisting of the timbers.
The ground on which the timbers are stacked should be drained and kept
dry.
The method of stacking is as follows: The timbers are laid upon
supports a few inches above the ground at sufficient intervals to allow
of a free circulation of air between them, and on these others are laid
at right angles. It is usual to put long strips of wood about ¾ inch
square between each row to prevent the timbers touching, as, after a
shower, the timbers cannot dry for a long time if one is resting on the
other.
Planks, deals, &c. can be stacked by laying them in one direction
throughout, provided that the space which is left between the boards
occurs in one layer immediately over the centre of the boards beneath
(see fig. 60).
In many cases very little drying takes place before shipment, but the
same methods of stacking should be observed whenever it takes place.
Stacking poles by placing them on end is not recommended, as they may
warp from insufficient support. This point is more important when the
poles are required for ladder sides.
[Illustration: FIG. 60]
Timber may be considered to be sufficiently seasoned for rough work
when it has lost one-fifth of its weight by evaporation.
[Illustration: FIG. 61]
The poles from St. Petersburg have a narrow strip of bark removed in
four equidistant longitudinal lines throughout their entire length.
This treatment assists drying, and tends to prevent dry rot.
Weather shakes sometimes form on the outside of the wood while
seasoning (see fig. 61).
They arise owing to the sapwood contracting more when drying than the
heartwood. Unless they extend to a considerable depth they do not
affect the quality of the wood. Balk timbers, where the sapwood is
uncut, and whole timbers principally suffer in this manner.
_Water seasoning_—that is, having the timbers completely immersed
in water for a short time before drying. This is a common practice. It
is frequently carried out at the docks, where the balks may be seen
floating about on the surface of the water. This is a bad method, as
the wood is at the same time under the influence of water, sun and air.
Water seasoning may make the wood more suitable for some purposes, but
Duhamel, while admitting its merits, says: 'Where strength is required
it ought not to be put in water.’
=Description.=—Pine or northern fir (_Pinus sylvestris_) is
light and stiff, and is good for poles and scaffolding purposes, but
only the commonest of the Swedish growths are used for this purpose.
The hardest comes from the coldest districts. It has large red knots
fairly regularly placed, inclined to be soft, and starting at acute
angles. It grows chiefly in Northern Europe.
White spruce or white fir (_Abies excelsa_).—The knots are small
and irregularly placed, are dark in colour, and start at an obtuse
angle, showing an absence of clean wood. It is used principally for
scaffold poles and ladder sides. It will snap under live loads, and is
not so strong as pine. It also chiefly grows in Northern Europe.
Larch (genus _Larix_) is imported from Northern Europe and
America. It is yellow in colour, tough, and is suitable for poles,
very durable, free from knots, but warps easily.
Elm (genus _Ulmus_) grows in the British Isles. The colour varies
from the reddish-brown of the heartwood to yellowish white of the
sapwood. It bears considerable pressure across the grain, and is most
useful in balks, as it is liable to warp when in smaller sizes. Is
suitable where bolts and nails have to be used.
Birch (_Betula alba_).—Light brown in colour, hard, even grain,
which enables it to be easily and readily split in the direction of its
length. It is not considered durable. It is used chiefly for putlogs.
Is exported from Europe and America.
Ash (_Fraxinus excelsior_).—Very light brown in colour, extremely
tough, and makes excellent rungs for ladders. It is found in Europe.
Oak (genus _Quercus_), also known as the common British oak, is a
native of all parts of Europe, from Sweden to the Mediterranean. The
wood has often a reddish tinge; and the grain is fairly straight, and
splits easily. It is generally free from knots, and is most suitable
where a stiff, straight-grained wood is desirable. It also offers
considerable resistance to pressure across the grain.
=Selection.=—The importation of scaffolding timber commences in
March. In June the Russian timbers are on the market, and the arrivals
continue until October.
Poles are selected from spruce, pine, and larch trees. Balk timbers are
of elm and fir and spruce. The putlogs are from the birch.
Poles are known in two qualities—'prime’ and 'brack.’ These terms refer
to their straightness of grain, freedom from knots, regularity of
taper, which should be slight, and condition as to seasoning.
Buyers take them usually unbarked, as they rise from the stack, and
sort them afterwards for their different purposes.
As soon as possible the bark should be removed, as it holds the water
and insects, and encourages the growth of a white fungus which is the
precursor of dry rot.
The St. Petersburg poles are the most generally used for scaffolding.
They are weaker than the Christiania poles, breaking shorter under
cross strains, are whiter in colour, have a smoother bark, are
straighter in grain, and therefore make better ladder sides.
Christiania poles are more yellow in colour, and break longer under a
cross strain than other poles. They are only to be obtained early in
the year, and are soon bought up when on the market.
To test a pole remove the bark, then prise across the grain with a
penknife. If the fibres break up short and brown, the pole is decayed
and useless.
To test a pole for any local weakness lift one end, leaving the other
on the ground. Two or three sharp jerks will cause undue bending at any
spot that may be seriously defective.
Balk timbers ring, if in a sound condition, when struck with a hammer.
A fresh-cut surface should be firm, shining, and somewhat translucent.
A dull, chalky, or woolly appearance is a sign of bad timber.
Poles, unlike most converted timbers, are not branded. Those known as
_prime_ should certainly be used for scaffolding purposes. The
brack are rough and irregular in growth.
Balk timbers and smaller scantlings are, in the finest qualities,
branded, the different countries from which they are exported being
Russia, Norway and Sweden.
Russian woods are generally hammer branded, no colour being used.
Norwegian woods are marked a blue colour, and Swedish wood a red colour.
Straight-grained timber should be chosen. Twisting, which may occur
to the extent of 45°, is not so apparent in young trees. The beam is
thereby weakened as regards tension and compression.
Large knots should be avoided. They weaken the beam for tension and
cross strains, but serve good purpose when in compression.
Sap in the wood is denoted by a blue stain, and betokens inferiority
as to strength and lasting qualities. This blueness is more noticeable
when the wood is wet. It must not be confounded with dark weather
stains.
=The Decay of Timber.=—Timber exposed to the constant changes of
weather tends to decay early. It has been noticed that wood when dried
after being exposed to dampness not only lost the moisture it had
absorbed, but also part of its substance. This loss occurs in a greater
degree when these changes take place a second time. It is therefore
apparent that scaffolding timber, exposed as it is to the vicissitudes
of the weather, without any protection such as would be gained by
painting, will soon from this cause show signs of decay.
Quicklime, when wet, has also a most destructive effect upon timber;
the lime, by abstracting carbon, helping the decomposition materially.
Scaffold boards suffer most in this respect from contact with fresh
mortar, the lime in which has not had time to become mild.
From a similar cause, the ends of putlogs which are inserted in a
newly built wall, as when used in a bricklayer’s scaffold, tend to
decay rapidly.
Scaffolding poles, when used as standards, have an increased tendency
to decay at their butt ends, owing to their being imbedded in the
ground to a distance of two or three feet.
The presence of sap in improperly seasoned wood is also conducive to
early decay.
=Preservation of Timber.=—Scaffolding timber being comparatively
cheap has little attention paid to it in regard to preservation.
When out of use the poles should be stacked, and a free current of air
around each ensured, not laid carelessly on the ground, which is too
frequently the case. The same remarks apply to the care of boards.
Nails which have been driven into the timber should, after use, be
carefully drawn. When left in the wood they rust, and set up a new
source of decay.
As before noted, the butt ends of standards tend to decay early. A
fairly effective method of preventing this is to bore the butt end
upwards to a distance of about 2 feet. The bore, which should be about
1/2 inch in diameter, should then be filled with pitch and plugged. The
pitch will find its way by capillarity into the pores of the wood and
tend to keep the dampness out. Coating the outsides of the butts with
pitch is also useful, and a combination of the two would no doubt be
effectual; this is especially recommended where the standards have to
remain in one place for a long time.
Ladders and fittings that are expensive are generally painted.
=The Durability of Wood.=—In Young’s 'Annals of Agriculture’ it is
stated that experiments were made on some 1½-inch planks of 30 to 45
years’ growth. They were placed in the weather for ten years, and then
examined, with the following result:
Larch—heart sound, sapwood decayed.
Spruce fir—sound.
Scotch fir—much decayed.
Birch—quite rotten.
This experiment, while useful to show the natural resistance of the
wood to the weather, does not take into account the effect of wear and
tear.
It is almost impossible, in fact, to arrive at any data from which the
life of scaffolding timbers can be gauged; but, roughly speaking, poles
may be expected to last from six to ten years according to the care
exercised. Balk timbers, being usually cut up after a time for other
purposes, have only a short life on a scaffold, and therefore seldom
decay while in use.
[Illustration: FIG. 62]
=The Use of Scaffolding Timber.=—Poles vary up to 40 and 50
feet in length and up to 8 inches in diameter at the butt end. As
decay, which usually commences at this end, sets in, the poles can be
shortened and made into sound puncheons or splicing pieces for the
ledgers. Balks are used up to 50 or 60 feet in length; and their period
of service on a scaffold is often an interval during which they become
well seasoned and suitable for other requirements. Putlogs are about 6
feet in length and 4 inches square in cross section, tapering sharply
to 2-1/2 by 3-1/2 inches at the ends where required for insertion in
the wall. Being of square section they are not liable to roll on the
ledgers. They should be split, not sawn, in the direction of their
length. The fibres are thus uncut and absorb moisture less easily.
This procedure it is found increases their durability. When treated in
this manner they are also stronger, as the fibres, being continuous,
give greater resistance to a load, the strength not depending wholly
on the lateral adhesion, but also upon the longitudinal cohesion. When
partly decayed they can be shortened and make good struts for timbering
excavations.
The boards are usually from 7 in. to 9 in. wide and 1½ in. to 2 in.
thick. Their length averages up to 14 feet. The ends are sawn as fig.
62 and strapped with iron to prevent splitting. When decayed they are a
source of danger for which there is only one remedy—smash them up.
CHAPTER V
_CORDAGE AND KNOTS_
The fibres used for the ropes and cords for scaffolding purposes are of
jute and hemp. The strongest rope is made of the latter variety, and
the best quality is termed the Manilla.
The cords are generally tarred for preservation, but this process has a
bad effect upon the strength of the fibre, more especially if the tar
is impure. The process of tarring is as follows:—
The fibres before they are formed into strands are passed through a
bath of hot tar. Immediately afterwards, and while still hot, the
material is squeezed through nippers, by which means any surplus tar is
removed.
The ropes, as used in scaffolding, are known as shroud laid or three
strand.
A shroud-laid rope consists of three strands wound round a core, each
containing a sufficient number of fibres to make an equal thickness. A
three-strand rope is similar, but has no core.
The following gives the breaking weights of scaffold cords and falls
according to circumference[2]:—
Size Cwts. qrs. lbs.
Tarred hemp scaffold } {2 inches in circumference 20 0 0
Cords } {2¼ inches in circumference 25 0 0
Tarred jute scaffold } {2 inches in circumference 14 0 0
Cords } {2¼ inches in circumference 18 0 0
White Manilla scaffold } {2 inches in circumference 32 0 0
Cord } {2¼ inches in circumference 39 0 0
{2½ inches in circumference 44 0 0
White Manilla rope for } {3 inches in circumference 70 0 0
pulleys and block ropes} {3½ inches in circumference 98 0 0
{4 inches in circumference 126 0 0
{4½ inches in circumference 158 0 0
It should be noted that all these weights are the actual breaking
strains of the ropes tested to destruction. In practice, one sixth
of the breaking weights only should be allowed, in order to leave a
sufficient margin for safety.
The strength of a rope is the combined strength of each separate yarn.
Therefore, if the fibres are not carefully twisted together, so that
each bears an equal strain, the rope is unsafe. Care should be taken
when choosing a rope that the strands are closely, evenly and smoothly
laid.
The rope is strongest when the fibres are at an angle of 45 degrees
to the run of the rope. When at a greater angle than this, the fibres
are apt to break, and when at a less angle, the friction between the
parts—upon which the strength of a rope greatly depends—is lessened.
The durability of tarred ropes is greater than when untarred, but not
to such an extent if kept dry.
A splice weakens a rope about one eighth.
Ropes are adulterated by the admixture of rubbish fibre termed 'batch.’
To test a rope as to condition, untwist it and notice whether any short
ends break upwards. If so, or if the tarring has decayed internally,
the rope must be viewed with suspicion.
Hemp ropes, after four to six months’ wear, are often one fourth weaker
than new ones.
Scaffold cords are from 15 to 18 feet in length.
Moisture will cause a shrinkage of 6 inches in an 18-foot cord.
A good hemp rope is more reliable than an iron chain, as the latter
sometimes snaps on surgeing.
The following diagrams show the various knots used in scaffolding,
both in the erection of the scaffold and for attaching materials to a
hoist. They are shown in a loose condition as being more useful for
study by a pupil. Several knots are, however, too intricate to explain
by diagram, and in these cases an attempt has been made, with but scant
success, to instruct as to their method of creation in the notes. It
will, however, be found by the student that half an hour with an expert
scaffolder, preferably an old sailor, will afford more instruction than
hours spent in studying diagrams.
The art of knot making is governed by three principles. First, a knot
must be made quickly. Secondly, it must not jam, as this prevents it
being undone easily. Lastly, under strain, it should break before
slipping.
[Illustration: PLATE III.]
=Plate III.=—No. 1. _Overhand or thumb knot._ Prevents the
end of a rope opening out or passing through the sheaves of a block.
No. 2. _Figure of eight knot._ Used as No. 1.
No. 3. _Square or reef knot._ Will sometimes jam with small ropes.
No. 4. The knot we all make until we learn better, known as the granny,
and will both slip and jam.
No. 5. _The bend or weaver’s knot_, used for joining ropes
together or securing a rope through an eye splice.
No. 6. _Wolding stick hitch_, is serviceable only in connection
with a pole used as a lever.
No. 7. _Bale sling_, for hanging on to hook of lifting tackle.
No. 8. _Magnus hitch, or rolling hitch_, for lifting material.
No. 9. _Two half hitches, or builder’s knot, or clove hitch._ Used
for tying ledgers to standards.
No. 10. _Loop knot_, used where ends of rope are not available.
No. 11. _Loop knot_, used where ends of pole are not available.
No. 12. _Boat knot._
No. 13. _Sheepshank, or dogshank_, a method of shortening a rope
without cutting it or reducing its strength.
[Illustration: PLATE IV.]
=Plate IV.=—No. 14. _Blackwall hitch_, very powerful, but
requires watching or may slip.
No. 15. _Midshipman’s hitch_, used as shown with a rounded hook.
No. 16. _Catspaw_ is an endless loop, and is used where great
power is required.
No. 17. _Capstan knot, or bowline._ After tightening it will not
slip.
No. 18. _Timber hitch_, for carrying scaffold poles. Take one turn
round the pole and standing part, and finish with jamming turns.
No. 19. _Artificer’s knot_, or half hitch and overhand.
No. 20. _Topsail halliard bend_, used as a timber hitch.
No. 21. _Bowline on a bight._ A board across the loops makes a
useful seat.
No. 22. _Racking or nippering_ is a method of temporarily joining
two ropes for lengthening purposes. The ends are laid side by side for
about 18 inches, and the marline or spunyarn is taken for about a dozen
turns round both, then by round turns over all and fastened with a reef
knot.
Nos. 23 and 24. _Round seizing._ With a slip knot at the end of
the spunyarn take a turn round the ropes to be nipped together. This
turn should be pulled tight, and continued for about a dozen turns
(No. 22); then take the end through the last turn, and take turns over
the first laid, finishing by carrying the spunyarn two or three times
between the rope and the seizing. Knot the end by jamming turns,
keeping the whole well taut (No. 24).
[Illustration: PLATE V.]
=Plate V.=—No. 25. _Butt or barrel sling_ when placed
horizontally.
No. 26. _Butt or barrel sling_ when placed vertically.
No. 27. _Double overhand knot._
No. 28. _Running bowline._
Nos. 29 and 30. _Band tie, marrying or splicing._ Commence as No.
29, and after continuing the turns until near the end of the rope, take
the rope twice between the poles and round the turns first laid, and
finish with jamming turns. Tighten with a wedge.
Nos. 31, 32 and 33. _Tying between standard and ledger._ Commence
with two half hitches as No. 31. Then twist ropes together as far as
they will go, and place ledger in position above the hitches (No. 32).
The twisted ropes are then drawn up in the front of the ledger to the
left of the standard, taken round the back of the standard, brought
again to the front and round ledger at the right of the standard, then
cross in front of the standard and round the ledger at the left of the
standard, and brought up and carried round the back of the standard.
This process is repeated until the end of the rope is nearly reached,
when it is given two or three turns round the ledger and fastened off
with jamming turns (No. 33). To tighten, drive a wedge at back of
standard.
[Illustration: PLATE VI.]
=Plate VI.=—No. 34. _Portuguese knot_ for shear legs, made by
several turns of the rope round the poles, and interlaced at the ends.
No. 35. _Running bow knot_—inferior to No. 28.
No. 36. _Bowline_—inferior to No. 17.
No. 37. _Double bend_—useful where a small rope is bent on to a
larger. The end of the rope is given one extra turn round the bight of
the other, with the consequence of a great increase of strength.
No. 38. _Fisherman’s knot._
No. 39. _Lark’s head_, fastened to a running knot.
No. 40. Where increased strength is required a small rope can be
attached to a larger one by means of a rolling hitch. The whole
arrangement comes apart as soon as the strain is removed.
No. 41. A method of lifting scaffold poles in a vertical position by
the use of the timber hitch and half hitch. If it is required to free
the upper end while the pole is being carried, the half hitch can be
replaced by a cord tied round the pole and the lifting rope.
CHAPTER VI
_SCAFFOLDING ACCESSORIES AND THEIR USE_
=Ladders.=—A ladder consists of a pair of sides connected by a
series of oak or ash treads or rungs. The sides of long ladders are
formed by cutting lengthwise a straight fir pole into two pieces. Short
ladders are made with square sides, but they are heavier and, as the
grain seldom runs continuously throughout their length, also weaker.
The pole for the sides is selected for its straightness of fibre, for a
twisted pole cannot be sawn lengthwise without cutting across part of
the grain, which as before stated is a source of weakness.
The pole has to be bored for the rungs, and this may be done either
before or after it has been cut lengthwise. It is better to bore first,
as then the holes occur at coincident intervals on each side; the rungs
by this means are kept parallel and level.
The rungs are placed 9 inches apart, and are from 1 in. to 1-1/2-in.
in depth in the centre, decreasing to 5/8-inch diameter at the ends.
The ends of the rungs are painted with red lead before insertion, and
any projections are afterwards cut off flush with the sides. They can
be fastened at each end with wedges (see fig. 63), or pins of 1/4-inch
diameter can be cut through the sides, as fig. 64, to fix every fifth
or sixth rung.
The first method is the better, as cross boring of the sides greatly
weakens them; but as the wedges may work out, an iron rod 5/16-inch in
diameter should be placed below every eighth or ninth rung, and bolted
on the outside for extra security.
The iron rods should not be used as treads instead of the wooden rungs
as they offer an insecure foothold. The rungs are considered to be
dangerous for use when they have been reduced by wear to one half of
their original depth. The best rungs are made from old wheel-spokes, as
they are well seasoned. The sides, which may be of sufficient length
to receive 100 rungs, are 9 inches apart at the top and from 12 to 13
inches apart at the bottom, according to the length of the ladder.
[Illustration: FIG. 63]
[Illustration: FIG. 64]
Extension ladders are useful where, owing to the varying heights of the
work, different lengths of ladders are required. The two halves of the
ladder are connected in various ways, but if well made they are easily
raised and lowered. They should be used only for the very lightest
work, such as painting, cleaning down, &c.
=Trestles.=—Trestles are used chiefly by painters plasterers, and
mechanics engaged on work that is not at a great height from the ground
or floor level, and for which a platform is required. They stand from
6 to 12 feet in height, and the rungs should be sufficiently wide to
carry three boards for the working platform. They are made of yellow
deal, with mortised joints and wrought-iron hinges (fig. 65).
[Illustration: FIG. 65]
=Steps.=—Steps are built up with two sides of the required height,
about 5 inches wide and 1 inch thick; the top and bottom are sawn to a
bevel so that they stand inclined.
The steps, which are grooved into the sides and fixed with screws,
are about 6 inches wide by 1-1/2 inches thick, and increase slightly
in length as they descend. This increase adds to the stability of the
steps as the width of the base is increased. The distance between each
step is from 7 to 9 inches.
At the back of the top step two legs about 2-1/2 inches wide by 1 inch
thick are secured by strong flap hinges. The legs are framed together
by two cross pieces, 3 or 4 inches wide and 1 inch thick.
The back legs, by opening out on the flap hinges, enable the entire
framework to stand upon an even surface. To prevent the legs opening
too far, they are connected to the sides of the steps by cords.
=Cripples.=—The simplest form of cripple is shown on fig. 66,
which sufficiently explains the design.
This cripple forms a fixed angle with the ladder, which, in order to
keep the platform level, can be laid only at one slope against the
wall. The defect is removed if the cripple is hinged and fitted with a
quadrant and pin, as shown on fig. 67. The platform in this case can be
kept level by adjustment irrespective of slope of ladder. The bracket
should be long enough to carry a platform three boards wide, but as a
rule it carries two.
[Illustration: FIG. 66]
[Illustration: FIG. 67]
Cripples may project from either side of the ladder, and are usually
hung on the rungs. An advantage is gained if, in addition to this,
clips are provided to clutch the sides of the ladder.
=Buckets and Skips.=—Besides the ordinary pail, which needs no
description, larger buckets are commonly used for carrying concrete,
mortar, earth, or any other moist or friable material.
Fig. 68 shows the tipping bucket, or skip, which balances on its hinges
at A. The hinges are so placed that they are above the centre
of gravity of the bucket when empty, and below the centre of gravity
when full. This position allows the bucket to remain upright when
empty, but it will make half a revolution and empty its contents when
full. To prevent this action occurring before it is required, a catch
on hinges is fixed on the rim of the bucket at B.
[Illustration: FIG. 68]
While the catch is in the position shown, the bucket cannot tilt, but
if it is turned back the bucket makes the half revolution required, and
after emptying its contents, swings upright of its own accord.
Buckets are constructed of steel, and the standard sizes vary in
capacity from 1/4 to 1 cubic yard.
For a similar purpose a steel box is used. In this case the bottom of
the box is hinged, and on the catch being released, drops out, allowing
the material to fall over any desired spot. The catch can be released
from above or below by means of a chain connected thereto, and the
bottom of the box regains its position when lowered to the ground for
refilling.
Each box is fitted with a bow for chain hook, or lugs for chain slings;
has a capacity of about 3 cubic feet, is made of steel plates, and may
be round or square on plan.
=Baskets.=—Baskets (as shown in figs. 69-73) have a capacity of
about 1 cubic foot.
There are three qualities of cane used in their construction: 'Mackerel
back,’ recognised by its peculiar markings, 'Short Nature,’ and
'Squeaky.’ Of these, the first is the best, the others following in the
order named. It is a defect of the baskets as ordinarily constructed
that their handles and bottoms give way after very little wear. Several
improvements have been put on the market, the best of which are shown
as follows.
[Illustration: FIG. 69]
[Illustration: FIG. 70]
In fig. 69 the black line represents an iron hook bent to the shape
required, and the cane plaited round as for the ordinary basket.
It is claimed that the handles and bottoms of these baskets cannot give
way, and it is a claim that is probably correct.
Owing to the difficulties of construction due to the rigidity of the
iron hoop, they cost more than the ordinary basket, and this, with
their extra weight, is unfortunately against their general adoption.
Variations of the same idea are shown on figs. 70 and 71.
[Illustration: FIG. 71]
[Illustration: FIG. 72]
In the first case (fig. 70) the iron is in two parts, which
theoretically would allow of weakness, but in practice the basket
answers its purpose well.
In fig. 71 the rigid ironwork is placed by a wire rope spliced to make
a complete circle. This kind of basket is easier to make and less in
weight than those just mentioned, but the cost of the rope keeps the
price high.
[Illustration: FIG. 73]
Fig. 72 shows another safety arrangement. A is a tarred hemp
rope built into the basket as shown, and the ends fitted with eyelets
for hoisting purposes, the handles being kept for use by the workmen.
The arrangement is a practical one, and gives the required element of
safety to the baskets so long as the rope remains sound.
Ordinarily constructed baskets can be made temporarily safe by passing
the slinging rope or chain through the handles and round the bottom of
the basket, as shown on fig. 73. To prevent the rope slipping, and to
give the basket a flat bottom, pieces of wood can be fitted as shown.
=Navvy Barrows.=—Navvy barrows (fig. 74) are of hard wood, wrought
and cast iron fittings and steel axles. They are fitted with iron, or
wooden wheels bound with iron, and vary in weight from 60 to 75 lbs.,
and have a capacity of about 1/10 of a cubic yard.
A barrow of this class can be slung by passing a hook through the wheel
and rings round the handles.
=Stone Bogies.=—Stone bogies (fig. 75) can be fitted with plain
wheels for running on flat surfaces, or flanged wheels for rails. They
are of oak, with steel axles and cast-iron wheels. The handles for
pulling are detachable and adjustable to either end.
=Hand Barrows.=—Hand barrows as fig. 76 are useful for carrying
light loads, and, when bearing material that cannot roll, may also be
slung.
=Hods.=—Hods (fig. 77) are used on small jobs in which to carry
mortar, bricks, &c. In capacity they will hold 2/3 of a cubic foot
of mortar or twenty bricks, but an ordinary load is 16 walling or 12
facing bricks, the weight of which is considered to be enough for a man
to carry up a ladder.
[Illustration: FIG. 74]
[Illustration: FIG. 75]
[Illustration: FIG. 76]
[Illustration: FIG. 77]
[Illustration: FIG. 78]
[Illustration: FIG. 79]
=Timber Trucks.=—Timber trucks (fig. 78) are used for carrying
timber balks, iron girders, &c. They are usually 3 or 4 feet in length,
with a width of about 24 inches and a height of 22 inches. They are
made sufficiently strong to carry 6,000 lbs.
[Illustration: FIG. 80]
=Sack Trucks.=—Sack trucks (fig. 79) are constructed of hard wood,
with fittings of wrought and cast iron and steel axles. They vary in
length up to 4 feet 4 inches, and the foot iron projects from 6 to 9
inches.
=Crates=, as shown on fig. 80, are constructed of oak with iron
bindings. They will carry a weight of 1,500 lbs. and hold 350 bricks.
They can be filled in the builder’s yard and transferred direct to the
working platform without disturbing the material, which, for saving
time, is often of great advantage. The absence of sides facilitates
loading, but on the other hand, if any materials, say bricks, are put
in loosely, they may fall out during transference, causing danger to
the workmen.
When used to carry rubble work which cannot be stacked, it is better
that sides should be fitted.
When used to carry a roll of lead, a stay should be placed, as shown
by dotted line on figure. This will prevent the crate buckling at the
bottom.
These crates are sometimes fitted with wheels to run on rails.
=Ashlar Shears.=—The shears (figs. 81 and 82) are useful for
lifting dressed work, the points fitting into small holes which have
been cut out for their reception in the ends or sides of the stone.
There is danger in their use if the points drag upwards and outwards.
To prevent this as far as possible, the holes should be cut low, but
not below the centre of gravity of the stone, or else it would turn
over and perhaps fall.
[Illustration: FIG. 81]
[Illustration: FIG. 82]
Fig. 82 is a bad form of shears, as, owing to the sharp curve, the
points can only clutch near the top of the stone.
[Illustration: FIG. 83]
[Illustration: FIG. 84]
[Illustration: FIG. 85]
=Stone Clips and Slings.=—The clips (fig. 83) are useful for
lifting stone slabs. The hook rings slide along the chain, and the
clips are therefore adjustable to any stone not exceeding in width half
the total length of the chain.
The chain slings have a ring at one end and a hook at the other, and
are useful for a similar purpose; but the manner of slinging depends
upon the thickness of the stone. For instance fig. 84, known as jack
slinging, answers well with a slab, say, of over 6 inches in depth,
but a thinner slab lifted in this way would be liable to break in the
middle. If, however, the chain were placed as fig. 85, and which is
known as figure-eight slinging, this risk would be removed.
=Stone Lewises.=—Lewises may be divided into two classes, curved
and straight-sided.
Fig. 86 shows the first, and fig. 87 the second class.
[Illustration: FIG. 86]
[Illustration: FIG. 87]
[Illustration: FIG. 88]
[Illustration: FIG. 89]
The first class is the inferior, as, when fitted into the stone, any
jerk of the supporting chain would act at the points A as a
blow on the stone, thus increasing any tendency to fracture.
The hole for the reception of the lewis is cut, so that a line down
its centre would run across the centre of gravity of the stone; and it
is made as deep as may be required by the weight and hardness of the
material.
The side or splayed pieces of the lewis shown on fig. 87 are fitted
first, and the centre piece last. A bolt through the top fixes their
position and also the ring by which it is to be lifted.
Care should be taken that the sides of the second class of lewis fit
accurately, for if they fit as fig. 88 they may flush the edge and
break out, or if they fit as fig. 89 the risk of fracture, as in the
first class, presents itself. In any case there is always a danger of
mishaps occurring, especially where the stone is not free from vents.
Their use with safety can only be left to the judgment of the mason.
=Stone Cramps.=—The cramps tighten on the stone by means of a
screw thread, as shown on fig. 90.
[Illustration: FIG. 90]
They are useful for lifting light finished work. Packing should be
placed at AA to prevent damage.
The ring by which it is slung is movable to preserve equilibrium.
=Wire and Chain Scaffold Lashings.=—Wire rope scaffold lashings
are now to be obtained for use in place of fibre cords. They are made
in lengths from 12 to 18 feet, and are fitted at one end with an
eyelet. In fixing, they commence with a clove hitch, the knot being
continued as with a fibre cord until near the end, when the lash is
taken through the eyelet (see fig. 91) and finished with jamming turns.
[Illustration: FIG. 91]
It is claimed that no wedges are required for tightening wire rope
lashings, as they do not shrink or swell; on the other hand, owing to
their small circumference, they cannot be pulled very tight by the
workmen, and it is questionable if they would bear being twisted round
the scaffolder’s hammer without injury.
Accidents also might happen if the poles shrank at all after being
fixed.
Tests have been made from which it has been estimated that each lashing
will carry a direct load of 6 tons.
A chain and bracket arrangement for tying ledgers to standards is
shown on fig. 92. It is easily and rapidly adjusted, and is tightened
by means of screw nuts at A and B.
[Illustration: FIG. 92]
Permanent injury might, however, be done to the standards by the
cutting in of the brackets when screwed up, especially after regular
use. The possible loss of the parts and their weight and consequent
disadvantage in transport are against their general adoption.
=Tightening Screws.=—Tightening screws or coupling links (fig. 93)
are fixed in the length of chain that connects the guys of the Scotch
cranes to the base of the queen legs.
[Illustration: FIG. 93]
Under the continuous vibration of the scaffold they run down and
release the chain considerably.
This can be prevented to some extent by inserting a piece of wood as
shown on fig. 93, and tying its other end to a rigid member of the leg.
In any case the chain requires frequent examination, and, if necessary,
retightening.
=Rollers.=—Rollers (fig. 94) are used for moving heavy material
along a smooth surface.
[Illustration: FIG. 94]
Pegs should be fixed at their ends, as shown on fig. 94, to form a
handle by which they can be moved when under the material without
danger to the workman’s hands, or better still, they should always be
longer than the load is wide.
=Levers.=—Levers of ash, fitted with iron shoes, as fig. 95, are
used to prise heavy material off the ground, to facilitate removal on
rollers or otherwise. In this case the lever acts as one of the first
order. By connecting the weight to the rings A and B
it can be used as a lever of the second or the third order.
[Illustration: FIG. 95]
=Dog Irons.=—Dog irons (fig. 96) are bars of flat or round wrought
iron, turned up at the ends, which are pointed. If both ends point in
the same plane they are termed 'male,’ if otherwise 'female.’ The shank
is about 12 inches long. Besides holding the timbers together, they
exert a certain power of compression upon the joint they enclose. This
is gained by hammering the inside of the spikes to a splay, leaving the
outside to form a right angle with the shank.
[Illustration: FIG. 96]
[Illustration: FIG. 97]
[Illustration: FIG. 98]
[Illustration: FIG. 99]
They may be described as inferior straps, and their holding power is
from 600 to 900 lbs. per inch in length of spikes, as deduced from
experiments by Captain Fraser, R.E. Dog irons have the advantage that
their use does not injure the timber to any extent, and so depreciate
its value. Dogs are fixed according to the joint to be enclosed. If
the joint is at right angles to the run of the timbers, they are fixed
as fig. 97.
If the timbers are at right angles they are fixed as fig. 98.
If both these joints occur the irons are placed as fig. 99.
They should be fixed on both sides of the timbers joined.
=Bolts.=—Bolts (fig. 100) are of wrought iron, and their different
parts should be in the following proportions:
Thickness of nut = 1 diameter of bolt
Thickness of head = 3/4 diameter of bolt
Diameter of head or nut over sides = 1-3/4 diameter of bolt
Size of square washer for fir = 3-1/2 diameter of bolt
Size of square washer for oak = 2-1/2 diameter of bolt
Thickness of washer = 1/3 diameter of bolt
There are disadvantages to the use of bolts in scaffolding. For
instance, the beams are weakened by the cutting of the fibres; and, if
the timber shrinks, the bolts may become loose. On the other hand, they
can be easily tightened after the framing has settled into position.
[Illustration: FIG. 100]
Their strength depends upon the quality of the iron, but varies between
20 and 25 tons of tensile strain per square inch of the smallest
sectional area (Anderson).
Washers are used to prevent the nut sinking into the wood when
tightened, and are equally necessary, but not always seen, under the
head. They should not be cut into the under side of timbers subjected
to a cross strain, as the cutting of any fibres is a source of
weakness. Bolts are used where dogs and spikes are of insufficient
length or holding power.
=Straps.=—Straps are wrought-iron bands of different designs, and
are used to form a connection between timbers. Branched straps (fig.
101) are used to strengthen angle joints. They are usually fixed in
pairs, and being fastened on the surface of the timbers they have an
advantage over bolts in that they do not cut into the material. If the
timbers settle at all, the straps may become subject to cross strains.
[Illustration: FIG. 101]
=Wire Ropes.=—Wire ropes are now in general use for heavy purposes.
They are stranded and laid similarly to fibre ropes. They should be of
mild plough steel wire. The number of wires in a strand varies from 12
to 37, and the number of strands is usually 6.
The following table gives the breaking strains of the ropes according
to their circumference, and the least diameter of barrel and sheaves
around which they may be worked at slow speeds.
In the table (p. 110) the diameters of the pulleys, &c. may be slightly
reduced for the more flexible ropes, but better results can always be
gained by using pulleys and sheaves of larger diameters.
A few points on the working of these ropes may be useful.
To remove a kink throw a turn out; it cannot be taken out by strain.
The ropes should be ungalvanised, and kept greased with any oil that
does not contain acid or alkali.
A rope running in a V groove has a short life.
A rope that is allowed to ride, chafe on its own part or to overlap,
will be almost immediately crippled.
The sign of an overloaded rope is excessive stretching.
+-------+-----------------------------+--------------+---------------+
| | | Extra | Special Extra |
| | Flexible Rope. |Flexible Rope.| Flexible Rope.|
| | 6 strands, each 12 wires | 6 strands, |6 strands, each|
| | |each 24 wires | 37 wires |
+-------+---------------+-------------+--------------+---------------+
| |Diam. of barrel| | | |
| Size |or sheave round| Guaranteed | Guaranteed | Guaranteed |
|Circum.|which it may be| Breaking | Breaking | Breaking |
| | worked at a | Strain | Strain | Strain |
| | slow speed | | | |
+-------+---------------+-------------+--------------+---------------+
|Inches | Inches | Tons | Tons | Tons |
| 1-1/2 | 9 | 4 | 7-1/2 | 8 |
| 1-3/4 | 10-1/2 | 5-1/2 | 9-3/4 | 11 |
| 2 | 12 | 7 | 13 | 14-1/2 |
| 2-1/4 | 13-1/2 | 9 | 16-1/4 | 17-1/2 |
| 2-1/2 | 15 | 12 | 20-1/2 | 22 |
| 2-3/4 | 16-1/2 | 15 | 24 | 26-1/2 |
| 3 | 18 | 18 | 28-1/2 | 32-1/4 |
| 3-1/4 | 19-1/2 | 22 | 34 | 37-1/2 |
| 3-1/2 | 21 | 26 | 39 | 43 |
| 3-3/4 | 22-1/2 | 29 | 45-1/2 | 50 |
| 4 | 24 | 33 | 51-1/2 | 56-1/2 |
| 4-1/4 | 25-1/2 | 36 | 59 | 65 |
| 4-1/2 | 27 | 39 | 65 | 70-1/2 |
| 4-3/4 | | | 74 | 79 |
| 5 | | | 82-1/2 | 88 |
+-------+---------------+-------------+--------------+---------------+
(_Bullivant & Co. Ltd._)
=Chains.=—The strength of a chain depends upon the diameter and
quality of the iron of which the links are formed, governed by good
workmanship. The safe load for working can be calculated approximately
by the following method:—
Square the number of eighths of an inch which are contained in the
diameter of the iron of which the link is made, and strike off the last
figure as a decimal.
For example, where the iron is of 1/2-inch diameter, square the number
of 1/8 in the diameter, i.e. 4 × 4 = 16 = 1·6 tons.
Generally before leaving the factory, chains are tested up to half the
weight they should break under, and which is about double the load they
are intended to carry in practice. This test cannot be relied upon for
the future working of the chain, as any stretching of a link, which
would ultimately result in fracture, would probably not be apparent
under it. The links should therefore be examined periodically for any
appearance of weakness or stretching.
A stretched link should at once be cut out, as it may break with much
less load than that which it was first tested to carry. A chain during
use also deteriorates in quality, and it is a good rule to have it
periodically annealed.
The links should then be re-tested up to double the weight they are
again required to carry.
A reliable chain is made of the treble best Staffordshire scrap iron.
Crane and pulley chains should be made with the shortest link possible,
according to the diameter of the iron used, as there is a considerable
leverage exerted on a long link when running round a pulley, more
especially where the diameter of the pulley is small.
=A Slater’s Truss.=—Slaters’ trusses (fig. 102) are used in pairs
by slaters and tilers when laying their material. Boards are laid
across the trusses and form an effective platform on which the workman
can kneel without damage to that part of the roof already covered.
They are slung from the ridge or other suitable fixture, and can be
pulled higher as the work proceeds. An old sack or similar material
laid under the truss will prevent any possible damage during the
progress of the work.
[Illustration: FIG. 102]
=Duck Runs.=—Duck runs (fig. 103) are laid upon slate and tile
roofs to give footing to, and to prevent damage being done by, the
workmen.
They should be firmly fixed, either by slinging from the ridge or
butting against a solid resistance.
[Illustration: FIG. 103]
=Mortar Boards.=—A mortar board is used as a bed on which mortar
can be mixed or deposited. It is roughly made of four or five 9-inch
boards each 3 or 4 feet long, framed together on the under side. Their
use prevents the new mortar coming in contact with the scaffold boards
with an injurious effect.
=Wedges.=—A wedge is a movable, double-inclined plane, used for
separating bodies, and by this means, tightening any connections
between the bodies they tend to separate. For scaffolding purposes they
should be of oak, or other wood which gives considerable resistance
to pressure across the grain. For tightening cordage wedges should be
about 12 inches in length and, as far as possible, split to shape.
In cross section they should be semicircular. Their taper should be
gradual and not too sudden, as otherwise they might work out. When
used in pairs as for shoring purposes, they are rectangular in cross
section, and are termed folding wedges.
=Nails.=—Cut nails stamped out of plates are best for scaffolds.
These nails have the advantage of being easily drawn out of timber.
When driven with their flat sides the way of the grain, they do not
tend to split the wood. They are used to fix platform boards, and
sometimes guard boards, on edge.
=Spikes.=—Spikes are nails above 4 inches in length. They form
a cheap method of fixing. Captain Fraser, R.E., has computed from
experiments that their holding power in fir is from 460 to 700 pounds
per inch of length the depth of cover plate being deducted.
[Illustration: FIG. 104]
=Scaffolder’s hatchet.=—The scaffolder’s hatchet (fig. 104) is an
ordinary shingling hatchet with a hammer head. It is practically the
only tool used by the scaffolder. With it he can shape the ends of the
putlogs, drive wedges, nails, &c., and, by giving the cord a turn round
the middle of the handle, tighten knots by using it as a lever.
CHAPTER VII
_THE TRANSPORT OF MATERIAL_
The transportation of material is not altogether within the province of
a scaffolder, but it is so intimately connected—indeed, it is difficult
to say where his connection with the lifting and carrying of material
commences and finishes—that the subject is here briefly commented upon.
=Crane Engines.=—The engines of the crane are so arranged that
all motions in connection with the derrick are under the control of
the driver. The engines are double cylinder with link-motion reversing
gear. The gearing is single and double purchase for lifting; the jib
barrel is fitted with steel catch wheel and double-lock safety catch
to prevent the jib running down. The slewing gear is worked from the
crank shaft, connected to the upright shaft from bottom race or spur
wheel, and is wrought by worm and worm wheel with double-cone friction
slewing gear. This arrangement permits of slewing the crane in either
direction without reversing the engine. It might also be mentioned that
the clutch for the jib motion is hooped with malleable iron to prevent
the possibility of its bursting.
Crane engines can be worked by electrical, steam, or manual power. The
smaller cranes are now so made that either steam or manual power can be
used as required. It is of recent date that these engines have been
supplied with electrical power, and of course their use is restricted
to where this power is obtainable.
=The Crane.=—The crane consists of four parts, the mast, jib,
sleepers, and guys or stays. The mast, which rises vertically, is
connected at its base to the platform on which the engine stands;
and at the top, to the guys by a pivot which allows of rotation in a
horizontal plane. It may be of iron, balk timber of oak or pitch pine,
or in two pieces of the same, strutted and braced. The jib, which
may be built of the same materials as the mast, is fastened to the
lower end of the mast by a joint which allows of rotary movement in a
vertical plane. The steel rope or chain which supports the weight runs
from the drum placed near the engine and over the top of the mast and
jib. Wheels are placed at these points to lessen friction.
The combination of movement allowed by the pivot of the mast and joints
of the jib, enables the load to be carried to any point commanded by
the effective length of the jib, except that it cannot be placed behind
the guys. Jibs are used up to 70 feet in length. To prevent slewing
under wind pressure, jibs over 50 feet long should be fitted with wind
brakes, especially on exposed situations.
The crane will stand the greatest strain when the jib is most upright,
and, reversely, less strain as it approaches the horizontal. It is a
good rule, and one which works for safety, not to allow the top end of
the jib to reach a lower level than the top of the mast, whatever the
weight of the load carried may be.
Cranes are made suitable for derrick staging to carry a weight of 7
tons. If the boiler is attached to the rotating platform of the crane,
it helps to counterbalance the load.
Cranes, while offering the readiest means of dealing with heavy
weights, do not give the best results when used for placing material
in its final position on the building. The vibration of the engine,
the swaying of the supporting rope from the jib, and the unevenness of
lowering under the band brake, prevent that steadiness of the material
which is necessary for good fixing.
[Illustration: FIG. 105]
Fig. 105 shows a small building crane; it is worked by manual power,
and is very suitable for light work. The illustration shows the general
method of construction, but there are other patterns which give greater
power.
The crane is fitted with two hoisting ropes which are wound on the drum
at A. One rope rises while the other descends. The ropes
pass through the arms B, and when the catch C rises
against the slot, it lifts the arm up. The base of the jib to which
it is connected then rises in the sliding groove and swings inward,
carrying the load well over the platform where it is to be deposited.
When the new load begins to rise, the jib swings outward and downward,
the rope paying out as required. By this means the jibs are in use
alternately for lifting.
=Pulleys.=—The pulley (fig. 106) is a circular iron disc which
revolves freely on an axle fitted into an iron box. The circumferential
edge is grooved to receive the rope or chain which passes round it.
[Illustration: FIG. 106]
Fig. 107 is the section of a groove suitable for fibre rope driving.
The rope is gripped at its sides, thus increasing its driving power.
Fig. 108 is the section of a groove where the pulley is used as a guide
only, the rope being allowed to rest on the bottom.
Fig. 109 is the section of a groove used for wire rope. The groove is
lined on the bottom with pieces of wood or leather to give greater
friction, as the rope would be injured if it were gripped in a groove
as fig. 107.
Figs. 110 and 111 are the sections of grooves suitable for chains, the
groove receiving every other link, the alternate links lying flat.
Fig. 110 is suitable where the pulley is used as a guide only, and fig.
111 is used for driving pulleys as in Weston’s blocks.
The part elevation shown in fig. 112 is known as a sprocket wheel, and
shows the sprockets cast in the groove upon which the links catch. It
is used for driving purposes.
[Illustration: FIG. 107]
[Illustration: FIG. 108]
[Illustration: FIG. 109]
[Illustration: FIG. 110]
[Illustration: FIG. 111]
[Illustration: FIG. 112]
When the pulley cannot rise or fall, it is termed a fixed pulley,
otherwise it is considered movable.
The fixed pulley, sometimes known as a gin wheel, can only change the
direction of a force, and gives no mechanical advantage, but when used
in conjunction with a movable pulley a mechanical advantage is gained.
Fig. 113 is an illustration of the single movable pulley. The rope
is connected to the beam at A, and passes round the pulley
B and over the fixed pulley C Now, if power be
exerted on the free end of the rope sufficient to move the machine, it
will be seen that for every one inch W rises, P will
descend two inches, therefore the mechanical advantage is 2. In other
words, a workman pulling at P with a force equal to 100 lbs.,
will lift W weighing 200 lbs., but the weight will rise at
only half the speed at which P falls.
Various combinations of pulleys are possible, but the most common in
use on buildings is shown in fig. 114, which is an illustration of the
system of pulleys known as block and tackle.
[Illustration: FIG. 113]
[Illustration: FIG. 114]
The power obtained by this machine is calculated as follows:
Divide the weight by double the number of pulleys in the lower block;
the quotient is the power (in the same units as the weight) required to
balance the load.
Theoretically, there is no limit to the number of pulleys and
consequent mechanical advantage, but the friction produced, and want
of perfect flexibility in the rope, prevent any great increase in the
number.
=Differential Pulleys.=—A differential block on Weston’s principle
(fig. 115) consists of a compound pulley of two different diameters
but of one casting, and therefore rotating together. The chain is an
endless one, and passes in turn over each diameter of the pulley. One
of the loops thus formed carries a single movable pulley, while the
other loop hangs loose (see fig. 116). The power which may be applied
to the loose loop on the side which comes from the largest diameter
will cause rotation of the pulley.
The chain must be four times in length the distance through which it is
required to raise the load.
These pulleys are tested to 50 per cent. above the weight they will
have to lift in practice, and the maximum load they will carry is
stamped on the castings. The mechanical advantage derived depends upon
the difference of diameter in the compound pulley. Usually with these
machines two men are required to lift one ton.
Another common form of differential pulley is known as the worm
block, and consists of two cast-iron toothed wheels at right angles
to each other, connected by a worm thread of case-hardened, mild
steel forging. The wheel upon which the power acts is worked by an
endless chain, and the lift wheel may be fitted with a chain or wire
rope to which the load is attached. Pulleys of this kind possess in
a great degree steadiness in lifting or in lowering. This is due to
the great mechanical advantage that can be gained by their method of
construction. By these pulleys one man can lift up to 3 tons. They are
tested and stamped with the maximum safe load, as are those on Weston’s
principle. The friction between the parts is sufficient to sustain
the loads when the power is removed. The steadiness of action allows
of their use to lift stones of great weight, and lower direct into
position for fixing on the building.
[Illustration: FIG. 115]
[Illustration: FIG. 116]
=The Winch.=—A winch is a hoisting machine in which an axle is
turned by a crank handle, and a rope or chain wound round it so as to
raise a weight. It is actually a form of lever whereby a weight may be
moved through the distance required.
Fig. 117 gives a type of winch in its simplest form. The mechanical
advantage gained by its use depends upon the difference between
the radius of the driving wheel and the radius of the axle; or the
circumference of the wheel and the circumference of the axle.
If the radius of the axle were the same as the radius of the wheel, no
mechanical advantage would be gained by its use. The advantage that is
gained by the arrangement can be calculated as follows:
As the radius of the wheel is to the radius of the drum so is the
weight that can be lifted to the power applied to the handle.
_Example._
Let radius of wheel = R,
radius of drum = _r_,
power applied to handle = P,
and weight lifted = W.
(R × P)/_r_ = W.
Then if R = 12 inches,
_r_ = 4 inches,
P = 60 lbs.
(12 × 60)/4 = 720/4 = 180 lbs. weight that can be
lifted, giving a mechanical advantage of 3.
It is obvious that, owing to the restriction of size, the mechanical
advantage that can be gained by the simple machine shown on this figure
is limited. To overcome this difficulty one, two, or three pairs of
toothed wheels are introduced into the machine, being thus known as a
single, double, or treble purchase winch or crab. The difference in
the number of teeth between the pinions and wheels gives the increased
mechanical advantage that is required. The method by which to find the
mechanical advantage gained is as follows:
The pressure exerted on the handle is to the weight lifted as the
radius of the drum multiplied by the number of teeth in the pinions is
to the radius of handle multiplied by the number of teeth in the wheels.
Winches, besides lifting from the barrel, are also used in conjunction
with pulley wheels to change the direction of the force and to gain
additional mechanical advantage. If a double rope be used, considerable
time will be saved in the progress of the work.
[Illustration: FIG. 117]
Fig. 118 shows the double rope. The middle of the rope is given a few
turns round the drum, and the ends are continued over the pulleys, one
sufficiently far to reach the ground. On raising the load the higher
end of the rope descends, and is ready to carry up the next load by
the time the first has been taken off. The illustration also shows the
winch at work in connection with one movable pulley; but unless the
material is to be raised to different heights, the same system of
pulleys should be used on each rope.
[Illustration: FIG. 118]
An improved winch with an advantage over those ordinarily used has the
drum grooved in three diameters, so that, with a minimum of trouble,
a choice of mechanical advantages is gained. There is no need with
these winches to pass the rope several times round the drum, for as the
rope works in a groove, greater friction is set up; and the clutches
provided to keep the rope in contact with the barrel for three quarters
of its circumference, still further prevent any likelihood of slipping.
[Illustration: FIG. 119]
=Jibs.=—For the fixing of the gear a jib (fig. 119) is sufficient
to carry a fixed pulley. A jib is a single pole attached horizontally
to the standards or ledgers above the platform upon which it is
intended to deposit the material. It should project slightly more than
half the width of the load to be hoisted, usually from 18 inches to 2
feet.
[Illustration: FIG. 120]
It is useful at times to have a pulley block fixed over the work in
hand, especially for fixing purposes. In these cases, the loads being
usually heavy, the jib requires extra support, which is obtained by
fixing it as a continuous beam supported at both ends (fig. 120). This
is arranged by carrying up on the interior of the building a series of
standards and ledgers; these rise from each floor as the work proceeds.
The jib can be carried right across the work in hand and the gear fixed
as required.
[Illustration: FIG. 121]
Another form of jib known as the 'mason’s’ is shown in fig. 121. It
is of framed timber from 9 in. by 3 in. to 11 in. by 3 in., leaving
a 4-inch opening down the centre, and rests across the ledgers. It
allows the pulleys which are hung on to the iron movable axle, to be
shifted horizontally throughout its length. For heavy material this
is invaluable, as the load can be raised, moved to its position for
fixing, and lowered as required.
[Illustration: FIG. 122]
=Shears.=—The shears or shear legs is another contrivance for
supporting heavy weights. It consists of two legs forming two sides
of a triangle, and may carry a pulley at the apex as shown in fig.
122, or a jib as shown in fig. 123. In the first case the legs are
not rigidly fixed, but are kept in position by ropes, A and
B, which, on being slackened, allow the shears to move from
the perpendicular (fig. 124). In this manner loads can be lifted and
placed in a different position other than that immediately over the one
they first occupied. The range is, however, limited, as in practice
the shears should not move more than 18 inches to 2 feet from the
perpendicular.
[Illustration: FIG. 123]
[Illustration: FIG. 124]
Shears are useful for raising and lowering the machinery on Scotch
derricks, and are often fixed on gantries to carry jibs.
For heavy weights, the legs and also the jib can be of two or three
poles tied together.
=Gin.=—The gyn, or gin, consists of three legs usually from 12
to 13 feet long. They are set up and joined together at the top, thus
forming a triangulated frame. A pulley wheel or block is fastened at
the apex, and extra power can be gained if they are furnished with
a crab winch standing between two of the legs. They are useful in
lifting or in lowering material through a well or opening in the
working platform, as shown in fig. 125.
[Illustration: FIG. 125]
[Illustration: FIG. 126]
=Rails.=—Scaffolds of a particularly strong construction have,
when necessary, rails laid upon them, in order that light trucks may be
moved freely from place to place.
=Sack trucks= are also used on platforms to carry cement, &c.,
where required.
Other accessories for carrying purposes, the uses of which are obvious,
are described in the chapter on Scaffolding Accessories.
The attachment of material to the transporting power is within the
province of the scaffolder. To take each class of material separately:—
=Ironwork.=—Ironwork is principally used in the form of girders
and columns. These are sometimes slung by a chain round the middle, and
as evenly balanced as possible. There is considerable danger of the
chain slipping, however well balanced; more especially is this the
case if the load is tilted when swinging. This may happen by the load
receiving a jar through touching some part of the erection, and thus
allowing the material to fall. To prevent this 'softeners’—i.e. old
bags, sacks, or even pieces of wood—are placed between the chain and
load. Then, with the chain turned twice round the whole tightly, the
danger is minimised. An extra chain may also be run from each end of
the load to a point some distance up the supporting chain, as shown on
fig. 126.
=Timber.=—Timber in lengths can be carried in the same manner
as ironwork, but, owing to the greater friction set up, it is not so
likely to slip as the former. The same precautions should be taken.
To carry timber or ironwork vertically, the supporting chain is given
a timber hitch round one end of the pole, and a half hitch round the
end which is meant to rise first. It is sometimes advantageous to
substitute a cord lashing for the half hitch. Then, when the highest
end of the pole reaches the platform, the lashing can be removed and
the pole received horizontally. This method is useful where the load
has to be passed through a window.
=Bricks=, =slates=, &c., are slung in crates and baskets, and
on small jobs are carried in hods by labourers. These accessories are
described in Chapter VI.
Note should be taken that these fittings are in the first instance
strongly made, kept in proper repair, not overloaded, and that spring
hooks are used on the slings.
=Stone.=—Stone-work can be slung by means of the lewis, slings,
cramps, clips, or shears. Another method is to pass the chain several
times round the material, as for girder lifting. It is only suitable
for rough work, as any finished edges or chamfers may be flushed even
if 'softeners’ are used.
CHAPTER VIII
_THE STABILITY OF A SCAFFOLD_
A scaffold, considered as a whole, is in a stable condition when,
under the forces that may act upon it, it remains in a state of rest
or equilibrium. Two forces which tend to create a loss of equilibrium
are: the pressure of wind which acts from any direction in a horizontal
plane, and the force of gravity due to the weight of the scaffold and
that of attendant loads.
=Wind Pressures.=—_The effect of wind upon a pole scaffold_:
The effect of wind acting on a single scaffold pole, erected as a
standard, can be first considered. For this purpose the pole shall be
taken as 32 feet long, 2 feet of which are below ground level. The
force of wind depends upon its velocity, and it is measured by the
pressure it exerts on a square foot of surface normal to its direction.
_If a point in a body is fixed, so that the body cannot move out of
its place, but may rotate about that point; a force which acts at any
other point, but in a direction that does not pass through the fixed
point, will produce rotation._
In applying this principle to the effect of wind on the scaffold pole,
the ground level will be the fixed point about which the standard may
rotate.
The wind acting upon the exposed surface of the pole may be likened to
a series of parallel forces that, not acting through the fixed point,
tend to produce rotation.
An advantage is gained if, instead of taking the wind as a series of
parallel forces, it is considered as a resultant force of proportionate
magnitude exerting a pressure upon the centre of the exposed surface.
In practice it will be sufficiently correct to take the centre of
surface of the pole at a point at half its height.
The tendency of a force to produce rotation about a fixed point is
termed its moment about that point. It is measured by multiplying the
units of force exerted by the units of the distance between its point
of application and the fixed point.
Example: If at the centre of surface of the pole under consideration,
that is at 15 feet above the fixed point, the resultant force of the
wind is equal to a pressure of 100 lbs., the moment about the fixed
point will be 15 by 100 = 1500.
In like manner the moment of resistance due to the weight of earth
packed round that portion of the pole below ground can be estimated.
For the pole to remain in equilibrium it will be necessary for the
moment of resistance to equal the moment of the overturning force,
assuming that the fixed point is stable.
No practical good can result by pursuing this calculation further.
It may be taken for granted that as the wind occasionally exerts a
pressure of over 50 lbs. per square foot, and a pressure of 40 lbs. per
square foot is the least for which calculations should be made, it will
always be necessary when scaffolding to any height to adopt special
measures to preserve stability.
When the ledgers are added to the standards they have some effect upon
the equilibrium of the erection, and this must now be considered.
The wind can be taken as acting from two directions, first directly
along the scaffold, and secondly across the scaffold. When at any other
angle to the structure it will have effect in both of these directions,
being greatest in the one with which it most nearly corresponds.
_When blowing along the scaffold._—The standards and ledgers form
with the ground level a series of rectangular parallelograms. The
connections between the sides of the parallelograms are not rigid. They
most nearly approach cup and ball joints, and as such, it will be wise
to regard them as entirely loose to a rotating force.
_The shape of a parallelogram with loose joints can be altered by a
force acting in the plane of its surface, and this alteration of shape
can take place without creating any strain on its joints or members._
From this it will be seen that the tyings and ledgers of the erection
may be considered as offering no resistance to a force tending to
rotate the standards about their fixed points; but by adding to
the surface upon which the force can act, the ledgers increase the
overturning moment about the fixed point.
In practice, although the scaffold may not entirely fail, any change
from regularity of structure due to wind pressure would cause the
members of the erection to offer a less effective resistance to the
other forces acting upon it. This being so, means must be taken to give
that rigidity to the standards, without which the scaffold may collapse.
_Although a parallelogram with loose joints will alter its shape
under pressure, a triangle under similar conditions cannot do so._
Advantage is taken of this fact to obtain the rigidity which is
necessary.
Taking a standard, and one of the ledgers, or the ground level as
forming two sides of a triangle, a pole, termed a brace, is fixed to
form the third side.
The triangle thus formed is a rigid figure offering resistance to any
force acting upon it in the same plane as its surface, and it will
remain rigid until the destruction of one of its joints or members. It
follows, therefore, that the standard forming one side of the triangle
becomes a sufficiently rigid body to withstand any pressure of wind
that may act upon it. The other standards in the erection, if not tied
to the brace, gain rigidity from the triangulated standard because of
their connection thereto by the ledgers.
_When the wind is blowing across the scaffold._—If the erection is
of the dependent type, the standards, putlogs, wall of building, and
ground level form a series of parallelograms which differ from those
previously noted in that a sufficiently rigid angle is formed between
the wall and ground level.
The effect of this inflexibility is to create rigidity throughout
the parallelogram, always providing that the other sides are firmly
connected at their points of juncture.
In practice this is not so; the putlogs, if tied to the ledgers,
which for this purpose is the same as being tied to the standards,
have no fixed connection to the wall of the building; but if they are
supplemented by poles tied from the standards to within the building,
they can be regarded as having, in effect, fixed joints.
If it be impossible to tie the standards within the building, the same
effect can be gained by strutting from the ground level.
This scaffold, if so treated, is sufficiently rigid to withstand any
wind force that tends to overturn the standard, either towards or from
the building.
If the erection is of the independent type, the cross section also
shows a series of parallelograms with loose joints, and so similar
conditions exist as in the first example, except that the overturning
force is acting in a different direction.
Any of the methods of gaining rigidity already given, and shown on
figs. 21 and 24, can be applied in this instance.
Guard boards, rails, face boards, &c., have no other effect than that
of increasing the surface upon which the wind can act. In consequence,
the overturning moment of the standards about their fixed point is also
greater.
Gantries form parallelograms with fixed joints with sufficient
strength—unless carried to a great height—to withstand any wind
pressure. If necessary, they can be braced in the same manner as the
pole scaffold.
Scotch derricks are so strongly built that, unless a wind force
exerting great pressure acted upon them, they could be considered safe
from destruction by that means.
The four pillars standing square to form each leg are crossed at right
angles by transoms which are bolted to the uprights. The parallelograms
thus formed have joints which allow of rotation; but the cross braces
fitted in each bay give rigidity in two ways. Besides triangulating
the frame, they offer a definite resistance to movement on the bolt by
butting against the transom, as will be seen by reference to fig. 1.
This resistance to movement is to some extent due to the resistance of
the timber to crushing.
The larger parallelograms formed by any two legs, the trussed beam and
the ground level, have joints that can only be destroyed by very great
force.
As the highest pressures noted in this country have equalled 80 lbs.
per square foot, and therefore have to be guarded against, it is wise
to triangulate the sides as shown on Frontispiece.
=The Force of Weight or Gravity.=—_The weight of a body equals
the force with which that body is drawn towards the earth’s centre._
The weight of a scaffold may thus be considered as a force acting
vertically downwards. The point at which the force acts is known as the
centre of weight or the centre of gravity.
As a first example, the effect of the forces of gravity may be
considered when acting upon an independent unloaded scaffold in its
simplest form, consisting of four standards, erected square on plan,
with ledgers and braces on each side.
This form of scaffold having regular sides, and its weight being
equally distributed throughout, may be considered as a single, evenly
disposed, rigid, rectangular body. The centre of such a body will
coincide with its geometrical centre, and may be found thus:—
Draw a diagonal from A to B and from C to
D (see fig. 127). The point of intersection will represent the
centre of gravity required.
[Illustration: FIG. 127]
In practice, it will be best to consider the scaffold as a regular
body, comprised as to height within the top and bottom ledgers. The
extra length of standards in this connection can safely be ignored.
_If a body rests on a hard surface, it will stand or fall according
as to whether a vertical line, drawn from the centre of gravity, falls
within or without its base. The base of a body is within a line drawn
round the points of support._
It will therefore be seen that so long as the scaffold is not acted
upon by any other force it will remain in equilibrium. But a scaffold
is erected to carry weights, and the effect of these weights upon
the stability must now be considered. The effect of a load upon the
scaffold is to alter the position of its centre of gravity.
[Illustration: FIG. 128]
Considering the scaffold and its load as two separate bodies, the point
or centre of gravity about which the two combined weights would act is
found as follows:—
Let A and B, fig. 128, represent two heavy bodies.
Join the centre of A to centre of gravity of B;
divide this line into as many parts as there are units of weights in
A and B together. Then mark off from A the
number of units there are in B, and the point thus found is
the point about which the combined weights act, i.e. the required
centre of gravity of the bodies.
The scaffold may, however, have several loads to carry, one or more of
which may be materials slung on the hoist.
=To find the Centre of Gravity of a number of Bodies.=—Find the
centre of gravity of two of the bodies A and B (fig.
129). From the point thus found, take a line to the centre of gravity
of a third, C. Divide the line into as many parts as there are
units of weight in all three bodies. Then from the centre of gravity
of A and B divide off a number of units of distance
equal to the number of units of weight in C. The point thus
found is the required centre of gravity. This process is continued
until all the bodies have been considered, care being taken that all
the units of weight in the bodies which have been considered are added
to the units of weight in the body under consideration when dividing
the line in which the centre of gravity is found. The final centre
of gravity found is the centre of gravity of the entire mass. If the
vertical line taken from it falls within the base, the scaffold is in
equilibrium and therefore stable.
[Illustration: FIG. 129]
When boards are laid on a scaffold, they may be treated as a load. If,
however, they are evenly distributed over the entire top of a scaffold,
their comparatively light weight may be usually neglected, as, their
centre of gravity occurring immediately above the centre of gravity of
the scaffold itself, would have no other effect than to slightly raise
the centre of gravity of the structure.
=To find the Centre of Gravity of Scaffold Boards laid to form a
Platform.=—A scaffolding platform, being of a slight depth in
comparison with its length and breadth, may be treated as a surface
usually rectangular.
[Illustration: FIG. 130]
The centre of gravity of a rectangular surface is the point of
intersection of its diagonals (fig. 130).
=To find the Centre of Gravity of a Dependent Scaffold and the Effect
of Loads upon it.=—A dependent scaffold, having only one frame of
standards and ledgers, to which are attached the putlogs, cannot be
considered as an evenly disposed regular body. Nevertheless, the rule
that the scaffold will not be in equilibrium unless a line from the
centre of gravity fall within the base still holds good. In the case
under consideration, as the wall of the building to which the scaffold
is securely attached by the putlogs and ties, carries its share of
the weight of the loads and putlogs, it must be taken as forming an
integral portion of the scaffold itself.
Therefore the centre of gravity of a dependent scaffold will be the
resultant centre of gravity of the outer frame, the putlogs, and the
wall, considered as separate bodies.
The centre of gravity of the frame may be found by taking it as a
rectangular surface. If necessary, the boards may be treated in like
manner.
The system of putlogs, if they are regularly placed, may be treated as
a regular body, and the centre of gravity found by the method already
given; but in practice their weight would have no effect towards loss
of equilibrium.
The wall may also be treated in like manner if of even thickness. If
of varying thickness, the centre of gravity of each portion of even
thickness should be found.
The resultant will be found by the method already given for finding the
centre of gravity of a number of bodies.
The base of the erection in this case should include the base of the
wall.
The effect of ordinary loads upon the stability of a scaffold of this
type is practically nil. No weight that the scaffold was capable of
carrying in itself could bring the resultant centre of gravity of the
scaffold and wall outside of the base; so that unless the scaffold
failed from rupture of its members or connections, it may be considered
safe from collapse due to instability.
=To find the Centre of Gravity of a Gantry.=—This can be found by
the method given for independent pole scaffolds.
=To find the Centre of Gravity of a Scotch Derrick.=—Owing to
the unevenly distributed weights about these scaffolds, they cannot
be taken as regular bodies. It will therefore be necessary to take
each part of the erection separately, and after finding the centre of
gravity of each, to find the resultant centre of gravity of the mass by
the method already given.
To find the Centre of Gravity of each Part.—Each leg can be treated as
an evenly disposed rigid body.
The mass of brickwork that is placed at the foot of the legs may be
treated similarly.
The platform may be considered as a surface. If triangular, the centre
of gravity is found by the following method:—
Bisect the base and join the point of bisection to the opposite angle.
The centre of gravity is at a point one-third of the length of the line
measured from the side divided (fig. 131).
[Illustration: FIG. 131]
The centre of gravity of the joists supporting the platform can be
taken with that of the platform itself, providing that the weights of
each are added together and the centre of gravity of the platform only
lowered to half the depth of the joists. The trussed girders supporting
the platform may be treated as rectangular surfaces.
The centre of gravity of the guys, sleepers, and jib will be at a point
in the centre of their length.
The centre of gravity of the engine may be somewhat difficult to find,
but it will be sufficient to treat it as a cylinder. The centre of
gravity of a cylinder is the middle of its axis.
Loads may be of various forms, but the centre of gravity will be found
on a line drawn downwards through the load immediately under the
supporting chain. In actual practice no load, the centre of gravity of
which, considered separately, falls within the base of the scaffold
which supports it, will cause instability. The greatest effect it can
have is to bring the centre of gravity of the entire mass nearly to
the edge of the base, so that a comparatively light load acting from
without may cause loss of equilibrium.
It has been so far assumed that, owing to the use of braces, ties,
struts, &c., the scaffolds considered have been rigid bodies, and where
this is so the principles given hold good. In practice, however, owing
to the lack of, or only partial use of, the members just mentioned,
scaffolds are often more or less flexible bodies. Where this is the
case, the lack of rigidity greatly increases the danger of collapse,
as the timbers, through yielding by flexure to the loads that act upon
them, allow such an alteration of the shape of the scaffold that the
centre of gravity may be carried outside the base. Even where this
does not occur, the racking movement allowed is dangerous, as the
connections are strained and become loose, creating an element of risk
that the otherwise careful scaffolder cannot altogether remove. For the
scaffolder the lesson to be learned is—that, whether the force he is
dealing with arises from the wind, loads, or a combination of both, he
must triangulate—TRIANGULATE.
It will be necessary to know the weight of material in working out
these problems. These have been given in the Appendix.
CHAPTER IX
_THE STRENGTH OF A SCAFFOLD_
The strength of a scaffold equals the resistance that its members
and their connections can offer to the strains that act upon it. The
timbers used may fail in various ways.
First.—As beams, they may fail to resist a cross strain.
Secondly.—As pillars and struts, they may fail to resist compression in
the direction of their length.
Thirdly.—As ties and braces, they may fail to resist tension; that is,
a strain tending to pull a beam asunder by stretching.
The loads that act upon a scaffold may be dead, that is to say, they do
not create a shock; or live, which means they are not stationary, and
may cause shock and vibration.
A live load causes nearly double the strain that a dead load produces.
If, therefore, both are of equal weight, the timber under strain
will carry as a live load only one half of what it would carry as a
dead load. The breaking weight is the load that will cause fracture
in the material. The safe load is the greatest weight that should be
allowed in practice. It is in proportion to the breaking load, and that
proportion is termed the factor of safety.
Experiments have been made to determine the resistance of timber to
fracture under the various forces that act upon it.
The result of these experiments is expressed by a given number, termed
a constant, which varies with the different growths of timber and the
different strains to which they are subjected.
The constants (C) for the strength of different kinds of timber under a
cross strain are as follows:
TABLE I.
+-----------------+-------------------+-----------+
| | C in lbs. for | |
| Material | rectangular beams | Authority |
| | supported at both | |
| | ends | |
+-----------------+-------------------+-----------+
| Spruce | 403 |} |
| Larch | 392 |} |
| Fir, Northern } | |} |
| Fir, Dantzic } | 448 |} |
| Fir, Memel } | |} |
| Fir, Riga | 392 |} Hurst |
| Elm | 336 |} |
| Birch | 448 |} |
| Ash, English | 672 |} |
| Oak, English | 560 |} |
| Oak, Baltic | 481 |} |
+-----------------+-------------------+-----------+
The factor of safety for beams under a cross strain is one-fifth.
TABLE II.—_Constants_ (D) _for compression_.
+-----------------+-----------------+-------------+-----------------+
| |D[3] in lbs. for | | D[4] in lbs. for|
| Material | crushing only | Material | flexure for long|
| | | | pillars |
+-----------------+-----------------+-------------+-----------------+
| | Wet Dry | | |
|Spruce |6,499 to 6,819 | Spruce | -- |
|Larch (fallen two| | Larch | 1,645 |
| months) |3,201 to 5,568 | Fir, Riga | 2,035 |
|Fir (white deal) |6,781 to 7,293 | Fir, Memel | 2,361 |
|Elm | 10,331 | Elm | 1,620 |
|Birch, English |3,297 to 6,402 | Birch | -- |
|Birch, American | 11,663 | | |
|Ash |8,683 to 9,363 | Ash | 1,840 |
|Oak, English |6,484 to 10,058 | Oak, English| 2,068 |
|Oak, Dantzic | | Oak, Dantzic| 2,410 |
| (very dry) | 7,731 | | |
| | | | |
|Pillars of medium length—the constant for bending is taken at 2·9 |
| for all timbers. |
+-----------------+-----------------+-------------+-----------------+
The working load on pillars should not be greater than one-tenth of the
breaking weight; but if the pillars are used for temporary purposes,
and are over 15 and under 30 diameters one-eighth, and under 15
diameters one-fifth may be taken as the factor of safety.
TABLE III.—_The Constants_ (E) _for Breaking Weight
under tensional stress_.
+----------+----------------------------------+-----------+
| Material | E in lbs. per unit of 1 sq. inch | Authority |
+----------+----------------------------------+-----------+
| Spruce | 3,360 | } |
| Larch | 3,360 | } |
| Fir | 3,360 | } Hurst |
| Elm | 4,480 | } |
| Ash | 4,480 | } |
| Oak | 6,720 | } |
+----------+----------------------------------+-----------+
The working load should not exceed one-fifth of the weight that would
cause rupture.
=Beams subject to a Transverse Strain.=—The loads that act upon a
beam may be concentrated, that is, acting at one point, or distributed,
which means that the load is evenly placed over the entire length of
the beam or a portion of the beam.
An evenly distributed load is considered to act at a point immediately
below its centre of gravity.
If a beam carries several loads they are considered to act at a point
immediately below the resultant centre of gravity of the whole.
Beams may be fixed, loose, or continuous. When fixed they are built
into the structure; if loose they are supported only; and are
continuous when they have more than two points of support in their
length.
For practical purposes, a continuous beam may be considered one-half
stronger than when supported at two points only.
The strength of a solid rectangular beam varies directly as its
breadth, the square of its depth, and inversely as its length.
The formula for finding the breaking weight (B.W.) of a solid
rectangular beam supported at each end and loaded at its centre is as
follows:
B.W. = (_bd_^2)/_l_ × _c_
where _b_ = breadth in inches;
_d_^2 = depth multiplied by itself in inches;
_l_ = length of beam between supports in feet;
_c_ = the constant found by experiment on the timber in use.
_Example._—Find the B.W. of a solid rectangular beam of Northern
fir, 9 in. by 9 in., and 10 feet between supports, loaded at its
centre.
The constant for Northern fir (Table I.) is 448.
B.W. = (9 × 9^2)/10 × 448
= (9 × 9 x 9 x 448)/10
= 14 tons 11cwt. 2qrs. 11-1/5 lbs.
One-fifth of this must be taken as a safe load, say 2 tons 18 cwt.
* * * * *
The strength of a solid cylindrical beam varies directly as its
diameter cubed, and inversely as its length.
The formula for finding the breaking weight (B.W.) of a solid
cylindrical beam supported at each end and loaded at its centre is as
follows:
B.W. = _d_^3/_l_ × _c_ × 10/17
where _d_^3 = least diameter in inches cubed;
_l_ = length of beam between supports in feet;
_c_ = the constant found by experiment on rectangular beams;
10/17 = the proportion that cylindrical beams bear to square beams,
_Example._—Find the B.W. of a solid cylindrical beam of spruce
fir 6 inches in diameter and 4 feet between supports.
The constant C for spruce fir is (Table I.) 403.
B.W. = (6 × 6 × 6 × 403 × 10)/(4 × 17)
= 12,801-3/17 lbs.
= 5 tons 14 cwt. 1qr. 5-3/17lbs.
One-fifth of this must be taken as a safe load, say 1 ton 2 cwt. 3
qrs.
[Illustration: FIG. 132]
The above diagram (fig. 132) illustrates the supports and loading of
the beams just considered. The following diagrams show beams variously
supported and loaded, and their relative strength to the first.
By adding to the formulæ already given, the proportion that these
following bear to the first, the same formulæ can be used for all.
[Illustration: FIG. 133]
Beam supported at both ends and load equally distributed bears double
of fig. 132.
[Illustration: FIG. 134
Beam fixed, load in centre, bears half more than fig. 132.]
[Illustration: FIG. 135
Beam fixed, load evenly distributed, bears triple of fig. 132.]
[Illustration: FIG. 136
Beam fixed one end only, load on free end, bears a quarter of fig. 132.]
[Illustration: FIG. 137
Beam fixed one end only, load evenly distributed, bears half of fig.
132.]
To find the effective length of a beam supported at both ends when the
position of the load is varied:
_Rule._—Divide four times the product of the distances of the load
from both supports by the whole span, in feet.
_Example._—Find the effective length of a beam the supports of
which are 6 feet apart, the load acting at a point 1 foot from its
centre.
The distances of the load from each support in this case are 2 feet and
4 feet.
Therefore (2 × 4 × 4)/6 = 5-1/3 feet, effective length of beam.
=Posts and Struts subject to Compression.=[5]—Posts and struts,
when above 30 diameters in length, tend to fail by bending and
subsequent cross breaking. When below 30 and above 5 diameters in
length, their weakness may partly be in their bending and partly by
crushing, and when below 5 diameters in length by crushing alone; but,
as these latter are rarely met with in scaffolding, they need not be
considered.
_On the resistance of long posts._—To find the greatest weight, W,
that a square post of 30 diameters and upwards will carry:
Multiply the fourth power of the side of the post in inches by the
value of D (Table II.), and divide by the square of the length in feet.
The quotient will be the weight required in pounds.
The formula is as follows:
W = (_d_^4 × D)/L^2 = weight in lbs. for square posts to resist bending,
where _d_ = depth in inches,
D = constant in pounds, for flexure,
L = length in feet.
_Example._—Find the weight that may be placed upon a post of elm
12 feet long and 4 inches square.
The value of D (Table II.) being 1,620,
(4^4 × 1,620)/12^2 = (256 × 1,620)/144 = 2,880 lbs.
One-tenth of this must be taken as a safe working load = 288 lbs.
* * * * *
The strength of cylindrical posts is to square ones as 10 is to 17, the
rest of the formula remaining the same, except that _d_ = diameter
of post in inches.
* * * * *
_When the pillar is rectangular._—Multiply the greater side by the
cube of the lesser in inches, and divide by the square of the length in
feet. The quotient multiplied by the value of D (Table II.) will give
the weight which the post will carry in pounds.
The formula is as follows:
W = (BT^3 × D)/L^2,
where B = breadth of post in inches;
T = least thickness in inches, and the rest of notation as before.
_Example._—Find the weight that may be placed on a rectangular
post of Dantzic oak 8 in. by 6 in. and 18 feet long.
The value of D (Table II.) being 2,410 lbs.,
(8 × 6^3 × 2,410)/18^2 = (8 × 216 × 2,410)/324 = 12,853 lbs.
The safe working load = say, 1,285 lbs.
_On the strength of posts of medium length._—When a post is less
than 30, but over 5, diameters in length, and therefore tending to fail
by crushing as well as by bending, the resistance to crushing is a
considerable proportion of the strength. This must, in consequence, be
allowed for in any calculation for the breaking weight.
To find the weight that would break a square or rectangular post of
between 5 and 30 diameters in length:
Multiply the area of the cross section of the post in square inches by
the weight in pounds that would crush a short prism of 1 inch square
(Table II.), and divide the product by 1·1 added to the square of the
length in feet, divided by 2·9 times the square of the least thickness
in inches.
The formula is as follows:
W = DS/(1·1 + L^2/T^2 2·9)
where D = the constant for the resistance to crushing.
S = the area of the cross section of the post in square inches.
1·1 is a modification introduced in order that the result
may be in accord with the result of experiments.
L = the length of the post in feet.
T = the least thickness in inches.
2·9 = the constant for the resistance to bending, and which
is taken at that figure for all timbers (Table II.).
_Example._—Find the breaking weight of a post of Dantzic oak 10
feet long and 6 inches square.
The constant for Dantzic oak (Table II.) is 7,731.
W = (7,731 × 36)/(1·1 + 100/(2·9 × 36)) = 278,316/2·05 = 135,763 lbs.
The factor safety is 1/8. Therefore the safe load will be 16,970 lbs.
The strength of cylindrical pillars is to square ones as 10 is to 17.
=Braces and Ties subject to a Tensional Strain.=—The weight that
will produce fracture in a beam strained in the direction of its
length, is in proportion to the area of the cross section of the beam
multiplied by the weight that would fracture a unit of that area.
The formula is as follows:
B.W. = E S
where E = the cohesive force in lbs. per unit of 1 square inch, as
in Table III.
S = the sectional area of the beam in square inches.
_Example._—Find the B.W. of a rectangular elm brace 9 in. by 3 in.
under a tensional strain.
The value of E (Table III.) is 4,480.
B.W. = 4,480 × 9 × 3 = 120,960 lbs.
One-fifth of this should be taken as a safe load,
= 24,192 lbs. = 10 tons 16 cwt.
which is the safe load required.
To find the sectional area of a cylindrical beam square its diameter
and multiply by ·7854.
The effect of fracture of a member of a scaffold depends upon its cause
and upon the importance of the member destroyed.
If the fracture is caused by a live load, say a heavy stone being
suddenly placed over a putlog, it is probable that the suspending
rope, if still attached, would prevent more damage being done. If the
fracture arose from an increasing dead load, say a stack of bricks
being gradually built up by labourers, the mass would probably tear its
way through all obstructions. Nevertheless, the entire scaffold, if
well braced and strutted, should not come down, the damage remaining
local.
The result of fracture of a standard under direct crushing would be
somewhat different, as, providing that the scaffold is rigid, the
greater strain thrown upon the ledgers, due to the increased distance
between supports, would probably cause them to fracture. In this
case the damage would probably still remain local. If, owing to the
fracture, the effect of the bracing were lost, the whole scaffold would
probably fail, as shown in the chapter on Stability.
It should be noted that the ledgers, together with the putlogs when
fixed at both ends, apart from carrying the loads, have an important
effect upon the standards, as, when securely connected, they divide the
uprights into a series of short posts, thus dispelling any likelihood
of failing by flexure.
CHAPTER X
_THE PREVENTION OF ACCIDENTS_
The safety of workmen depends not altogether upon the stability and
strength of a scaffold, but also upon the use of certain precautions
which, while not requisite for the progress of the work, are most
necessary for the prevention of accidents.
[Illustration: FIG. 138]
Put briefly, the principal of these precautions are:
Ladders should rise at least 6 feet 6 inches above the top platform
they serve.
If the ladder is too short for this height to be allowed, a T piece as
fig. 138 should be fixed across the top of the ladder to give warning
to the workman that he has no higher hand hold.
Ladders above 25 feet in length between foot and rest should be stayed
in the centre to prevent sagging. The stay should be a wooden shaft
with an iron clip. By clipping the rung as shown in fig. 139 they do
not meet the workman’s hands and feet when climbing. The same effect is
gained when the top of the ladder rises considerably above the point of
rest, by staying as shown on fig. 140.
[Illustration: FIG. 139]
[Illustration: FIG. 140]
Ladders should have a level footing and be firmly tied to the point of
rest.
Working platforms should be fitted with guard rails along the outside
and at the ends, at a height of 3 feet 6 inches from the platform.
They may be temporarily removed for the landing of the workmen and
material, although it is not always necessary to do so in the latter
case.
[Illustration: FIG. 141]
If a well is left in the working platform through which to hoist
material, the opening should be guarded with rails as for the outside
and ends.
A well hole, if likely to be of permanent use, should be fitted with a
hinged flap door, that can be shut down as required.
Boards on edge should be fitted on the outside and ends of working
platforms, and should rise above the platform at least 7 inches. This
will allow of a 9-inch board being used standing on the putlogs. They
should not be fixed near ladders where the workmen land. Additional
boards should be placed at the back of any stack of bricks or other
material in order to prevent it falling off the scaffold.
[Illustration: FIG. 142]
Edge boards are usually nailed to the standards. On exposed situations
it is better to tie them, as the wind, continually acting on their
surface, will in time draw the nails.
Platform boards, when lapping, frequently lose their place, being
kicked by the workmen during their progress about the scaffold. When
this happens the boards assume the position shown on fig. 141, and what
is known as a trap is formed. The danger of a trap is shown on fig. 142.
[Illustration: FIG. 143]
Platform boards to be safe from tilting, should not project more than 6
inches beyond the putlogs. At this distance the weight of the workman
is most over the putlog, and even if he stood on the extreme edge,
experiments have shown that his weight is more than counterbalanced by
the weight and length of an ordinary board.
Where scaffold boards are used as a means of communication between
one part of the scaffold and another they should be laid in pairs, so
as to form 'runs’ at least 18 inches wide. To prevent unequal sagging
they should be strapped on the under side. It would be better to have
properly constructed gangways and most decidedly safer.
'Bridging runs’ for barrows are usually three boards wide. Five boards
wide is better, and, as previously shown, they should be joined to
prevent unequal sagging.
Centering should be carried on supports which rise from a solid
foundation (fig. 143). The practice of trusting the supports to keep
their position under pressure from the stay A (fig. 144), or
by being spiked to the new work, is to be regretted, as the only reason
for its use is to effect a very slight saving of timber.
[Illustration: FIG. 144]
[Illustration: FIG. 145]
The various knots, tyings, marryings, &c., should be carefully watched,
as, if used in a damp condition, the cordage relaxes considerably when
drying. The scaffolder should have instructions to examine carefully
all cordage in use and tighten the same as required.
The use of sound plant should be insisted upon. Defective plant should
be at once marked, so that its use cannot be unknowingly continued.
Only the scaffolder or his assistant should be allowed to erect, alter
or adapt the scaffolding for its different purposes. Many accidents,
again, occur owing to the scaffolding having been altered during a
temporary absence of the mechanic, and the reconstruction not having
been made safe by his return. This most frequently happens when the
scaffolding is not under the charge of one responsible person.
No working platform should be used by the mechanic until its
construction is complete. Sufficient plant should be on the job to
enable this to be done without disturbing the platform already in use.
Scaffolds should not be heavily loaded. Apart from the risk of the
timbers failing, the weight, in the case of the bricklayers’ scaffold,
has a bad effect upon the new work.
Fan guards, as shown on fig. 145, are usually erected in urban
districts to safeguard the public from falling material. There is no
reason why, for the safety of the workmen, they should not be always
fixed.
Due care should be exercised by the workmen themselves, and observance
made to the unwritten rules of experience.
The following instance is given as an illustration of what is meant.
[Illustration: FIG. 146]
A scaffolder requiring a pole carried it from point A on
fig. 146 to point B. He carelessly carried it upon his right
shoulder, and in turning the corner the pole hit against the standard
C, the recoil knocking him off the scaffold. If he had carried
the pole on his left shoulder he would have fallen inwardly on the
boards, and his life would not have been lost.
CHAPTER XI
_LEGAL MATTERS AFFECTING SCAFFOLDING. LOCAL BYE-LAWS_
Regulations governing the erection of scaffolding have been made by
many of the principal Local Authorities in the Kingdom. Their purpose
is to safeguard the public using the thoroughfares near which the
structures are built. Those issued by the City Corporation of London
are summarised as follows:—
CORPORATION OF LONDON
_REGULATIONS FOR SCAFFOLDS_
APPLICATIONS
_Each application for a scaffold is to be entered in a book, with
headings for the following information:—_
_Name of street or place, and number of house._
_Nature of work to be executed._ _Area of ground level of new
premises to be built, or old premises largely altered._
_Number of storeys, including ground floor, if new premises are to
be built or old premises altered._
_Length of scaffolding needed._
_Time for which license is requested._
_Name and address of Owner._
_Name and address of Architect._
_Name and address of Builder._
_Date of Application._
_Signature of Applicant._
REGULATIONS
_The Inspector of Pavements is to report in the application book
the time he thinks needful for the scaffold to be licensed; the
license is then to be made out, and the conditions entered in the
book by the Engineer’s Clerk._
_If there is disagreement between the applicant and the Inspector,
as to the time needed, the Engineer will decide._
_No scaffold is to project beyond the foot-way pavement where it
is narrow, nor more than 6 feet where it is wide enough to admit of
such projection; any deviation on account of special reasons is to be
stated upon the license._
_No scaffold is to be enclosed so as to prevent passengers passing
under it._
_The lower stages of scaffolds are to be close or doubly planked;
each stage to have fan and edge boards, and such other precautions to
be taken as the Inspector of Pavements requires, to prevent dirt or
wet falling upon the public, or for the public safety._
_No materials are to be deposited below any scaffold._
_Where practicable or needed, a boarded platform, 4 feet wide, and
as much wider as may be necessary for the traffic, with stout post
rails, and wheel kerbs on the outside of it, are to be constructed
outside the scaffold, as the Inspector may direct._
_Where it is necessary in the public interest, applicants shall
form a gantry, stage, or bridge over the public-way, if required, so
as to allow the foot passengers to pass beneath it. The gantry is to
be double planked, and so constructed as to prevent dust, rubbish,
or water falling upon the foot passengers, and the licensee shall
keep the public-way beneath it clean to the satisfaction of the
Inspector._
_Scaffolds are to be watched and lighted at night._
_All fire hydrants must be left unenclosed in recesses formed of
such size and in such manner as may enable the hydrant to be easily
got at and used._
_Public lamps are not to be enclosed without the permission of
the Engineer. When such enclosure is permitted, the applicant shall
put a lamp or lamps temporarily outside the scaffold, so that the
public-way may be properly lighted._
_The licensee shall undertake to employ and pay the Contractors
to the Corporation to make good the pavements, lamps, and all works
disturbed, to the satisfaction of the Engineer._
_Licenses are not allowed to be transferred._
Other cities have similar regulations, but are not generally so
complete in detail.
=Scaffoldings.=—The Burgh Police (Scotland) Act of 1903 (3 Edw.
VII., ch. 33) contains among its general clauses the following:—
=Sec. 32.=—_The dean of guild court shall on the application
of the burgh surveyor have power to prohibit and stop the erection,
use, or employment, and to order the alteration or removal of any
crane, scaffolding, staging, or shoring in or connected with the
construction and erection, or the demolition, alteration, repair, or
securing of any new or existing building, or in or connected with
any excavation for the purpose of any work authorised by the dean of
guild court, where such crane, scaffolding, staging or shoring is, or
is likely to be, in the judgment of the burgh surveyor, a source of
danger._
=Sec. 103.=—_'Dean of guild court’ shall, in this Act, as
regards burghs where there is no dean of guild court, mean the town
council._
The above Act applies to Scotland only, and the section mentioned first
is carried out only so far as it affects the safety of the public at
large.
_FACTORY AND WORKSHOP ACT, 1901_
Section 105 of the above Act, so far as it relates to buildings, reads
as follows:—
BUILDINGS
105.—(1). _The provisions of this Act with respect to_—
(1) _power to make orders as to dangerous machines (section 17)_;
(2) _accidents (sections 19-22)_;
(3) _regulations for dangerous trades (sections 79-86)_;
(4) _powers of inspection (section 119); and_
(5) _fines in case of death or injury (section 136) shall have
effect as if any premises on which machinery worked by steam, water,
or other mechanical power, is temporarily used for the purpose of the
construction of a building or any structural work in connection with
a building were included in the word 'factory,’ and the purpose for
which the machinery is used were a manufacturing process, and as if
the person who, by himself, his agents, or workmen, temporarily uses
any such machinery for the before-mentioned purpose were the occupier
of the said premises; and for the purpose of the enforcement of those
provisions the person so using any such machinery shall be deemed to
be the occupier of a factory._
(2). _The provisions of this Act with respect to notice of
accidents, and the formal investigation of accidents, shall have
effect as if any building which exceeds 30 feet in height, and which
is being constructed or repaired by means of a scaffolding ... were
included in the word 'factory,’ and as if ... the employer of the
persons engaged in the construction or repair ... were the occupier
of a factory._
It will be noticed that the provisions of the Act are more stringent
for buildings which are being constructed or repaired by machinery, and
that these buildings come within the provisions of the Act whether or
not they exceed the limit of 30 feet.
The provisions of the Act as mentioned in the beginning of this section
have been embodied in the following abstract, issued from the Home
Office, January 1902.
Form 57.[6]
_January 1902._
_FACTORY AND WORKSHOP ACT, 1901_
_Abstract of the provisions of the Act as to_
BUILDINGS IN COURSE OF CONSTRUCTION OR REPAIR
H.M. INSPECTOR OF FACTORIES, }
_To whom Communications and_ }
_Notices should be addressed_ }
H.M. SUPERINTENDING INSPECTOR }
OF FACTORIES }
H.M. CHIEF INSPECTOR OF } ARTHUR WHITELEGGE,
FACTORIES } ESQ., M.D.,
} Home Office, London, S. W.
CERTIFYING SURGEON
Certain provisions of the Factory and Workshop Act, including those
which are stated below, apply as if any premises on which machinery
worked by mechanical power is temporarily used in the construction of
a building, or in structural work in connection with a building, were
a factory, and as if the purpose for which the machinery is used were
a manufacturing process. For the purpose of the enforcement of those
provisions, the person so using (by himself, his agents, or workmen)
any such machinery is deemed to be the occupier of a factory.
In the case of buildings over 30 feet in height, which are being
constructed or repaired by means of scaffolding, paragraphs 4 and 6
apply in like manner, whether machinery be used or not; and for the
purpose of their enforcement the employer of the persons engaged in
the construction or repair is deemed to be the occupier of a factory.
The provisions stated below apply also to any private line or siding
used in connection with a building in course of construction or
repair as above.
[Sidenote: Dangerous Machinery or Plant.]
1.—If any part of the ways, works, machinery, or plant (including a
steam boiler) is in such condition that it cannot be used without
danger to life or limb, a Court of Summary Jurisdiction may, on
complaint of an Inspector, make an order prohibiting it from being
used, absolutely or until it is duly repaired or altered.
[Sidenote: Dangerous Processes.]
2.—If any machinery, plant, process, or description of manual labour
is dangerous or injurious to health, or dangerous to life or limb,
regulations may be made by the Secretary of State.
[Sidenote: Steam Boilers.]
3.—Every steam boiler must (_a_) be maintained in proper
condition, and (_b_) have a proper safety-valve, steam-gauge,
and water-gauge, all maintained in proper condition, and (_c_)
be thoroughly examined by a competent person every 14 months. A
signed report of the result of the examination must be entered within
14 days in a Register to be kept for the purpose in the premises
(Form 73[7]).
[Sidenote: Accidents.]
[7]4.—When there occurs in the premises any accident which causes to
a person employed therein such injury as to prevent him on any one
of the three working days next after the occurrence of the accident
from being employed for five hours on his ordinary work, written
Notice (Form 43[7]) must be sent forthwith to H.M. Inspector for the
district.
5.—Every such accident must also be entered in a Register to be kept
for the purpose in the premises (Form 73[7]).
6.—If the accident is fatal, or is produced by machinery moved by
power, or by a vat or pan containing hot liquid, or by explosion,
or by escape of gas or steam, written Notice (Form 43[7]) must
_also_ be sent forthwith to the Certifying Surgeon for the
district.
[Sidenote: Returns.]
7.—If so required by the Secretary of State, a return of the persons
employed must be sent to H.M. Chief Inspector of Factories at such
times and with such particulars as may be directed.
[Sidenote: Powers of Inspectors.]
8.—H.M. Inspectors have power to inspect every part of the premises
by day or by night. They may require the production of registers,
certificates, and other papers. They may examine any person found
in the premises either alone or in the presence of any other person
as they think fit, and may require him to sign a declaration of
the truth of the matters about which he is examined. They may also
exercise such other powers as may be necessary for carrying the Act
into effect. Every person obstructing an Inspector, or refusing to
answer his questions, is liable to a penalty.
The limiting height of 30 feet has been inserted for the reason,
apparently, that it was not considered desirable to bring those minor
accidents which might reasonably be expected to occur on the smaller
buildings into notice.
_NOTICE OF ACCIDENTS ACT, 1906_
Certain provisions of the Factory and Workshop Act, 1901, have recently
been repealed, viz. Sec. 19, which deals with the notification of
accidents. The repealing Act, and which contains clauses replacing the
section, is known as Notice of Accidents Act, 1906, which came into
operation on the first day of January 1907. The fourth section, which
applies to buildings, reads as follows:—
=Sec. 4.=—(1) _Where any accident occurs in a factory or
workshop which is either_—
(_a_) _an accident causing loss of life to a person employed
in the factory or workshop; or_
(_b_) _an accident due to any machinery moved by mechanical
power, or to molten metal, hot liquid explosion, escape of gas or
steam, or to electricity, and so disabling any person employed in the
factory or workshop as to cause him to be absent throughout at least
one whole day from his ordinary work; or_
(_c_) _an accident due to any other special cause which the
Secretary of State may specify by order, and causing such disablement
as aforesaid; or_
(_d_) _an accident disabling for more than seven days a person
employed in the factory or workshop from working at his ordinary
work,_
_written notice of the accident, in such form and accompanied by
such particulars as the Secretary of State prescribes, shall forthwith
be sent to the inspector of the district and also in the case of the
accidents mentioned in paragraphs (a) and (b) of this subsection, and
(if the order of the Secretary of State specifying the special cause so
requires) of accidents mentioned in paragraph (c), to the certifying
surgeon of the district._
(2) _If any accident causing disablement is notified under this
section, and after notification thereof results in the death of the
person disabled, notice in writing of the death shall be sent to the
inspector as soon as the death comes to the knowledge of the occupier
of the factory or workshop._
(3) _If any notice with respect to an accident in a factory or
workshop required to be sent by this section is not sent as so
required, the occupier of the factory or workshop shall be liable to a
fine not exceeding ten pounds._
(4) _If any accident to which this section applies occurs to a person
employed in a factory or workshop the occupier of which is not the
actual employer of the person killed or injured, the actual employer
shall immediately report the same to the occupier, and in default shall
be liable to a fine not exceeding five pounds._
(5) _The foregoing provisions of this section shall be substituted
for section nineteen of the Factory and Workshop Act, 1901._[8]
This amendment, as before stated, renders Part 4 of the Abstract
inoperative.
The Notice of Accidents Act also imposes new duties upon employers.
Section 5 reads as follows:—
=Sec. 5.=—(1) _If the Secretary of State considers that, by
reason of the risk of serious injury to persons employed, it is
expedient that notice should be given under this Act in every case
of any special class of explosion, fire, collapse of buildings,
accidents to machinery or plant, or other occurrences in a mine or
quarry, or in a factory or workshop, including any place which for
the purpose of the provisions of the Factory and Workshop Act, 1901,
with respect to accidents is a factory or workshop, or is included
in the word 'factory’ or 'workshop,’ or is part of a factory or
workshop, the Secretary of State may by order extend the provisions
of this Act requiring notice of accidents to be given to an inspector
to any class of occurrences, whether personal injury or disablement
is caused or not, and, where any such order is made, the provisions
of this Act shall have effect as extended by the order._
(2) _The Secretary of State may by any such order allow the
required notice of any occurrence to which the order relates, instead
of being sent forthwith, to be sent within the time limited by the
order._
The Secretary of State, acting under this section, issued an order in
December 1906 which stipulates that such an occurrence as the breaking
of a rope, chain, or other appliance used for raising or lowering
persons or goods by means of mechanical power should be forthwith
notified on Form 43 by the occupier of the factory to the Inspector for
the district.
_REPORT BY AN INSPECTOR OF THE HOME OFFICE ON BUILDING ACCIDENTS_
Under Sections 79-85 of the Factory and Workshop Act the Secretary of
State is empowered to make regulations for any description of manual
labour that is dangerous or injurious to health, or dangerous to life
or limb. No regulations for the building trade have as yet been made,
but in the Chief Inspector of Factories’ Annual Report for 1905 a
number of suggestions made by a member of the factory department were
printed for the guidance of those engaged in building operations. They
are as follows:—
1. _All working platforms from which it is possible for a workman
or material, tools, and plant to fall a distance of more than 8 feet
should, before employment takes place thereon, be provided throughout
their entire length both on the inside and outside, and at the
ends_—
(_a_) _with a guard-rail fixed at a height of 3 feet 6 inches
above the scaffold boards;_
(_b_) _with boards fixed so that their bottom edges rest on or
abut to the scaffold boards. The boards so fixed shall rise above the
working platform not less than 7 inches._
_Always providing_—
(i) _that where the working platform is fixed within a distance of
12 inches from the buildings, the guard-rail and boards need not be
supplied to the inward side;_
(ii) _that where it is necessary to deposit upon the working
platform material which could not be deposited thereon if the
guard-rail and boards were fixed, the guard-rail and boards may be
removed for this purpose;_
(iii) _that the guard-rail and boards need not be fixed within a
reasonable distance of each side of any ladder which provides a means
of access to the working platform;_
(iv) _that the guard-rail and boards may be removed between any two
standards between which material is being landed;_
(v) _that the guard-rail may be considered unnecessary where
additional boards on edge are fixed to a height of 3 feet 6
inches._
_The reason for the limit of 8 feet is as follows:—_
_The first platform is generally 5 to 6 feet above the ground and
the material required on that platform can be placed upon it from the
ground level. If this is done safeguards are in the way. But on the
second platform about 10 feet high this would not hold good, and from
that point safeguards should be provided._
_Guard-rails should be rigid, and not movable as would be the case
if a rope or chain were fixed. Workmen are used to rigidity in their
surroundings, and their sense of safety is increased by the use of
rigid rails. It would also be difficult to keep ropes at the required
height as they lengthen in dry weather._
_Boards to be placed on edge are usually 9 inches wide; therefore a
rise of 7 inches can easily be provided, even when the boards butt to
the platform by resting on the putlogs. In exposed situations they
are better tied into position than nailed as nails draw under wind
pressure._
_A scaffold is usually within a few inches of the building, except
where breaks in the wall of the erection occur, so that rails and edge
boards are not usually necessary on the inside of the platform._
_In allowing a reasonable distance on each side of a ladder,
consideration must be given not only for the landing of workmen but
also for material being carried; 4 or 5 feet should be sufficient._
2. _All bridging runs between different portions of a scaffold or
building, and from which a workman could fall a distance of more than
8 feet, should be not less than 18 inches wide. If composed of two or
more boards they should be fastened together in such a manner as to
prevent unequal sagging._
_The word 'bridging’ is used to differentiate from those runs which
are continuously supported along the top of a wall or in other ways.
Some discretion should be used as to the prevention of unequal sagging.
If the runs have frequent supports, say every 3 feet, the sagging
would be so slight as to be negligible. Where necessary, in lieu of a
properly constructed gangway, wooden straps screwed on the under side
and at right angles to the run would be satisfactory._
3. _Scaffold boards forming part of a working platform or run should
be carried at each end by a putlog or similar support and should not
project more than 6 inches beyond it unless lapped by other boards
which should rest partly on or over the same putlog and partly upon
putlogs other than those upon which the supported board rests._
_Always providing:—That this suggestion should not apply to any
projection at the ends of the working platform and similar places
effectively guarded._
_This suggestion is intended to prevent the formation of traps,
that is: boards so laid upon the putlogs that they would tilt under
pressure. Traps are the cause of accidents from which the victims have
little chance of escape, for when on a platform it is not easy to
ascertain how the platform is supported. Also, it must be remembered
that workmen do not always note the position of the putlogs which
support them. Even when originally well laid, boards often lose
their position unless they are butted, and this means that constant
supervision should be given to their position by the scaffolder._
_When butted, the putlogs at the board ends should be within 12
inches of each other and the boards given an equal projection,
otherwise they may tilt. Although this may not result in such a serious
accident as when the boards lap each other, it still remains a danger
to be avoided._
_4. Ladders used as a means of communication in, on, or about a
scaffold or building under construction or repair should rise at least
6 feet above the place they give access to._
_When climbing a ladder a man’s arms are at about right angles to
his body, but to land it is easier and therefore safer to raise them
somewhat higher. Therefore the limit of 6 feet would be safe except to
a tall man. A piece of wood or rope nailed at the top of a short ladder
would however give notice that there was no higher hand hold._
5. _Ladders used as a means of communication in, on, or about a
scaffold or building under construction or repair should have a level
and solid footing and be securely fixed at the top point of rest._
_A firmly fixed wedge will usually supply a level-footing. A sloping
pavement generally causes this danger when the ladder is not standing
at right angles to the incline._
_A solid footing is also most necessary. Ladders are sometimes seen
raised upon a stack of loose bricks or other similar material, which is
a most dangerous practice._
_Ladders can be fixed in various ways:_—
(_a_) _by lodgment on a suitable part of the building, say
within an embrasure;_
(_b_) _by tying to the highest point of rest;_
(_c_) _by the use of guy ropes to adjacent parts of the
building._
_As to_—
(_a_) _This is a fairly satisfactory method and is safer with
long and therefore heavy ladders, the weight being helpful to keep
the proper position._
(_b_) _is the most secure but not always possible._
(_c_) _allows of considerable play, but is fairly safe if the
ladder has a fixed footing._
6. _No ladder serving as a means of communication in, on, or about a
scaffold or building in course of construction or repair should have
an additional smaller ladder attached to or spliced on to it for the
purpose of obtaining extra length._
_Ladders are often spliced especially for light work. The danger
arises from the displacement of the usual handhold and also foothold,
if the tying slips or gives sufficiently, so that the rungs, where the
lap takes place, are not on the same level._
7. _Openings within the working platforms through which workmen
could fall a distance greater than 8 feet should be surrounded with
a guard-rail fixed at a height of 3 feet 6 inches above the working
platform._
_A guard-rail may at times interfere with the free use of the
platform, and where this occurs an alternative is to fit the well hole
with trap doors which can be closed when the well is out of use._
8. _No loose putlogs or other timbers should be allowed to remain
projecting from the face of the scaffold where hoisting or lowering of
material or plant is carried on._
_The danger from any projecting timbers is a real one. If loose the
transport of material may disturb it by contact, or the material itself
may fall. The face of the scaffold would be an imaginary line drawn
from standard to standard on their outside._
9. _Baskets when loaded should not be slung from the handles only,
but the sling should be passed round the bottom of the basket. Provided
always that if means have been taken in the construction of the basket
to prevent its handles and bottom breaking out, this suggestion need
not apply._
_Several baskets are now on the market with wire ropes &c. used
in their construction, and which prevent the handles &c. breaking
away. The life of an ordinary basket in constant use averages about
a fortnight, but much depends upon the material carried, slates, for
instance, having a particularly wearing effect._
10. _All poles before being used in the construction of the scaffold
should be barked._
_This is generally done, but is imperative when the poles are to be
used as standards. If not ripped off the bark acts as a sheath to the
pole, and having little adhesion thereto will slip under pressure such
as is exerted by the ledgers._
11. _No work should take place on a working platform until its
construction is complete, unless the portion incomplete is effectively
guarded from the men using it._
_In the course of building, the working platforms have to be raised
as the work progresses. It is at these times that the dangers arise
which the above suggestion is intended to prevent._
_As a rule only one platform is completely sheeted (i.e., boarded) at
a time, and in the transition stage of raising the platform the workmen
are tempted to proceed with their work before it is completed and
safeguarded. The remedy is for the use of additional timber._
12. _No alteration of or interference with the construction of the
scaffold should be made except with the authority of a responsible
person._
_On small jobs the general foreman should take responsibility in this
connection. On large works, where a foreman scaffolder is engaged, it
should be his business to superintend all alterations._
13. _All plant should be constantly examined, and if any be found
defective it should be disposed of in such a manner that its use cannot
be continued unknowingly._
_Many builders have no objection to the total destruction of minor
details of plant found defective when the same is ordered by the
foreman. No general system of marking defective plant is in use, but a
good plan is to arrange for its removal from the building, care being
taken that its return to the yard as defective should be made known to
those concerned._
14. _In such places where the scaffolding has not been erected by the
direct employer of the men using it, or where its management has not
been under his personal or deputed control or supervision, he should
satisfy himself either personally or by his agents before allowing his
employees to work thereon that the foregoing suggestions are kept in
force on that portion of the scaffold and building with which he is
concerned._
_This suggestion has a similar intention to the following clause
often embodied in building contracts. It reads as follows:_—
'_The sub-contractor shall have the use in common with the
workmen of the contractors and of other sub-contractors of existing
scaffolding only. The sub-contractor shall, however, satisfy himself
that any scaffolding used for the purpose of this contract is fit
and proper for his purpose and shall be solely responsible for any
accidents which may result from the user of such scaffolding or plant
to himself or men in his employ._’
_It is of course unnecessary to add that accidents will occur even
when the greatest attention and care are exercised. There is frequently
no other reason for mishaps than overwork, rushing, misdirected economy
in plant, and, it may be added, intoxicating liquors. Much depends upon
the foreman in these matters. They are often first-rate managers of
machinery, have an excellent knowledge of the best means of carrying
out work, and not content with their existing reputations attempt to
be also successful drivers of men. This is, perhaps, more likely to
occur when the job is not paying: a hint from the employer to that
effect will almost certainly result in an effort being made to retrieve
matters. This is only natural and praiseworthy, if to gain the desired
end the safety of the workmen is not jeopardised._
_The cost of properly safeguarding a scaffold is certainly an item,
especially on large works, but should not be difficult to calculate.
It is an expenditure that will not be in proportion to the cost of the
building, but would be a direct charge upon the scaffolding required.
I have made some calculations, and in estimating it would probably be
found not to exceed an increase of 5 per cent. upon the amount devoted
to the cost of the scaffold._
_THE WORKMEN’S COMPENSATION ACT, 1906_
In this Act, which came into force on July 1, 1907, no mention is
made of the limitations included in the superseded Act of 1897, and
which specially mentioned that compensation should only be paid for
accidents occurring in, on, or about buildings which exceeded 30 feet
in height and were either being constructed or repaired by means of
a scaffolding, or being demolished, or on which machinery driven by
steam, water, or other mechanical power was used for the purpose of the
construction, repair, or demolition thereof.
In the new Act the liability of employers to workmen for injuries is
expressed in Sec. 1 as follows:—
(1) _If in any employment[9] personal injury[10] by accident
arising out of and in the course of his employment is caused to a
workman, his employer shall, subject as hereinafter mentioned, be
liable to pay compensation in accordance with the first schedule to
this Act._
_Provided that—_
(_a_) _the employer shall not be liable under this Act in
respect of any injury which does not disable the workman for a period
of at least one week[11] from earning full wages at the work at which
he was employed;_
(_b_) _when the injury was caused by the personal negligence
or wilful act of the employer or of some person for whose act or
default the employer is responsible, nothing in this Act shall
affect any civil liability of the employer, but in that case the
workman may at his option either claim compensation under this Act,
or take proceedings independently of this Act[12]; but the employer
shall not be liable to pay compensation for injury to a workman by
accident arising out of and in the course of the employment both
independently of and also under this Act, and shall not be liable to
any proceedings independently of this Act, except in case of such
personal negligence or wilful act as aforesaid;_
(_c_) _if it is proved that the injury to a workman is
attributable to the serious and wilful misconduct of that workman,
any compensation claimed in respect of that injury shall, unless that
injury results in death or serious and permanent disablement,[13] be
disallowed._
DEFINITION OF EMPLOYER AND WORKMAN
=Sec. 13.=—_In this Act, unless the context otherwise
requires,_—
_'Employer’ includes any body of persons corporate or incorporate
and the legal personal representative of a deceased employer, and,
where the services of a workman are temporarily lent or let on hire
to another person by the person with whom the workman has entered
into a contract of service or apprenticeship, the latter shall, for
the purposes of this Act, be deemed to continue to be the employer of
the workman whilst he is working for that other person;_
_'Workman’ does not include any person employed otherwise than by
way of manual labour whose remuneration exceeds two hundred and fifty
pounds a year, or a person whose employment is of a casual nature and
who is employed otherwise than for the purposes of the employer’s
trade or business, or a member of a police force, or an out worker,
or a member of the employer’s family dwelling in his house, but, save
as aforesaid, means any person who has entered into or works under a
contract of service or apprenticeship with an employer, whether by
way of manual labour, clerical work, or otherwise, and whether the
contract is expressed or implied, is oral or in writing;_
_Any reference to a workman who has been injured shall, where
the workman is dead, include a reference to his legal personal
representative or to his dependants or other person to whom or for
whose benefit compensation is payable._
Both of these definitions have been altered from those given in the Act
of 1897. The definition of employer has been extended from the words
'deceased employer.’ The definition of workman is much more definite,
and does not allow of much theorising. With the few exceptions
mentioned it covers all classes of service, and should certainly cover
any persons whose duties are connected with scaffolding.
APPENDIX
_WEIGHT OF MATERIAL_
lbs. per
cubic foot
Asphalte (gritted) 156·00
Brick (common) from 97·31
Brick (common) to 125·00
Brick (red) 135·50
Brick (pale red) 130·31
Brick (Common London Stock) 115·00
Brick paving (English clinker) 103·31
Brick paving (Dutch clinker) 92·62
Brickwork in mortar, about 110·00
Cement (Roman) and sand (equal parts) 113·56
Cement alone (cast) 100·00
Chalk from 125·00
Chalk to 166·00
Chalk (Dorking) 116·81
Clay (common) 119·93
Clay (with gravel) 160·00
Concrete (lime) 130·00
Concrete (cement) 136·00
Earth (common) from 95·00
Earth (common) from 124·00
Earth (loamy) 126·00
Earth (rammed) 99·00
Earth (loose or sandy) 95·00
Firestone 112·00
Flint from 161·25
Flint to 164·37
Granite from 158·62
Granite to 187·47
Gravel 109·32
Iron (bar) from 475·00
Iron (bar) to 487·50
Iron (hammered) 485·18
Iron (not hammered) 475·00
Iron (cast) from 450·00
Iron (cast) to 475·00
Lead (milled) 712·93
Lead (cast) 709·50
Lime (quick) 52·68
Marble from 161·25
Marble to 177·50
Mortar (New) 110·00
Mortar (of river sand 3 parts, of lime in paste 2 parts) 100·93
Mortar well beaten together 118·31
Mortar (of pit sand) 99·25
Mortar well beaten together 118·93
Mortar (common of chalk, lime and sand, dry) 96·87
Mortar (lime, sand, and hair for plastering dry) 86·50
Sand (pure quartz) 171·87
Sand (river) 117·87
Sand (River Thames best) 102·37
Sand (pit, clean but coarse) 100·62
Sand (pit, but fine grained) 92·50
Sand (road grit) 93·37
Sand (River Thames inferior) 90·87
Slate, Welsh 180·50
Slate, Anglesea 179·75
Slate, Westmoreland (pale blue) 174·73
Slate, Westmoreland (fine-grained pale blue) 170·75
Slate, Westmoreland (dark blue) 173·43
Slate, Westmoreland (pale greenish blue) 173·00
Slate, Westmoreland (black blue for floors) 172·37
Slate, Welsh rag 172·00
Slate, Cornwall (greyish blue) 157·00
Snow from 8·00
Snow to 14·00
Shingle 95·00
Steel from 486·25
Steel to 490·00
Stone, Bath (roe stone) 155·87
Stone, Bath 123·43
Stone, blue lias (limestone) 54·18
Stone, Bramley Fall (sandstone) 56·62
Stone, Bristol 56·87
Stone, Caen 31·75
Stone, Clitheroe (limestone) 67·87
Stone, Derbyshire (red friable sandstone) 46·62
Stone, Dundee 58·12
Stone, Hilton (sandstone) 36·06
Stone, Kentish rag 66·00
Stone, Ketton (roe stone) 55·87
Stone, Kincardine (sandstone) 53·00
Stone, Penarth (limestone) 65·81
Stone, Portland (roe stone) 53·81
Stone, Portland 151·43
Stone, Purbeck 67·50
Stone, Woodstock (flagstone) 63·37
Stone, Yorkshire (paving) 56·68
Tile (common plain) 16·15
Water (sea) 64·18
Water (rain) 62·50
Wood (ash) 50·00
Wood (birch) 45·00
Wood (beech) 45·00
Wood (elm, common, dry) 34·00
Wood (fir, Memel dry) 34·00
Wood (fir, Norway spruce) 32·00
Wood (fir, Riga dry) 29·12
Wood (fir, Scotch dry) 26·80
Wood (larch, white wood seasoned) 22·75
Wood (larch, dry) 38·31
Wood (oak, English seasoned) 48·56
Wood (oak, Dantzic seasoned) 47·24
Wood (oak, Riga dry) 43·00
Zinc 450·00
FOOTNOTES:
[Footnote 1: _A Treatise on Shoring and Underpinning, and generally
dealing with Dangerous Structures._ By Cecil Haden Stock. Third
edition, revised by F. R. Farrow. (B. T. Batsford.)]
[Footnote 2: As derived from tests made by Messrs. Frost Bros., Ltd.,
rope manufacturers, London, E.]
[Footnote 3: Authority, Hodgkinson.]
[Footnote 4: These constants, in the absence of experiments, are
assumed to vary according to the modulus of elasticity of the wood.
(Tredgold, from Dr. Young’s _Nat. Philos._, vol. ii.)]
[Footnote 5: Tredgold’s _Carpentry_.]
[Footnote 6: These forms are to be obtained from Messrs. Wyman & Sons,
Ltd., Fetter Lane, E.C.]
[Footnote 7: NOTE.—Part 4 is now inoperative. See p. 167.]
[Footnote 8: 1 Edw. VII., c. 22.]
[Footnote 9: This brings about the inclusion of any workmen who
are defined as such in Sec. 13 of this Act. (See _post_, page
178C.)]
[Footnote 10: These words may bring an injury due to nervous shock
within the provisions of the Act.]
[Footnote 11: This is an alteration from the Act of 1897, in which two
weeks was the period of disablement necessary before compensation could
be claimed.]
[Footnote 12: There is a slight alteration in drafting here from the
Act of 1897, but in no way is the substance affected.]
[Footnote 13: The words 'unless that injury results in death or serious
and permanent disablement’ are new and considerably enlarge the scope
of the section.]
INDEX
Accidents Act, notice of, 167
Accidents, report on, by Home Office Inspector, 170
Ash, 70
Ashlar shears, 100
Attachment for hoisting, 129
Balks, 74
Barrows, 97
— hand, 97
Baskets, 95
Beams, strength of, 143, 145
— stiffest, 66
— strongest, 66
Birch, 70
Blocks and tackle, 120
Boards, 2, 26
— cutting of, 67
Boatswains’ boats, 45
Bogies, stone, 97
Bolts, 108
Braces, 2, 26
— strength of, 143, 151
Buckets, 93
Bye-laws, scaffolding, London, 161
Centering, supports to, 158
Centre of gravity, to find, 136
Chain lashings, 103
Chains, 9, 110
Chimney climbing, 33
Cleats, 51
Connections, 29
Constants, 143
Cordage, 76
— danger from damp, 158
— strength of, 76
Coupling links, 9, 105
Crabs, 16
Cradles, 43
Crane, 116
Crates, 99
Cripples, 93
Dangerous occurrences, notice of, 169
Dead shores, 58
Derrick platform, 6
— staging for traveller, 10
— stagings, 3
— travelling, 10
Dog irons, 107
— — fixing of, 107
Duck runs, 112
Edge boards, 156
Effect of weight, 136
— of wind pressure, 131
Elm, 70
Employer, definition of, 178B
Engines, 115
Face boards, 28
Factory Act, abstract of, 165
— — repeal of Sec. 19, 167
— — application of, 164
Fan guards, 159
Flying shores, 50
Footing blocks, 14, 17
— piece, 55
Gantries, 12, 16
Gin, 128
Guys, 9
Hatchet, 113
Height of building under Factory Act, 164
Hods, 97
Horizontal shores, 50
Jibs, 125
— masons’, 127
Joggles, 50
King legs, 3
Kites, 33
Knots, 78
Ladders, 90
— accidents from, 154
— fixing of, 8
Landing stages, 28
Larch, 69
Ledgers, 2
— connections between, 22
Levers, 106
Lewises, 102
Masons’ jibs, 127
Materials, weight of, Appendix
Mortar boards, 112
Nails, 113
Needles, 30
— for shoring, 50
Notice of accidents, 167
— — dangerous occurrences, 169
Oak, 70
Painters’ boats, 43
Pine, 69
Plant, danger from unsound, 158
— danger from insufficient, 159
Platforms, 6, 12
— for traveller, 15
Poles, 74
Posts, strength of, 143, 149
Pulleys, 118
— differential, 121
Puncheons, 20
Putlogs, 2, 74
— fixing of, 23
Queen legs, 3
Raking shores, 52
— — scantlings for, 54
— struts, 51
Repeal of Sec. 19 Factory Act, 1901, 167
Report by Home Office Inspector on building accidents, 170
Rider shore, 57
Rollers, 106
Runners, 2
Runs, 157
Scaffolders’ hatchet, 113
Scaffolding, definition of, 1
— foundations, 3
— application Burgh Police Scotland Act, 163
Scaffolding needle, 30
— proper control of, 158
— system of, 2
Scaffolds, bricklayers’, 19
— for chimneys, 31, 37
— for domes and arches, 42
— ladder, 46
— masons’, 27
— for steeples, 42
— swinging, 43
— for towers, 41
Shakes, cup, 62
— radial, 62
— weather, 69
Shear legs, 127
Shears, ashlar, 100
Shore, preparation of, 55
Shores, 2
Shoring, 49
Skips, 93
Slater’s truss, 111
Sleepers, 9
Sole piece, 55
Spikes, 113
Spruce, 69
Stagings, 17
Standards, 2
— fixing of rising, 21
Stone clips, 101
— cramps, 103
— slings, 101
Straining pieces, 51
Straps, 109
Struts, 2
— strength of, 149
— to shoring, 56
System, North country, 2
— South country, 12
Ties, 2
— strength of, 143, 151
Timber, branding, 71
Timber, classification and structure, 62
— conversion, 65
— decay of, 72
— defects in living tree, 62
— description, 69
— durability of, 74
— felling, 64
— preservation, 73
— scantlings, 65
— seasoning, 67
— selection, 70
— stacking, 67
— testing, 71
— use of scaffolding, 74
Tossles, 50
Transoms, 2
Traps, danger from, 157
Trestles, 92
Trucks, sack, 99
— timber, 99
Underpinning, 57
Wall plates, 50
Wedge driving, 57, 60
Wedges, 112
Weight, effect of, 136
— of materials, Appendix
Wells, safeguards, 156
Winch, 123
Wind pressures, 131
Window strutting, 58
Wire lashings, 103
— ropes, 109
Workman, definition of, 178C
Workmen’s Compensation Act, 178A
— — — liability of employers 178A
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