Transcriber’s Notes:
The spelling, hyphenation, punctuation and accentuation are as the
original, except for apparent typographical errors which have been
corrected.
Italic text is denoted _thus_.
Bold text is denoted =thus=.
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.