_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.