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.

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.

[_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.

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.

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.

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.

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.

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.

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.

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.

=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.

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.

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.

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.

=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.