279. 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 the work.

280. The most common form of scaffold is that used by the bricklayer. It consists of poles, usually of fir, from 25 to 40 feet in length, and from 6 to 8 inches in diameter at the butt or larger ends. These poles, which are called "standards," are planted in a row at intervals of 10 or 12 feet, and at a distance from the wall to be erected of about 5 feet in the clear. To the standards on the sides next the wall other poles called "ledgers," placed horizontally, are lashed with ropes, as the work proceeds, at intervals of about 4 feet in height. These support the " putlogs," which are pieces of squared timber about 6 feet long and from 4 inches by 3 inches to 4 inches by 3 1/2 inches in scantling. The putlogs are supported at one end on the ledgers and the other on the wall, a header or half-brick being left out for the purpose in building.

Putlogs are usually placed at about 3 1/2 or 4 feet apart. On them are laid the scaffold boards, which are about 9 inches wide by 1 1/2 inch thick. It is on these scaffold boards that the workmen stand, and the bricks and mortar are deposited. Fig. 86 shows the arrangement of a bricklayer's scaffold. When the scaffold has to be carried to a considerable height other poles are lashed to the standards with ropes tightened by wedges. Poles are also lashed diagonally across every three or four standards in the shape of a St. Andrew's cross; these are called "braces," and they serve to stiffen or brace the scaffold longitudinally.

Fig.86.

281. In buildings which do not admit of putlog holes in the walls, as where rubble stone or ashlar facing is used, and which do not require heavy machinery for hoisting or strong timbers in the scaffold, two rows of standards with ledgers are used, one row being close to the wall, and the other at the usual distance, so that both ends of the putlogs may rest on the ledgers.

282. Scaffolds, as described in Arts. 280 and 281, are sometimes used for heights of 90 or 100 feet from the ground, as in building church steeples and similar work. The men entrusted with the erection of scaffolds are seldom anything more than ordinary labourers, and are as a rule wholly ignorant of the principles of mechanics as taught in the schools.

283. In the erection of houses it is usual to construct a staging about 10 feet square on the outside of the scaffolding, for the purpose of hoisting materials, and from which they are distributed for use. This staging is usually formed with standards and ledgers in the same manner as the scaffold to which it is connected.

284. In the erection of large works in masonry, the materials used being blocks of stone, frequently weighing several tons, it is obvious that a different arrangement is required from that where the materials can be lifted and set by the hands of the workmen, as in the case of bricks and the small stones used in rubble work. The mason therefore uses, instead of a scaffold formed of round poles, one of squared timbers of large scantling, which being too large to be lashed with ropes are fastened together by bolts and dog-irons, and are kept quite independent of the walls, putlog holes as used in brickwork being inadmissible.

The standards were formerly planted in two rows, one being next to the wall, and a boarded platform was carried on the top similar to the bricklayer's scaffold, the heavy stones being hoisted and set by means of "shears" with blocks and tackle. This method is now almost superseded in large works by a staging formed of squared timbers in the same manner as the mason's scaffold, but with only one row of standards on each side of the wall. On these standards are laid the longitudinal timbers, which usually carry a line of rails on which a travelling platform containing the hoisting gear can move over the entire extent of the building. The standards and longitudinal timbers are made perfectly rigid by struts, disposed as shown by Fig. 87, which is a front view of one tier of the outer row of timbers.

The standards A A are in scantling usually from 8 to 12 inches square, according to the height of the scaffold and the weight to be supported. The distance apart is from 10 to 20 feet. Corbel or "cap" pieces B B are placed under the longitudinal timbers or "runners" C C, to give the latter better bearing on the heads of the standards. The runners C C are usually of the same scantling as the standards; but the struts D D are seldom more than one-half the sectional area of the standards. These struts usually pitch against a straining piece E, which is bolted to the under-side of the runners. The lower ends of the struts rest on cleats, and are secured to the sides of the standards either by iron spikes or bolts.

Fig. 87.

It is desirable to have as few bolt-holes as possible, and to avoid notching, mortising, or otherwise cutting into the timber, so that the deterioration in value at the completion of the work may be as little as possible. Therefore the several pieces are for the most part put together with dog-irons, which are pieces of square or round iron, about 3/4 of an inch in diameter, having the ends pointed and turned down at right angles. These are driven into the wood, and can be removed with little or no injury to it afterwards.

The distance at which each row of standards should bo placed from the wall will depend upon the general arrangement for conducting the works. In some cases a tramway leading from the quarry or stone depot is laid between the outer row of standards and the wall to admit of the stone being lifted directly from the truck on to the work. In this case a space of from 10 to 20 feet would be required between the standards and the wall.

In other cases, as in the streets of towns, or where the space is limited, the timbers are placed within a few feet of the wall on both sides, and the materials are lifted at some convenient part of the work, over which the "traveller" with its hoisting gear can be brought. Fig. 88 shows a section of a wall in progress with the travelling platform resting on the staging.

To prevent lateral movement in the staging, struts from the ground, as F, Fig. 88, are usually fixed to each standard.

FIG. 88.

The lower ends of the struts should always be fixed to "foot-blocks," as at G, by which they are prevented from sinking into the ground. A short pile should be driven at the outer end of the foot-block, which will prevent it from slipping. The usual practice, however, is that shown by Fig. 1, Plate XXXIII., in which the foot-block is sunk in the ground at right angles to the direction of the strut. The choice of these methods will depend on the nature of the ground.

Plate XXXIII.

SCAFFOLDS and GANTRIES.

Fig. 2

Elevvation

Fig; 1

Section

Sometimes the ends of the standards are framed with a short or "stub" tenon into a continuous sill of timber placed on the surface of the ground; this prevents the unequal settlement of the standards, which would be fatal to the stability of the staging.