When a wall is built as high from the ground as the bricklayer can conveniently reach he commences a scaffold by planting a row of poles or "standards," S S, about 10 or 12 feet apart (Fig. 389).
Across these standards, at the level of the work already done, are poles, called "ledgers" secured with lashings which are in many cases tightened up by wooden keys or wedges, and upon these are laid short transverse pieces called "putlogs" p, about 6 feet long and 3 inches thick, which form bearers to support the scaffold-boards, b.
The putlogs are from 4 to 6 feet apart, according to the strength of the scaffold-boards, which should be about 1½ inch thick; header bricks are temporarily left out, forming holes, h h, into which one end of each putlog is inserted, the other end resting upon the ledger.
Three or four scaffold-boards are laid across the putlogs; on these the bricklayer stands and his materials are deposited.
The materials are either carried up ladders in hods, or hoisted by means of a pulley or windlass and rope.
In many cases a platform for landing materials is erected in the same way as the scaffold, and close to it.
When the wall is so high that it can barely be reached from the scaffold-boards another row of ledgers is lashed to the standards, fresh putlogs laid, and the scaffold-boards are raised to the new level.
The ledgers and putlogs used at the lower levels are left in position to steady the scaffold, and if the building be very high and in an exposed situation the scaffolding must be stiffened by lashing long poles, called "braces," B B, diagonally across the outside of the standards and ledgers.
The scaffold for the inside of the brick or rubble walls of buildings sometimes consists merely of planks laid across the joists of the different floors, which are placed in position for the purpose; when the walling has risen more than 5 feet above a floor a fresh tier of planks is provided, supported on trestles, empty casks, or anything that may be available.
When there are no floors on the inside of the wall the scaffold then is constructed in the same way as for the outside.
Scaffolding for ashlar walls, of which the stones can be lifted without machinery, are formed with standards and ledgers; but as putlogs cannot conveniently be inserted in the face of the masonry, a row of standards is used on both sides of the wall, between which the putlogs are lashed, so that the scaffolding is entirely independent of the building.
In some parts of the country houses many stories in height are erected without scaffolds at all, the work being all done from the inside, and the men supported only by temporary platforms formed on the different floors in succession.
Scaffolds made from square scantling have been used under the supposition that the timber might eventually be used in the building, that cords would be saved, and that the scaffolding could be more quickly erected and taken down.
Practically, however, these advantages have not been found to exist, and, where the scaffolding is high, iron sockets for uniting the lengths of scantling and other expensive and awkward contrivances have been found necessary, so that the system may be considered a failure.
Gabers Scaffolds, made from pieces of flat timber or deals bolted together and well cross-braced, are sometimes used in Scotland. Their general construction will be understood from Figs. 390, 391, which are taken from an actual example.
When the stones to be lifted are very heavy, scaffolds of poles lashed with cords would not be safe, nor could they carry the necessary machinery for lifting the stones.
In such a case a staging or "gantry" is erected of balk timber, supporting a tramway, upon which runs a "traveller," extending across the gantry at right angles to the direction of its length, and consisting of a stage on wheels, along which moves a truck carrying a double purchase crab.
As the stage or traveller can move anywhere in the direction of the length of the scaffold, and the truck can move along the traveller across the width of the gantry, it is evident that the crab can be brought vertically over any point lying within the scaffolding.
The traveller consists of two trussed beams (see Part I.) fixed parallel to one another, and about 4 or 5 feet apart.
At each end the beams are united by a cast-iron carriage containing a pair of wheels, which run along the rails fixed upon the upper beams or sills of the gantry.
The traveller is worked along by turning these wheels, which is effected by machinery worked either from the platform carrying the crab, or by men stationed at the ends of the traveller.
The truck carrying the crab is moved along on the rails of the traveller, across the gantry, by machinery on its own platform.
The outer wheels of travellers moving on a circular gantry are made with very broad tires, so that they may not jam upon the rail.
Fig. 392 is a longitudinal view of a gantry constructed of balk timbers. The uprights, U U, are placed from 10 to 20 feet apart according to the size of timber available, and the capsills or "runners," R R, are supported by struts, S S, which butt against a straining piece, SP, and rest upon cleats, C. Corbel pieces are often introduced, as shown at CP.
The standards or uprights at the end of the gantry should be strutted as shown, and so should every standard be supported by struts on the outside to prevent lateral movement.
In order to keep the timber as perfect as possible bolts should be avoided, and the balks united by straps or "dogs."
The latter are pieces of iron about ¾ inch square, the ends of which are turned down and pointed by being splayed on the inside so as to draw the timbers together when driven home.
It frequently happens that a line of railway can be brought from the stoneyard right under the gantry, as shown, in which case the stone can be lifted off the trucks by the traveller and set at once.
Derrick Cranes of the form shown in Fig. 393 are sometimes used for lifting the materials required in building large houses, as well as other structures.
The crane is placed on a platform in a high and central position, so that it can reach the stones or other materials where they are deposited, and also by revolving the jib or derrick place them where they are required in the work. The derrick can be raised or lowered as well as revolved; this enables the length of its reach to be varied so that materials can be picked up or set down on any spot within its range.
It is manifestly of the utmost importance that stones to be hoisted should be simply and safely secured to the chain or "fall" by which they are to be lifted.
There are several ways of doing this: -
Rough stones may merely have a chain passed round them; this, however, would injure worked stones, and would be inconvenient while they were being set.
A hole is cut in the stone, being wider at the bottom than above (see Fig. 394); into it are fitted three pieces of iron of the shape shown ; the two side pieces are first placed, then the centre piece, and the three are united by a bolt passing through them. This bolt also secures a rin or shackle, into which the hook of the fall is inserted when the stone is to be raised; as the stone rises, the lewis, in virtue of its wedge shape, becomes tightly jammed into the hole.
An improvement on the ordinary lewis, frequently used, is shown in Fig. 395.
In this the chain is attached to a ring passing through the eye (e) of the centre piece, c, the lower part of which is wedge-shaped.
The side pieces are connected to one another by cross pieces, cp, of flat iron on each side of the centre bar; they are hinged to the ends of the cross pieces, and with it are free to move up and down the centre piece.
When the stress comes upon the lewis the centre piece is drawn up, and as the broader part of its wedge rises between the side pieces it forces them out upon the sides of the hole, and the greater the strain the tighter becomes the grip of the lewis.
When the stone has been lifted a smart tap on the head of the centre piece drives it downwards, the side pieces collapse, and the lewis can be withdrawn.
A great advantage in this lewis is that the pieces are all connected, which saves time, and prevents their being lost.
A modification of the lewis (shown in Fig. 396) is used for setting under water. A line is attached to the rectangular piece, after the stone is set this is pulled out, and then the wedge piece can easily be removed.
5. The following is a substitute for the lewis sometimes used for hard stone. Two short bars connected by a chain are let into holes inclined inwards towards the centre of the stone; when the strain comes upon the centre of the chain the bars are pressed inwards and grip the stone.
A pair of nippers is sometimes used thus (Fig. 397) : -
A small piece is picked out on each side of the stone so as to give the point of the nippers a hold.
The upper ends or "eyes " of the nippers are fixed to a short chain which passes through a ring attached to the fall. When the weight comes upon this chain its effect is to draw the eyes of the nippers inwards, which has a similar effect upon the points and tightens their hold upon the stone.
Care must be taken that these holes in the side of the stone are made so far from the top that the weight will not cause the points to drag up through the stone.
They should of course be so high that the centre of gravity of the stone is below the points, in order that it may not turn over in the nippers.
In all cases where worked stones have to be lifted by nippers care should be taken in working them to leave little projecting knobs to receive the points of the nippers so that the worked face may not be injured. These little knobs can be dressed off when or just before the stone is set.