This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol1", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It is best and safest to place the ladder communicating with the stages within the thickness of the scaffold, the upper stages being reached by a series of fairly short ladders. Long climbs are very fatiguing to the hodmen who carry the bricks and mortar to the workmen on the stages. If long ladders are used, however, they should be strutted from the standards or from the ledgers to prevent them from swaying. All ladders should be securely lashed to the scaffold. On large building works it is most economical in labour to erect temporary wood stairs to enable the workmen to get at their work rapidly.
In towns where the lower portion of a scaffold would be likely to interfere with the traffic, timber structures not less than 10 feet high, called Gantries, are erected, usually spanning the footpath upon which the scaffolding rests.
These gantries are built up in the following manner: Two timber sleepers, varying from 6 by 6 inches to 12 by 12 inches, are laid on either side of the path, and uprights of the same size as the sleepers are placed upon and fixed to them with dogs. Heads or transoms of the same scantling as the uprights are laid across these uprights and dogged to them (see Fig. 233). Joints in the heads are arranged to come centrally over the uprights, and a piece of timber is placed between the uprights and the head in order to increase the bearing of the latter.
The heads are strutted between the uprights by means of struts inclined to about 45 degrees, and varying from 4 by 4 to 6 by 6 inches, supported at the lower ends by cleats, and butted against one another at the upper ends; or, if the uprights be too far apart to permit these ends to be butted, a straining piece is introduced between them and spiked to the head. The uprights are also braced transversely by rows of 7 by 2-inch timbers spiked diagonally between them. The lower ends of the struts and braces should not be lower than 6 feet 6 inches, otherwise they may prove a nuisance to people passing beneath the gantry.
The stage is formed by placing transverse beams, from 7 by 2 inches to 9 by 2 inches, upon the heads, and by spiking 9 by 3-inch boards, laid flat, upon these beams.
Rails and guard boards are usually placed round the edges of the stages, for the reasons already explained.
If the stages are large, quantities of materials may be stored upon them, or masons' or other sheds may be erected upon them.
Large balks of timber, with the ends splayed, and called Fenders, are fixed against the outside sleeper to prevent vehicles from coming in contact with the uprights. Sometimes the sleepers are connected together by means of transverse timbers halved to them, in which case a floor must be constructed by laying 7 by 2-inch to 9 by 3-inch boards on top of the transverse timbers, these boards being sloped up from the pavement at each end of the gantry, fillets of wood being nailed across them to give a good foothold. Gantries are often erected for the transport of materials, and of this class of gantry there are three types, namely - 1. Fixed Gantries with Traveller. 2. Travelling Gantries. 3. Tower Gantries.
1. Fixed Gaiitrics with Travellers are framed similarly to the stages shown in Fig. 233, save that they cannot be braced transversely except at the ends. Each upright is prevented from swinging transversely by strutting, as shown in Fig. 234. On top of the heads iron rails are fixed as shown, upon which a travelling beam which carries a travelling hoisting machine is placed.
The rails are turned up at the ends to prevent the traveller from running off, and guards are fixed at the ends of the beam to keep the crab on.
2. Travelling Gantries. - In this type of gantry the whole framework runs along rails supported upon sleepers, and consists of two strong trestles spanned by a trussed cross beam upon which the crab rests. The usual method of framing these gantries is shown in Fig. 235.
3. Tower Gantries, or Derrick Towers, are lofty stages, supported upon three or four " towers," or legs built up of timber. Upon those platforms derrick cranes are supported for the transport of material to various parts of the building-. The towers, which are about 6 feet square, are disposed on plan in the form of an isosceles triangle, and are paced from 20 to 30 feet apart, and rest upon platforms formed of layers of 9 by 3-inch deals, well spiked together, and varying in size according to the nature of the soil upon which they rest. Uprights, usually built up of three layers of 9 by 3-inch deals of varying lengths bolted together, are placed at the corners of the towers and connected at intervals of from 10 to 12 feet by means of 9 by 3-inch cross struts or transoms securely bolted to the uprights. Diagonal struts, cut out of 9 by 3-inch deals, are then securely bolted between the transoms, as shown in Fig. 235A. The towers are carried up in a series of trussed bays until the desired height is reached. Trussed beams are built across from the tower, forming the apex of the isosceles triangle on plan to the other towers, these beams being built up as follows: Solid timbers, usually 12 by 12 inches, form the upper and lower booms, which are supported upon the two uppermost transoms, which are spaced from 5 to 6 feet apart vertically. The upper booms are halved at their intersection, carried on to project about 4 feet beyond the tower, and strutted from one of the transoms. These two booms are connected together by means of iron bolts spaced about 6 feet apart. Wooden uprights and cross bracing, cut out of 6 by 6-inch timber, is fitted between the booms, and securely fixed by screwing up the nut ends of the bolts. A 12 by 12-inch timber is thrown across the remaining side, resting upon the top transoms of the towers, and stiffened by means of struts supported at their lower ends upon one of the transoms of the towers, and butting at the upper ends against a straining piece spiked to the beam.
Joists are laid across these beams to support the stage. The crane is placed over the leg upon which the trussed beams meet, and this leg is sometimes furnished with a stout central standard strutted at intervals from the transoms. The crane consists of an upright member called a Mast, fixed at the lower end to a pivoted base and pivoted at the top to the ends of two Stays, which are fixed at their lower ends to Sleepers. These sleepers lie on the staging, and are connected to the base of the mast by a pivoted joint. A member called a Jib is connected by means of a hinged joint to the lower end of the mast, while the upper end is slung by a chain passing over a pulley at the top of the mast to a steam winch, thus enabling the jib to be raised or lowered in a vertical plane. A steel rope or chain for supporting the loads is passed over pulleys, one at the upper end of the jib and one at the upper end of the mast, and is then carried round the drum of a steam winch for raising or lowering the loads.
Stout chains are fixed to the sleepers beneath the lower ends of the stays, and securely fixed to the platform at the bottom of the supporting towers. These are weighed down with bricks, stones, or other suitable material. Care should be taken to keep these chains tight, otherwise they will tighten up with a jerk every time a load is lifted by the crane. For this reason a screw coupling link is inserted in the chains, which should be inspected every day and tightened if necessary.
The load at the bottom of towers is sometimes supported on platforms resting upon the lowest transoms to make the towers as stable as possible. The load at bottom of the towers beneath the stays should always be at least twice the heaviest load likely to be supported from the jib.
Before setting up a tower gantry over a site the plans of the proposed building should be consulted and the position which will give the maximum range to the crane determined.