The construction of Tower Gantries has already been alluded to in Volume I. Part III. Chapter XIII (The Decoration Of Domestic Buildings). The importance of carefully determining the exact position or positions of the "Scotsmen" is evident, having regard to the most convenient working of the cranes and jibs. The king leg (being the largest) upon which the crane is fixed is the first to be erected (see Fig. 235A, Vol. I.). The queen legs are then set out from it to form a triangle. The length of the sleepers required (usually from 25 to 30 feet) determines the distance between the king and queen legs. The legs should have a concrete or other solid foundation if possible, but where this cannot be obtained two thicknesses of 3-inch timber laid crosswise and fixed 2 feet below ground level should be laid to a dead level.

The framework of 12 by 2-inch timber to carry standards is then fixed. The legs may be from 6 to 10 feet square on plan, according to requirements. The standards (four to each leg) may be either solid or laminated; i.e. those of the king legs should not be less than 9 by 9 inches solid, or if laminated should consist of three pieces of battens 9 by 3 inches bolted together.

The queen legs should have balks of timber 7 by 7 inches, or three pieces of batten 7 by 2 1/2 inches bolted together.

If whole timbers can be obtained they ought to be used.

It is better that the king leg, having to carry the weight of the engine, should have balks of timber 14 by 14 inches, and if built up four deals, 16 by 4 inches each should be used.

The king leg should have an extra balk laid with the horizontal framing at bottom.

To prevent any winding or bending an extra standard up the centre of the leg should be fixed and strutted from each of the four outer standards, and behind each transom (see Fig. 266).

The central standard may, however, be supported from the foundation. If made in this way the legs would support a platform over 100 feet in height.

Derrick Stagings 298

Fig. 266.

The legs are connected by trussed beams thus. Two pieces of timber, each 12 by 8 inches, are laid one above the other between the king leg and each queen leg on the two top transoms (see Fig. 267). They are from 6 to 9 feet apart, the top bay being made lower than the others.

The lower balks are secured to the centre standard of the king leg by wrought-iron straps.

The top timbers have a projection from the king leg of from 6 to 8 or 10 feet beyond the king leg, and are halved at intersection. These projecting ends are connected to each other by 8 by 6-inch balks, and again to the return balk by pieces of similar scantling. The top balks are supported by struts from the central standard (see Fig. 267). The upper and lower balks are connected as shown by iron bolts, and each bay is diagonally braced, as are the bays to legs. These iron bolts should be covered by pieces of the same scantling as the cross pieces or braces.

Derrick Stagings 299

Fig. 267.

The top balks having been continued, a larger area to the platform round engine is thus afforded. The boards, usually 9 by 1 1/2 inches, are laid at right angles to the joists, or on poles about 3 feet apart.

These joists or poles are laid parallel to one another and at right angles to the truss, forming the back support of the platform.

Although it may only be necessary at times, and according to circumstances, to partially cover the space between the legs (see Fig. 267), it is very desirable to adopt the plan of thus securing a larger platform as a space for the storage of engine coal, the weight of which would help to keep the framework of the structure steady. Where coal or other heavy materials are stored the floor of the staging or platform should be of double thickness.

The platform is reached in various ways. The ladders are sometimes fixed to the inside of king leg or on the outside of queen leg, but to avoid the danger of climbing upright ladders, as would arise in those instances, it is desirable, if possible, to fix the ladders as shown in Fig. 268.

Derrick Stagings 300

Fig. 268.

It should be observed that additional security against any possible lateral motion, consequent upon the effect of wind, etc., can be made by cross-bracing by poles or scantlings between each leg, as shown in Fig. 269.

In the erection of long buildings it is not unusual to construct the derrick on a travelling bogie rather than to reconstruct it elsewhere on the site. This arrangement (see Fig. 270) applies to small derricks, and is usually adopted when the crane is erected outside the building.

Derrick Stagings 301

Fig. 269.

A platform for travelling cranes should be constructed as follows, and as shown in Fig. 271. As these platforms or gantries are used in builders' yards, and in those of stone masons and timber merchants, as well as upon extensive building works, it is obvious that they should be firmly and solidly built, framed, and braced. Balks of timber forming standards in two parallel tiers are placed about 9 or 10 feet apart longitudinally, and from 20 to 30 feet transversely. The heads to standards and the sleepers also should be of same scantling at least as standards themselves, and be thoroughly braced in every way possible to render the structure firm and free from any likelihood of displacement. Head pieces as shown should be fixed between to distribute the pressure. The runners are laid on the head pieces, and consist of railway metals resting in chairs bolted to heads. Along these runs a small but strong carriage called the " traveller," formed of two stout trussed beams, rested on and bolted to two short cross beams which are mounted on pairs of flanged wheels. The traveller moves longitudinally along the gantry, and carries the crane, winch, or crab. In this way a universal motion for any load carried by the crab is obtained. It is absolutely necessary, in constructing these erections, that the ends of the rails should be turned up some inches to prevent the traveller or winch carriage running off the metals; and in order to render additional strength and firmness to the gantry, it is desirable to brace it on the inside, as shown in Fig. 272.