In the case of a three-towered gantry it is necessary to ballast only the two queen legs. Weighting of the king leg is unnecessary. With a square gantry having four legs, all should be weighted, and in calculating the ballast necessary for the crane towers the weight of the engine should be considered. Access to the platform is obtained by ladders fixed either inside or outside one of the queen legs. With the exception of the boards forming the working plat form, which are usually spiked down, the timbers of a tower gantry should all be connected by screw bolts and nuts.
In steel framed buildings the derrick tower is unnecessary, the revolving jib, base and engine being supported by the steel work and raised to succeeding heights as the work proceeds.
Swinging scaffolds or cradles, used in connection with painting, renovating and light repair work consists of light rigidly framed platforms suspended from cantilever supports. They are capable of being raised or lowered by the occupants of the cradle by means of block and fall tackle. A newer type of cradle is a modified form of the suspended scaffolding already described, the raising and lowering being done by machines attached direct to the metal frames which hold the platform and guard rails, instead of by "block and fall." Scaffolding for Chimneys.—Tall chimney shafts may be erected by internal scaffolding only, or by a combination of ex ternal and internal staging. The latter method is often adopted
when the lower part of the shaft is designed with ornamental brick work, string courses, panels, etc., and it is important that this work should be carefully finished. An external scaffold is therefore car ried up until plain work not more than 2 or bricks thick is reached, when the remainder can be completed by "overhand" work from an internal scaffold. The offsets made in the brickwork on the inside are used to support the timbering. For the repair of tall chimneys, light ladders are erected one above the other by a steeplejack and his assistants, each being lashed to the one below it and secured to the brickwork by dog-hooks driven in the joints. When the top of the chimney is reached balk timbers are raised by pulleys and laid across the top. From these are swung cradles from which the defective work is made good. If the work or weather demand a more stable scaffold, a light but strong frame work of putlogs held together with iron bolts is fixed on each side of the shaft with iron holdfasts, and a platform of boards is laid upon them. For circular chimneys pieces of timber cut to a curve are clamped with iron to the putlogs to prevent them from bend ing when the bolts connecting the two frames are screwed up.
See J. F. Hurst, Tredgold's Carpentry; A. G. H. Thatcher, Scaff old ing; C. F. Mitchell, Building Construction; G. Ellis, Modern Practical Carpentry.