Fig. 1.—Elevation of Staging for Derrick Crane
In the case of the queen legs an extra balk is placed under the framework, as shown in [fig. 1.]
The legs are from 6 to 10 feet square on plan, the king legs being the larger. Each leg comprises four standards, either of whole timbers or battens bolted together. The standards for the king legs should measure not less than 9 in. by 9 in.; if of battens, then three pieces 9 in. by 3 in. should be used.
For the queen legs, balks 7 in. by 7 in., or three pieces 7 in. by 21⁄2 in., are sufficient sectional area. Where battens are used they are given a lap equal to one-third of their full length, and are bolted together by 3⁄4-inch wrought-iron bolts.
Whole timbers are used when they can be obtained in one piece of sufficient length.
As the standards rise they are divided into bays by transoms. These are made out of about 9-in. by 3-in. deals, placed from about 6 to 10 feet apart.
The bays are triangulated by cross-braces 7 in. by 21⁄2 in.; both of these are usually placed on the outside of the bay, but one or both may be placed on the inside. The first method is the better, as the braces, by butting against the transoms, give an increased resistance to movement.
The king leg, having to carry the weight of the engine, requires greater strength. This is gained by running an extra standard up the centre of the leg. If it is whole timber, balks about 14 in. by 14 in. are required; if built up, four deals 16 in. by 4 in. are used. It should stand upon an extra balk laid with the horizontal framing at the bottom of the leg, and should rest on a solid foundation.
To prevent any tendency to flexure this extra standard should be strutted from all four of the outer standards behind each transom (see [fig. 2]).