A combination of both methods is commonly seen, and found useful in practice.
In scaffolding, the vertical timbers are known as standards or uprights. The horizontal timbers between the standards are known as ledgers when of cylindrical section, but as transoms and runners when of rectangular section. Braces, shores, struts and ties of any section are pieces used to stiffen the structure. The putlogs, or joists as they are called when of greater length, carry the boards which form the working platform.
The Northern System.—This scaffolding can be divided into two parts. First, the derrick staging from which the transporting power acts; and, second, the platforms, which bring the workmen within reach of their work.
Derrick Stagings.—These stagings, also known as Scotch derricks and ‘Scotchmen,’ are erected to carry the power required, usually a steam crane.
They consist of three or four timber towers or legs supporting a platform upon which the crane stands. The number of legs depends upon the area over which the power is required to act.
When one crane is to be erected, three legs are sufficient to carry the platform.
If the building is a large one, several such stagings may be constructed; but in some cases two cranes are required where the size of the building will not allow of two stagings. In these cases the platform is square and supported at each angle by a leg. The cranes are then fixed diametrically opposite each other.
In determining the position of the legs they must be placed where the effective range of the crane is most required, and also where they will cause the least possible obstruction to the progress of the building. The position of the tower that carries the crane, and which is known as the principal or king leg, is first fixed. The secondary or queen legs are set out from it in the form of an isosceles triangle. The distance between the king and queen legs depends upon the length of the sleepers. These run from below the engine to the lower ends of the guys, and average from 25 to 30 feet in length.
The legs, especially the king legs, if intended to rise from the earth, must have a foundation of two thicknesses of 3-inch timbering laid crosswise. This is unnecessary if there is a concrete or other solid foundation.
Apart from the necessity for any foundation, the standards should rise from a framework of balk timbers of about 12 in. by 12 in., laid on the ground, and halved at their intersection ([fig. 1]).