Ironwork.—Ironwork is principally used in the form of girders and columns. These are sometimes slung by a chain round the middle, and as evenly balanced as possible. There is considerable danger of the chain slipping, however well balanced; more especially is this the case if the load is tilted when swinging. This may happen by the load receiving a jar through touching some part of the erection, and thus allowing the material to fall. To prevent this ‘softeners’—i.e. old bags, sacks, or even pieces of wood—are placed between the chain and load. Then, with the chain turned twice round the whole tightly, the danger is minimised. An extra chain may also be run from each end of the load to a point some distance up the supporting chain, as shown on [fig. 126].
Timber.—Timber in lengths can be carried in the same manner as ironwork, but, owing to the greater friction set up, it is not so likely to slip as the former. The same precautions should be taken.
To carry timber or ironwork vertically, the supporting chain is given a timber hitch round one end of the pole, and a half hitch round the end which is meant to rise first. It is sometimes advantageous to substitute a cord lashing for the half hitch. Then, when the highest end of the pole reaches the platform, the lashing can be removed and the pole received horizontally. This method is useful where the load has to be passed through a window.
Bricks, slates, &c., are slung in crates and baskets, and on small jobs are carried in hods by labourers. These accessories are described in Chapter VI.
Note should be taken that these fittings are in the first instance strongly made, kept in proper repair, not overloaded, and that spring hooks are used on the slings.
Stone.—Stone-work can be slung by means of the lewis, slings, cramps, clips, or shears. Another method is to pass the chain several times round the material, as for girder lifting. It is only suitable for rough work, as any finished edges or chamfers may be flushed even if ‘softeners’ are used.
CHAPTER VIII
THE STABILITY OF A SCAFFOLD
A scaffold, considered as a whole, is in a stable condition when, under the forces that may act upon it, it remains in a state of rest or equilibrium. Two forces which tend to create a loss of equilibrium are: the pressure of wind which acts from any direction in a horizontal plane, and the force of gravity due to the weight of the scaffold and that of attendant loads.
Wind Pressures.—The effect of wind upon a pole scaffold:
The effect of wind acting on a single scaffold pole, erected as a standard, can be first considered. For this purpose the pole shall be taken as 32 feet long, 2 feet of which are below ground level. The force of wind depends upon its velocity, and it is measured by the pressure it exerts on a square foot of surface normal to its direction.