FOUNDATIONS, GASWORKS BUILDINGS, ETC. 59
a thickness greater than 3| inches, the best material will be made from 4 parts of aggregate, 2 parts of sand, and 1 part of Portland cement. If the stresses on the floor are computed, th.e concrete should be neglected and merely regarded as filling, whilst in the case of large retort-house floors the machine loaded to its maximum should be dealt with. as a rolling load. The weight, furthermore, must be assumed to act on three wheels only, this often being the temporary condition of affairs, due to a faulty rail, or to some external influence. The centre columns (see Fig. 13) usually associated with these heavy floors must be strong enough to take the rolling load and any eccentricity of loading which. may be caused by the above-mentioned factors.
In the erection of ironwork, bolts should only be employed wliere riveting is impossible or unsuitable ; and, when used, bolts should be burred over after tighten-ing up. The question as to whcther holes should be drilled or punched is one of long-standing argument, and it should be remembered tliat punching, though quicker, gives a tapered hole in addition to setting up crystallization of the metal around the hole, thus causing weakness. Drilled lioles are certainly to be preferred, and they should be made y^-inch larger in diameter than the bolt up to j-inch. bolts, and -g-inch larger for bolts above j-inch.