As already stated, cast-iron is generally used in the construction of pillars or columns, this material being calculated to bear a high strain of compression, which tends, as shown in fig. 481, to crush or break the particles, as at the points a, b, and c; wrought-iron, although in one sense stronger, being more liable to bend, as shown by the exaggerated dotted lines d. The strongest form or section of cast-iron columns is a hollow cylinder, as at e, and this is that generally adopted - with the exception of columns of diameter of 4 inches or less, which are solid - for those of 5 to 6 inches external diameter, the thickness of metal should be from 6/8 to 7/8 of an inch, and for those from 8 to 10 inches diameter, 1 inch to 1⅛. In another part of this section we have given various points connected with columns, as, for example, the influence which the length has upon the strength, or the resisting powers; we now give the formula for finding the dimensions.
According to Hodgkinson, the strength of cast-iron columns is found by using the following formula: - The ends being flat and fixed for solid columns, the breaking weight -
W = 44 D3.6 - d 3.6/l 1.7 ; for hollow columns W = 44 D 3.6 - d 3.6/l 1.7, in which D is the diameter exterior in inches of solid columns, d the internal diameter of hollow do., l length in feet, W crushing weight in tons. When I exceeds the D 30 times the thickness of metal in hollow columns should not be less than one-twelfth of the diameter.