For large spans and heavy weights it will be often necessary to use iron or steel columns with wooden girders. If cast-iron columns are used, they should not be made with a shell less than three-quarters of an inch thick. This is necessary on account of the danger of an unequal thickness in the shell of the column. To obtain the hollow column, the casting must be made about a "core," and although this core is accurately centered, there is danger of its being displaced by the pouring in of the molten iron, as, being of a lighter composition, it will have a tendency to "float." This may result in an added thickness to one side of the shell of the column and -a corresponding lack of metal on the opposite side. To guard against this defect, cast-iron columns should always be tested by boring a small hole on opposite sides, and if more than one-fourth of the thickness of shell is wanting in any column it should be rejected. The outer surface of cast iron should be smooth and clean, with sharp angles; and all projections, such as lugs, caps, or bases, should be closely examined to detect the presence of cracks which may occur at these points.

Fig. 172. Cast-iron Pintle.

Fig. 173. Box Anchor on Wall.