All floor timbers having a span of more than sixteen feet should be crowned, that is, the top of the joist is cut to the shape of an arc of a circle, having a rise of one-quarter inch to every sixteen feet of span. This is necessary to allow for the ordinary sag of the timber, so that a level floor may result. Steel Girders. With the use of steel girders new considerations will arise. If a single beam is set entirely below the floor timbers, it will give a better bearing if the timbers lap and spike to each other. With two or more beams, the timbers may be brought end to end as on the heavy wooden girder. If the steel girder is set flush with the beams, they may be cut so as to run into the trough formed by the flanges of the beam, but should be supported by stirrup-irons or hangers, Fig. 181, as the sloping flange of the steel beam does not afford a good bearing. If the steel beam is deeper than the floor timbers, a common method is to bolt a timber to each side of the beam for a bearing; and of course an angle bar can be used in the same manner. (Fig. 182.) In any case, the floor beam should be fastened so that there is no danger of slipping out of the hanger. Most of the patent hangers have a lug, or bolt, to secure the timber in place, and where timbers come opposite, common iron dogs turning down into each timber may be used.

Fig. 179. Hanging of Joists.

Fig. 179. Hanging of Joists.

Fig. 180. Wooden Girder and Joists.

Fig. 180. Wooden Girder and Joists.