FLOORS After the wall, the next important part of the house frame to be considered is the floors, which are usually framed while the wall is being put up and before it is finished. They must be made not only strong enough to carry any load which may come upon them, but also stiff enough so that they will not vibrate when a person walks across the floor, as is the case in some cheaply-built houses. The floors are formed of girders and beams, or "joists," the girders being large, heavy timbers which support the lighter joists when it is impos sible to allow these to span the whole distance between the outside walls.
Girders. Girders are generally needed only in the first floor, since in all the other floors the inner ends of the joists may be sup ported by the partitions of the floor below. They are usually of wood, though it may sometimes be found economical to use steel beams in large buildings, and even in small buildings the use of steel for this purpose is increasing rapidly. Wrought iron was once used, but steel is now cheaper and has taken its place. However, when this is not found to be expedient, hard pine or spruce girders will answer very well. The connections used in the case of steel girders will be explained later. The girders may be of spruce or even of hemlock, but it is hard to get the hemlock in such large sizes as would be required, and spruce, too, is hardly strong enough for the purpose. Southern pine, therefore, is usually employed for girders in the best work.
The size of tue girder depends on the span, that is, the distance between the supporting walls, and upon the loads which the floor is expected to carry. In general, the size of a beam or girder varies directly as the square of the length of the span, so that if we have two spans, one of which is twice as great as the other, the girder for the longer span should be four times as strong as the girder for the smaller span. In ordinary houses, however, all the girders are made about SX 12 inches in sections, although sometimes an S X S-inch timber would fice, and sometimes perhaps a 12-inch piece would be required.
It should be remembered in deciding upon the size of this piece, that any girder is increased in strength in direct tion to the width of the timber (that is, a girder 12 inches wide is twice as strong as one 6 inches wide), but in direct proportion also to the square of the depth (that is, a girder 12 inches deep is four times as strong as one 6 inches deep). Hence, the most economical girder is one which is deeper than it is wide, such as an SX 12-inch stick; and the width may be decreased by any amount so long as it is wide enough to vide sufficient stiffness, and the depth is sufficient to enable the piece to carry the load placed upon it. If the piece is made too narrow in proportion to its depth, however, it is likely to fail by "buckling," that is, it will bend as shown in Fig. 132. It has been found by experience that for safety the width should be at least equal to of the depth.
There are at least three ways in which the joists may be sup ported by a girder. The best, but most expensive, method is to support the ends of the joists in patent hangers or stirrup irons which connect with the girder. This method is the same as was described for the sill, except that with the girder a double stirrup iron, such as that shown in Fig. 133, may be used. These stirrup iron hangers are made of wrought iron, 2.1.
or 3 inches wide, and about t inch thick, bent into the required shape. They usually fail by the crushing of the wood of the gir ders, especially when a single hanger, like that shown in Fig.
134, is used. Fig. 135 shows a double stirrup iron hanger in use. Patent hangers as shown in Fig. 136 are the most suit able.
If hangers of any kind are used, there will be no cutting of the girder except at the ends, where it frames into the sill, and even there a hanger may be used. The girder may be placed so that the joists will be flush with it on top, or so that it is flush with the sill on top.