In brick dwellings girders are generally used only for supporting the first floor, and in the same way as described in Section 47. In heavier buildings the load on the girders becomes so great that it is necessary to utilize the full section of the girder, and very often steel beams are required.
Heavy Wooden Girders. - When heavy wooden girders must be kept flush with the joists the latter should be supported by joist hangers or stirrups.
Generally, however, it is necessary to make the girder deeper than the floor joists, in which case the latter may be supported by bolting long pieces of 3-inch hard pine plank to the bottom of the girder, as shown in Fig. 52.
. The depth of these pieces should be at least 4 inches for 10-inch joists, 5 inches for 12-inch joists and 6 inches for 14-inch joists. Three-quarter-inch bolts, spaced from 16 to 20 inches, may be used for 10-inch joists, and 7/8-inch bolts, spaced 20 and 16 inches respectively, for 12 and 14-inch joists. The bolts should be placed a little above the centre of the bearing strips, and the ends of the joists should be spiked to the sides of the girder, as shown in the figure.
The joists may also be supported on 3½x4 inch angle bars, bolted to the girder, as shown in Fig. 53. This method is undoubtedly better than that shown in Fig. 52, as there will not be as much settlement from shrinkage, and there is also less bending moment in the bolts.
In a great many small cities and towns, however, such angles are not carried in stock, and the expense of getting them will prohibit their use. Ordinary bolts can be readily obtained in almost any locality and at a comparatively small price.
Figs. 52 and 53 also show the manner of strapping the girders for lathing.
In designing the girders it should be remembered that deep girders are more economical than shallow ones, and when framed flush, or nearly so, on top, they prevent passage of fire through the floors.
Built-up Wooden Girders. - In many localities it is impossible to obtain large timbers without making a special order for them from the mills, while planks of almost any size can be readily obtained, and generally at a less price per thousand feet. In such cases the girders may be built up of several planks placed side by side, as in Fig. 52, and bolted together. The planks, however, should always break joints over a support. The bolts need not be larger than 5/8 inch in diameter, and spaced 2 feet on centres, in staggered rows, two bolts being placed at each end of the girder. Where each plank is the full length of the girder, the only use of the bolts is to keep the planks together and to distribute the load on all the planks.
The author believes that girders built up in this way are better than solid girders, and of equal if not greater strength. They certainly are not as likely to warp, and there is less chance of using decayed timber.
6l. Steel Beam Girders. - When it is desired to keep steel girders flush with the floor joists the latter may be supported by bolting a plank to the side of the beam and inserting joist hangers into holes bored in the plank, as shown in Fig. 54. For ordinary construction this is probably the cheapest and simplest method of framing, as the planks also afford nailings for the furring strips.
Another method of supporting the ends of the floor joists is to rivet angle irons to the sides of the beam, as shown in Fig. 64. With this method there can be no settlement of the joists from shrinkage, tut either method may be recommended. On account of the almost inevitable shrinkage of the joists, it is better that the depth of the steel beam be 1 inch less than that of the wooden beams, so that the top of the steel beams may be dropped 1 inch.
When the steel beam is deeper than the floor joists the latter may be supported by bolting 3x4 hard pine strips to the beam, as shown in Fig. 55. As the strips have a bearing on the lower flange of the beam, they need not be more than 4 inches deep, even for 14-inch, joists. Angle irons may also be used for supporting the joists, but the wood strips are generally preferred where the ceiling is to be plastered, as they are much more easily furred for lathing or casing.
Whichever of these methods of supporting the joists is used, enough bolts or rivets should be used to sustain the load carried by the strips or angles. [Each \-inch bolt may be allowed to support 3,000 pounds on each side of the girder, and each 7/8 inch bolt 4,000 pounds.]