Buildings of several stories having the floors supported by posts and girders without partitions are not very rigid, hence if the building is very high, so as to be effected by wind pressure, or contains machinery, it is very desirable to brace the posts and girders at the angles formed by their intersection, as shown in Figs. 473 and 475. Such bracing also adds materially to the strength and stiffness of the girders, but not to the posts.
A very interesting paper by Prof. Edgar Kidwell, of the Michigan Mining School, entitled " Comparative Tests of Bracing for Wooden Bents," was published in Volume IV., of the Proceedings of the Lake Superior Mining Institute. From the tests therein described, Prof. Kidwell found that for wooden braces, the best method of framing the brace to the post and girder, all things considered, is that shown in Fig. 473. Cast iron knees of the pattern shown in Fig. 474 were found to add little to the stiffness of the bent, until the latter had deflected more than would be admissible in a building.
Fig. 472a.-Van Dorn Joist Hangers and Post Cap.
As a result of his studies and experiments on the bracing of wooden bents, Prof. Kidwell has patented the brace shown in Fig. 475, which consists of a piece of ordinary gas or steam pipe, fitted by right and left hand screw threads into cast iron shoes, designed to be secured to the posts and girders as shown. For structures containing very heavy machinery, the shoes should be secured by bolts (preferably passing outside of the post), but for stationary loads, lag screws may be used. The drawing shows the brace as applied in the top story, or where the girder passes over the post, but it is equally efficient when the girder rests on a post cap, as in Fig. 473, the ends of the girders, of course, being well tied together.
The obvious advantages of this brace over the wooden brace are greater rigidity of connection, less weakening of the post and girder, and opportunity for adjustment in case of shrinkage in post, or settlement of girder. Other advantages are that the brace can be applied at any time and to old as well as to new buildings; it is also less clumsy than the wooden brace.
The sizes of pipe to be substituted for various sizes of wooden braces are as follows:
For 4x4 brace, 2½-inch pipe; for 4x6 brace, 3-inch pipe; for 4x6 brace, 4-inch pipe; for 6x8 brace, 4 1/8-inch pipe.