When the roof to be supported is not too long, or posts can be placed near the end walls, it is often cheaper to support the roof by longitudinal trusses, particularly where there is to be no ceiling, or where it is desired to have a greater clear height in the centre than at the sides. Fig. 126 shows the manner in which the roof of a large car house and stable at Avon-dale, Ohio (James W. McLaughlin, architect), is supported by longitudinal trusses. As will be seen, the top chord of the trusses forms a support for the rafters at a point a little above their centre, and the upper lengths are supported entirely by the trusses, the rafters being mutually supported at the ridge, as in a small pitch roof. With such construction, however, the tops of the trusses should be connected frequently by horizontal tie-beams, to prevent them from being pushed outward by the upper rafters. These tie-beams should also be of such a section that they may act as struts when under wind pressure. In this particular building one of the trusses forms the ridge of the front portion of the roof, while the other truss supports one side of a sort of tower. In this instance it was probably considerably cheaper to support the roof in the manner shown than it would have been by trusses extending across the building.
Fig. 125A. - Section of School House Roof.
The construction shown at the right in Fig. 126 is that commonly employed for supporting the roof of large barns.
Figs. 127 and 128 show a method of supporting the roof and ceiling over a large room which, while embodying no new principle, is yet an uncommon arrangement. The construction shown by these figures was designed by Messrs Boring and Tilton, architects, for supporting a school building at Newtown, New York. The full construction shown by Fig. 127 may be called a pyramidal truss. It consists essentially of two Warren trusses, set on a slant, the same beam forming the top chord of both trusses. It makes a very strong and stiff truss, requiring no lateral bracing (see Section 59), and gives a greater clear height for the room than any other form of construction. The same principle could be adopted in roofing a square area with supports at four corners only, using a pair of King rod or Queen trusses in the place of the Warren truss, Fig.
127. This latter construction was used by the same architects in roofing two halls in the East Orange, N. J., Town Hall, drawings of which were published in the Engineering Record of Oct. 26, 1901. The truss shown in Fig. 127 has been patented by Mr. Boring, and should any one desire to avail himself of this idea, he should obtain permission from Mr. Boring, which we understand will not be a difficult matter.
Fig. 127. - Pyramidal Truss.