For open roofs, of wide span - 100 feet or more - the segmental arched rib, with an iron tie, is probably the most economical wooden truss that can be used, as well as the most pleasing in appearance. It has been quite extensively used in this country for supporting the roofs of exhibition buildings, armories, skating-rinks, etc.

The truss shown in Fig. 58, which is over the large hall of the Mechanics' Charitable Association Building in Boston, is a good example of this type. In this truss the braced arch, possesses in itself sufficient strength and rigidity to transmit the roof load to the supports in the same way that a brick arch would, while the horizontal tie resists the thrust of the arch. The framework above the arch does not form a part of the truss, but is merely a series of braced posts to support the purlin.

Fig. 58.

Fig. 59 shows one-half of a truss, which, with seventeen others, was designed for supporting the central bay of Sanger Hall, Philadelphia,* Messrs. Hazelhurst & Huckel, architects.

This building was erected, in 1897, for the use of the Eighteenth National Sangerfest, and was to be taken down and removed immediately after the four days' session. It was therefore but a temporary building, although built in accordance with the building ordinance, and the illustration is given as showing what is undoubtedly the cheapest method of supporting such a roof without the use of objectionable columns. The trusses were spaced 20 feet from centres.

Fig. 59.