Fig. 221. - Plan Showing Horizontal Trusses.

Enlarged sections of the ribs and web members of the arched trusses are shown in Fig. 222. It should be noticed that the vertical web members have an iron rod through their center, so that they act as ties rather than struts.

Fig. 223 shows an enlarged detail of the iron skewbacks and posts which receive the ends of the arched trusses.*

•The construction of this building was originally illustrated in the American Architect of May 1, 1880.

94: When steel trusses are used for roofing building's of this class, either the three-hinged braced arch or a truss like that shown in Fig. 103 is most commonly selected, because it requires no posts, and is comparatively easy of erection. There are several examples of segmental and elliptical braced arches, however (see Fig. 98), and for auditoriums having several galleries, the quadrangular truss has been frequently used, as in the Madison Square Garden, and the Kansas City Auditorium, see Figs. 88, 90, and 92.

For armories and gymnasiums not exceeding 80 feet in width the French truss, shown in Fig. 74, is an economical and desirable type.

Figs. 224 and 225 show a plan and elevation of the trusses and bracing of a drill hall, 80'x 120', roofed as in Fig. 74.

Fig. 103 shows one of the trusses used in roofing the Exposition Hall, at Providence, R. I., and Fig. 226 shows a plan of the roof framing.*

95. Examples Of Armory Roofs

As showing the quantities of steel required in roofs of from 82 to 118 feet in width, the following data, given by Mr. H. G. Tyrrell, C. E., is of much practical value:

95 Examples Of Armory Roofs 300225

Fig. 222.

*Figs. 74, 103, 224-226, are reproduced from drawings by Mr. H. G. Tyrrell, C. E., and published in the Architects' and Builders' Magazine for October, 1901.

"All of the following actual cases were proportioned for slate and plank roofing-, resting on wood rafters 2 feet apart. Steel purlins occur about 10 feet, apart. The unit stresses used were 12,000 and 15,000 pounds per square inch, in compression and tension, respectively. The trusses are all similar in general outline to that shown in Fig. 103, Chapter II (Foundations On Compressible Soils). Thespans given are center to center of side bearings, and are 4 to 5 feet less than the outside width of building.

"The assumed loads were as follows:

"For trusses, dead weight of roof and covering = 25 lbs. per sq. ft. of sloping surface.

"For purlins, dead weight of roof and covering= 18 lbs. per sq. ft. of sloping surface.

" Dead weight of snow = 10 lbs. per sq. ft. of sloping surface. "Horizontal wind = 40 lbs. per sq. ft., = 28 lbs. normal."

95 Examples Of Armory Roofs 300226

Fig. 223