The building consists of the usual administrative portion, about 56 feet deep, by 160 feet wide and of a drill hall covering the rest of the ground area and having an unbroken floor space of 35,000 square feet of asphalt. The side walls of the building are carried about 5 feet above the point where the roof meets them, and alongside this wall is a walk where soldiers can patrol from one tower to another, should it ever become necessary to defend the armory to that extent.
*From the Engineering Record, Aug. 24, 1902.
Fig. 228. - Detail of End Sections and Crown.
The drill hall has a roof supported by twelve steel trusses 20 feet apart on centers, which have a span of 156 feet, out to out, and a rise of 52 feet to the top of the upper chord. The trusses are hingeless arches, Fig. 227, with horizontal square lower ends seated on top of the abutment piers and connected by horizontal tension members under the floor. The parallel top and bottom chords are 5 feet apart, out to out, and are false ellipses, composed of circular arcs, described from three centers with a long radius of 104 feet and two short radii, of 39 feet each. The trusses are wholly within the side walls and are seated, just below the floor level, on extensions of the concrete wall footings capped with 2 x 6-foot blue stone pedestals. They are connected by seven lines of longitudinal lattice girder struts 5 feet deep with their webs in radial planes, which are field riveted to lateral connection plates on the top and bottom chord flange angles. The roof trusses are also braced together in alternate panels by lateral diagonal rods in the planes of the top chords, with pin connections and sleeve-nut adjustments.
Both chords of the arch trusses are made throughout of pairs of 6 x 6 x 7/16-inch angles riveted together back to back and reinforced with 16-irich flange cover plates from the haunches to the springing line. The panel lengths are nearly uniform, varying from about 4 feet 8 inches to 5 feet 10 inches, and all web members are pairs of zigzag angles from 5.x 3 x 3/8 inches at the skewbacks to 3 x 2 x 3/8 inches at the crown. Except where the truss sections are field spliced, all web members are riveted directly to the vertical flanges of the chord angles without connection or gusset plates. Each truss was shipped from the bridge shops in six sections, which were field riveted together before erection. The splices were made at the panel points in the top chords and in the middles of the opposite panels of the bottom chords, and the joints were covered with flange and double web plates.
The end sections of the trusses are about 20 feet long and have a solid web plate in the two lower panels, as shown in the detail, Fig. 228. The 3/4-inch planed base plate has two slotted holes for large anchor bolts, and projects beyond the inside of the truss and its pedestal to receive the connection pins for the two 1 5/8-inch horizontal bottom tie rods, which are made in five lengths connected by sleeve nuts. The base plate rests on a 1-inch bed plate nearly 4 feet long, and the anchor-bolt nuts are screwed down tight on ring washers which bear on the bed plates and clear the holes in the base plates, thus preventing any pressure or friction on the base plates. The lateral connection plates are in two pieces at each splice, riveted to the under side of the horizontal flanges of the chord angles, and are notched to clear the web members of the truss as shown in the detail sketch. At the crown splice the top chord joint is riveted solid, but the bottom chord splice is made with cover plates which are riveted fast to one section of the truss and have slotted holes for bolts in the other section so as to allow for temperature displacements. The roof is proportioned for a wind load of 30 pounds per square foot of the vertical projection, and the main trusses are designed for an additional load of 20 tons at each end from the gallery floor, which is calculated for a uniform loading of 150 pounds per square foot.
On every truss, at the sixth panel point from each end, there is riveted to the top chord a cast saddle from which are suspended two 1 1/2-inch vertical pin-connected rods with sleeve-nut adjustments. These rods clear each side of the truss and are bent to join each other under the bottom chord, where they are pin-connected to a single rod 2 1/8 inches square, which supports the side gallery. Pairs of short 6 x 6-inch angles are riveted across the top chord at intervals of about two panel lengths, and between them are secured by two bolts through their vertical flanges, 10 x 14-inch yellow pine purlins, about 12 feet apart. These purlins support 3 x 8-inch rafters 24 inches apart on centers, which are sheathed with matched 1 1/2-inch spruce boards planed and beaded on the under side. The boards are covered with five-ply slag roofing, laid by the Warren Ehret Company, except where the slopes are steep and in the gutters, where they are tinned, with standing seams.