38 UNITED STATES NATIONAL MUSEUM 



In the ranges the typical roof trusses have a span of 52 feet 2 

 inches in the clear. The lower member is 2 inches above the third 

 story ceiling line, from which level to the apex of the truss the 

 height is 7 feet f inch, this being the maximum height of the range 

 attics. The outside ends of all trusses rest upon built-up struts hav- 

 ing a 10 by f-inch plate stiffened by four 3| by 3| by i%-inch angles 

 and bearing upon the third story girders inside the walls of that 

 story. The inside ends of all typical trusses bear directly upon 

 cast-iron shoes bolted to the top of the court walls. The top and 

 bottom members of the typical trusses are formed of double 6 by 4 

 by iV"iiicli angles riveted together. The interior members are of 

 lighter material, as are also certain special trusses at the northeast 

 and northwest corners of the building, which do not completely span 

 the ranges but have an intermediate bearing in the single interior 

 column at each of these points. The trusses are stiffened by latticed 

 purlins extending at right angles to them and spaced 12 feet 6 inches 

 on centers, and these spaces in turn are traversed by beams, 4 inches 

 deep, spaced 6 feet 2 inches on centers, which are riveted to the 

 purlins. The bottoms of the purlins are 6| inches, and of the beams 

 2 inches, above the third story ceiling, the metal lathing of which is 

 fastened to the beams and lower members of the trusses. 



Roof shell and covering. — The construction to which the outside 

 covering of slate is attached, on the mansards of the ranges and the 

 outer slopes of the wings, consists of a 5-inch shell of reenforced 

 cinder concrete supported between I beam purlins at the upper edge 

 and the wall at the lower. Elsewhere, except where skylights occur, 

 the construction is a combination of concrete frames and terra cotta 

 arches covered with copper, built in the following manner: Keen- 

 forced concrete rafters, 4 inches wide, spaced 16 inches on centers, 

 and having intermediate points of support in the I beam purlins of 

 the steel framev/ork, run in a transverse direction across the roof. 

 The spaces between the rafters are filled with terra cotta blocks, but 

 as the entire construction is flush and the materials were placed in 

 the one operation the result is a continuous slab or shell reenforced 

 by- the rafters. The usual thickness of this construction is 6 inches, 

 but over wide spans blocks 10 inches thick and rafters of the same 

 depth occur. Embedded in the concrete work of alternate rafters, 

 and therefore on lines 2 feet 8 inches apart, are galvanized iron wires 

 in the form of loops, the ends of which project sufficiently above the 

 concrete to fasten small galvanized iron T's running with the rafters 

 and standing about ^\ inches above them. The entire surface is 

 furthermore topped off with a 1-inch coating of cement mortar which 

 covers all wires and the lower flanges of the T's, leaving the upright 

 arm of the latter projecting. The sheet copper, which is of 12-ounce 

 weight, is laid on this surface and the parallel standing joints follow 



