NATURAL HISTORY BUILDING 37 



outer and court walls, one for each of the side spaces and one for the 

 intervening skylight area. For the former, which measure 32 feet 

 5 inches between the columns and the outer walls and 4 inches less be- 

 tween the columns and the court walls, the trusses are diamond- 

 shaped, resting at the wall ends on cast-iron shoes at a level of 16 

 inches below the attic floor and riveted at the other ends to the tops 

 of the columns 10 feet 2 inches above the floor. For the intervening 

 space of 51 feet 6 inches the trusses employed are triangular in shape, 

 being riveted to the columns 7 feet above the floor, their apexes reach- 

 ing a height of 15 feet 4 inches above the same level. The various 

 members of the trusses are built of angle iron riveted together in 

 pairs. The top members of the outer trusses are' two 5 by 3| by 

 ^-inch angles, and the bottom members two 3^ by 3^ by f -inch angles ; 

 while in the middle trusses the top members consist of two 6 by 6 

 by f -inch, and the bottom of two 5 by 3 by f -inch angles. All columns 

 are braced at the top by 15-inch I beams extending between them, and 

 in other ways the entire framework is secured and reinforced in a 

 manner to obtain the necessary rigidity. 



On top of the triangular trusses is a raised platform or deck of 

 9-inch I beams and 3 by 2^-inch angles designed to support the sky- 

 light construction over the central part of each wing. A line of these 

 beams extends continuously above the apexes of the trusses, and 

 another line 8 feet from the first on each side of the ridge, while the 

 angle curb construction carries the lower edges or eaves of the sky- 

 lights. 



Projecting 9 feet 9 inches into the lighting space on all sides is a 

 series of 12-inch beams, which are framed into the columns at one end 

 and .suspended from the trusses at the other, the outer ends being 

 connected and the construction thus stiffened by 12-inch channels. 

 Riveted to the channels and suspended from the trusses by angles, 

 8-inch I beams, spaced 18^ feet, span the 32 feet from channel to chan- 

 nel, the approximate width of the skylight, and this framework, with 

 some subordinate members, carries the ceiling light construction 

 which is described in another connection. 



At the northern end of the north wing, in the space between the 

 stair well and the elevator shaft, 12-inch I beams are used instead of 

 trusses as roof supports, and the floor construction of the third story 

 in this section, therefore, differs somewhat from that at the outer 

 ends of the east and west wings. Here I beams carry the weight of 

 the floor arches directly to the outer walls instead of indirectly per- 

 forming that function through the trusses as is tj^pical elsewhere. 



All roof truss members in the side and end sections of the wings 

 which are not protected within the roof and floor construction are 

 enclosed in expanded metal jackets and fireproof ed with a casing of 

 lime plaster gauged with cement. 



