20 STANDARD SPECIFICATIONS 



Depth of girders, center to center of gravity of chords. 

 Depth of trusses, center to center of gravity, or center to 

 center of pins, of chords. 



76. General Proportions. The width between centers of 

 trusses carrying a single straight electric railway track shall 

 not be less than 15 ft., or for any bridge less than one-twentieth 

 (-^5-) of the span. 



The depth of plate-girder spans shall preferably not be less 

 than one-twelfth (^ of the span, the depth of lattice girders 

 and pony trusses shall not be less than one-tenth (-fa) of the span, 

 and the depth of riveted and pin connected through trusses shall 

 preferably not be less than one-eighth () of the span, or less than 

 the panel length. 



77. Floor Framing. Steel joists and stringers shall pref- 

 erably be riveted to the web of the floor beams. Rolled beams 

 used as joists shall be spaced not to exceed 3 ft. center to center 

 and shall not have a depth of less than one-twentieth (-fa) of 

 the span. 



If the floor plank be continuous each joist may be assumed 

 to cany only two-thirds (J) of the concentrated load. The 

 top flanges of stringers must be provided with securely bolted 

 wooden shims for the purpose of spiking the planking thereto. 



When end floor beams are not used over the masonry the 

 joists shall have their ends rigidly connected by means of struts 

 and the stringers by means of cross frames. 



Floor beams shall preferably be arranged so as to be per- 

 pendicular to the girder or truss at the panel points; they may 

 rest upon the top chord in deck bridges, but in through bridges 

 they shall be riveted to the verticals. 



Rolled beams used as joints, stringers, or floor beams shall be 

 proportioned by their moment of inertia. 



78. Plate Girders. For plate girders used as stringers, floor 

 beams, and main girders, see Part IV. 



79. Beam Bridges. When bridges carrying electric rail- 

 ways are built of rolled beams they shall be braced as follows: 

 With a single beam under each rail and for spans under 20 ft. 

 the bracing shall consist of cross channels framed at each end at 

 intervals not exceeding 5 ft., for spans over 20 ft. a cross channel 

 framed at each end with an intermediate diagonal bracing. When 

 two or more beams are placed under each rail the beams shall 



