PART II 



HIGHWAY BRIDGES 



GENERAL REQUIREMENTS 



49. Classification. Highway bridges may, based upon traffic 

 conditions, be divided into three classes, viz.: 



Class A City bridges, subject to heavy traffic. 



Class B City, Interurban or Country bridges, subject 



to medium traffic, and 

 Class C Country bridges, subject to light traffic. 



50. Type of Bridge. The following types of bridges shall 

 preferably be used: 



For spans up to 20 f t . , wooden beams or rolled beams. 



" " from 20 to 40 ft., rolled beams or plate girders. 



" 40 to 70 ft., plate girders. 



" " " 80 to 100 ft., plate girders or riveted trusses. 

 " " " 100 to 160 ft., riveted trusses. 

 " long spans, 160 ft.and over, pin-connected trusses. 



Pony trusses shall be avoided wherever possible, but may 

 be used for spans from 40 to 90 ft. (Par. 80.) 



51. Materials. All parts of the superstructure, except the 

 flooring and paving, shall be of structural steel, or rivet steel. 



Cast iron may be used for minor parts and for ornamental 

 purposes; all other castings shall be of steel. 



52. Clearances. All through bridges carrying electric cars 

 shall have a clear head-room, above the top of the rail, of at 

 least 15 ft., for a width of 6 ft. over the center of the track. 

 Where the track is straight there shall be clear width of at least 

 7. ft. on each side of the center of the track at a height of 12 in. 

 above the top of rails. Where the tracks are curved, the addi- 



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