GENERAL SPECIFICATIONS FOR STEEL HIGHWAY BRIDGES.* 



BY 



MILO S. KETCH UM, 



M. Am. Soc. C. ET. 



THIRD EDITION, 



1918 



PART I. DESIGN. 

 GENERAL DESCRIPTION. 



1. Classes. Bridges under these specifications are divided into eight classes, as follows; 

 Class A. For city traffic. 



Class B. For suburban or interurban traffic with heavy electric cars. 



Class C. For country roads with ordinary traffic and light electric cars. 



Class DI. For country roads with heavy traffic. 



Class Dj. For country roads with light traffic. 



Class Ei. For heavy electric street railways only. 



Class Ej. For medium electric street railways only. 



Class E 3 . For light electric street railways only. 



2. Material. All parts of the structure shall be of rolled steel, except the flooring, floor 

 joists and wheel guards, when wooden floors are used. Cast iron or cast steel may be used in the 

 machinery of movable bridges, for wheel guards, and in special cases for bed plates. 



3. Types of Truss. The following types of bridges are recommended: 

 Spans up to 30 ft. Rolled beams. 



Spans from 30 to 80 ft. Riveted plate girders, or riveted low trusses for classes A, B, Ei, 

 Ej and E 3 ; and riveted low trusses for classes C, Di and D 2 . 

 Spans 80 to 1 60 ft. Riveted or pin-connected high trusses. 



Spans 160 to 200 ft. Pin-connected trusses of the Pratt type with inclined chords. 

 Spans over 200 ft. Pin-connected trusses of the Petit type or K-type. 



4. Length of Span. In calculating the stresses the length of span shall be taken as the 

 distance between centers of end pins for pin-connected trusses, centers of end bearing plates for 

 riveted trusses and for girders, and center to center of trusses for floorbeams. 



5. Form of Trusses. The form of truss shall preferably be as given in paragraph 3. In 

 through trusses the end vertical suspenders and the two panels of the lower chord at each end 

 shall be made rigid members if the wind load produces a reversal of stress in the lower chord. In 

 through bridges the floorbeams shall be riveted above or below the lower chord pins. 



6. Lateral Bracing. All lateral and sway bracing shall preferably, and all portal bracing 

 must be, made of shapes capable of resisting compression as well as tension, and shall have riveted 

 connections. Low trusses and through plate girders shall be stayed by knee braces or gusset 

 plates at each floorbeam. 



7. Spacing of Trusses. For bridges carrying electric cars the clear width from the center of 

 the track shall not be less than 7 ft. at a height exceeding one foot above the track where the 

 tracks are straight, and an equivalent distance when the tracks are curved. The distance between 

 centers of trusses shall in no case be less than one-twentieth of the span between the centers of 

 end-pins or shoes, and shall preferably not be less than one-twelfth of the span. 



8. Head Room. For classes A, B, C, Di, Ei, E 2 and E 3 the clear head room for a width of 

 eight (8) ft. on each track, or eight (8) ft. on the center line of the bridge shall not be less than 

 15 ft., and for class D 2 not less than I2| ft. 



9. Footwalks. Where footwalks are required, they shall generally be placed outside of the 

 trusses and be supported on longitudinal beams resting on overhanging steel brackets. 



10. Handrailing. A strong and suitable handrailing shall be placed at each side of the bridge 

 and be rigidly attached to the superstructure. 



n. Trestle Towers. Trestle bents shall preferably be composed of two supporting columns, 

 two bents forming a tower; each tower thus formed shall be thoroughly braced in both directions 

 and have struts between the feet of the columns. The feet of the columns must be secured to 

 an anchorage capable of resisting one and one-half times the specified wind forces (89). 



* Reprinted from the author's "The Design of Highway Bridges." 



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