152 



STEEL RAILWAY BRIDGES. 



CHAP. IV. 



For the relative weights of railway bridges built of carbon and of nickel steel, see paper 

 entitled " Nickel Steel for Bridges," by Mr. J. A. L. Waddell, M. Am. Soc. C. E., printed in Trans. 

 Am. Soc. C. E., Vol. 63, 1909. 



10 20 30 40 50 00 70 80 

 Span in Teet. 



FIG. 5. WEIGHT OF SINGLE TRACK THROUGH 



PLATE GIRDER SPANS. TYPE C4 (FLANGES 



OF 2 ANGLES AND COVER PLATES, Two 



STRINGERS). CHICAGO, MILWAUKEE 



& ST. PAUL RY. 



10 W 30 40 50 00 70 80 90 

 in FeeL 



FIG. 6. WEIGHT OF THROUGH PLATE GIRDER 



SPANS^ TYPE Cs (FLANGES OF 2 ANGLES 



AND COVER PLATES, SHALLOW FLOOR, 



4 STRINGERS). CHICAGO, MIL- 



WAUKEE & ST. PAUL RY. 



LOADS. The dead load of a railway bridge is assumed to act at the joints the same as in a 

 highway bridge. The dead joint loads are commonly assumed to act on the loaded chord, but 

 may be assumed as divided between the panel points of the two chords, one-third and two-thirds 

 of the dead loads usually being assumed as acting at the panel points of the unloaded and the 

 loaded chords, respectively, see discussion of specifications in the last part of this chapter. 



The live load on a railway bridge consists of wheel loads, the weights and spacing of the 

 wheels depending upon the type of the rolling stock used. The locomotives and cars differ so 

 much that it would be difficult if not impossible to design the bridges on any railway system for 

 the actual conditions, and conventional systems of loading, which approximate the actual con- 

 ditions, are assumed. The conventional systems for calculating the live load stresses in railway 

 bridges that have been most favorably received are: (i) Cooper's Conventional System of Wheel 

 Concentrations; (2) the use of an Equivalent Uniform Load; and (3) the use of a uniform load 

 and one or two wheel concentrations. In addition to these some railroads specify special engine 

 loadings. The three Methods will be briefly described. 



