CHAPTER XIII. 

 STRESSES IN RAILWAY BRIDGES FROM WHEEL LOADS. 



140. Kinds of Stress. The live load on railway bridges 

 produces stress in various ways. 



1st, Vertical static stresses due to the weight of the locomo- 

 tive and train in any position. 



2nd. Vibratory stresses due to various causes, generally in- 

 cluded under the term * ' Impact. ' ' 



3rd. Horizontal static stresses due to the centrifugal force 

 of the train, if the track is on a curve. 



4th. Longitudinal static stresses due to the momentum of 

 the train, and friction on the rails when the brakes are applied. 



The vibratory stresses cannot be calculated, but are provided 

 for in a more or less arbitrary way by taking a certain percentage 

 (varying with different specifications) of the static live load stress 

 as "impact stress," or by using small working stresses (28). No 

 account, however, is usually taken (except in the general design 

 of the bridge) of vibratory stresses which do not act vertically. 

 Horizontal vibratory stresses also occur even when the track is 

 not on a curve, when the wind is not blowing and when the brakes 

 are not applied. Stresses in bridges having curved track will be 

 treated in Art. 176. The stresses produced by the friction 

 between the rails and the wheels is of particular importance in 

 viaducts (steel trestles). 



\ ' 141. Loading. The live load on a railway bridge all consists 

 of wheel loads, but nearly all railway bridge specifications require 

 it to be assumed to consist of two consolidation engines followed 

 by a uniformly distributed load representing the train. 1 The 

 weights on the wheels and their distances apart may represent the 

 actual engine in use which will produce the maximum effect on 

 the bridges, but they usually represent typical engines supposed 

 to be heavy enough to provide for future increase of weight. 



Such a typical loading is shown in Fig. 198, which is Class 

 #40 of Cooper's " General Specifications for Steel Railroad 

 Bridges." In Cooper's loadings the class number is the same as 



1 The Specifications of the Pennsylvania Lines West of PiUsburg give a 

 uniform load combined with a single concentrated load. Some specifications 

 also give a special load on two axles 6 ft. to 8 ft. apart which governs for sliort 

 spans. 



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