280 LOADINCJ. Art. 141. 



UK- load on a pair of driving wheels in thousands of pounds, and 

 ten times that of the uniform load per foot. The other classes 

 given in the specification are #27, #30, #35 and .#50, but any 



/ 2 3 4 S 6789 IO II ^ 13 14 IS 16 17 16 



* * * * mo Ibs. per lineal ft 

 LQ QQQO opooZoOOOO oo oo r 



rWs 9' 5^6 ' S' 6' 8' 5-S'S' 9- S-'G-S-S' 



Cooper's C/ass E ^O Loading 

 Fig. 198. 



intermediate class may be made by taking the loads in proportion 

 to the class numbers. Since the spacing of the wheels is the same 

 for all classes, and since the loads on corresponding wheels are in 

 direct proportion to the class numbers, any stress due to the #50 

 loading, for example, is to that due to the #40 loading as 50 is 

 to 40. 



The #40 to #60 loadings are those oftenest used for steam 

 railways: One locomotive (engine and tender) of the #40 load- 

 ing weighs 142 tons and covers a distance of 56 ft. ; this is an 

 average load of about 5000 Ibs. per foot, while that of the drivers 

 alone is much greater. For spans somewhat less than the length 

 of two locomotives, the loading giving maximum stresses consists 

 entirely of wheel loads because these give a much greater load 

 than the uniform load following. The average weight of loco-mo- 

 tive now specified by a number of the principal railway systems 

 is over 200 tons. 1 



142. Equivalent Loadings. On account of the laborious- 

 ness of calculating stresses from wheel loads as usually specified, 

 many " equivalent loads" have been proposed, the idea being to 

 substitute a simpler load which would give practically the same 

 stresses as the wheel loads, or at least none that would be mark- 

 edly smaller. Engineers are pretty well agreed that an equiva- 

 lent load will give abundantly accurate results. 2 In order to 

 approximate closely to the stresses given by the specified wheel 

 loads, a number of different uniform loads arc used for each 



1 For a comparison of different specifications see article by the author in 

 Eng. News, Vol. 50, page 444. 



2 For a thorough discussion of this subject see Trans. Am. Soe. C. E., 

 Vol. 42, p. 180. Also Kiif/. AV?/-*. Also Johnson's " Modern Trained Struc- 

 tures," Chapter VI. 



