140 



STEEL HIGHWAY BRIDGES. 



CHAP. III. 



Class 2- For Medium Electric Railways Only. On each track a series' of concentrations 

 consisting of two pairs of trucks, the axles of the pairs being spaced 5- ft. centers, while the distance 

 between centers of interior axles is 10 ft., the pairs of trucks being spaced 15-ft. centers. The 

 axles are loaded with a load of 25,000 lb., making a total load of 100,000 Ib. Or a uniform load 

 of 3,500 lb. per lineal foot for all spans up to 50 ft., reduced to 2,000 lb. per lineal foot for spans 

 of 200 ft. and over, and proportionately for intermediate spans. 



Class EZ. For Light Electric Railways Only. On each track a series of concentrations 

 consisting of two pairs of trucks, the axles of the pairs being spaced 5-ft. centers, while the distance 

 between centers of interior axles is 10 ft., the pairs of trucks being spaced 15-ft. centers. The 

 axles are loaded with a load of 20,000 lb. making a total load of 80,000 lb. Or a uniform load of 

 2,500 lb. per lineal foot for all spans up to 50 ft., reduced to 1,500 lb. per lineal foot for spans of 

 200 ft. and over, and proportionately for intermediate spans. 



TABLE I. 

 LIVE LOADS FOR THE TRUSSES 



30. Wind Loads. The lateral bracing in the unloaded chords of truss bridges shall be designed 

 for a lateral wind load of 150 lb. per lineal foot of bridge, considered as a moving load. The lateral 

 bracing in the loaded chords of truss bridges shall be designed for a lateral wind load of 300 lb. per 

 lineal foot of bridge, considered as a moving load. For spans over 300 ft. each of the above load- 

 ings shall be increased 10 lb. for each 20 ft. increase in span. In highway bridges not carrying 

 electric cars the end-posts of through and deck bridges and the intermediate posts of through 

 bridges shall be designed for a combination (i) of the dead load stresses and the total live load 

 stresses; or (2) of the dead load stresses, the live load stresses, the impact and centrifugal stresses, 

 and one-half the total wind load stresses. In low truss bridges and plate girders not carrying 

 electric cars the wind load on the unloaded chord may be omitted and the lateral bracing be de- 

 signed for a lateral wind load of 300 lb. per lineal foot treated as a moving load. In bridges with 

 sway bracing one-half of the wind load may be assumed to pass to the lower chord through the 

 sway bracing. 



31. In trestle towers the bracing and columns shall be designed to resist the following lateral 

 forces, in addition to the stresses due to dead and live loads: The trusses loaded or unloaded, the 

 lateral pressures specified above; and a lateral pressure of 100 lb. for each vertical lineal foot of 

 trestle bent. 



32. Temperature. Stresses due to a variation in temperature of 150 degrees shall be pro- 

 vided for (81). 



33. Centrifugal Force of Train. Structures located on curves shall be designed for the 

 centrifugal force of the live load acting at the top of the rail. The centrifugal force shall be calcu- 

 lated by the following formula: C = (0043 0.003 D) W'D; where C = centrifugal force in lb.; 

 W ' = weight of train in lb. ; and D = degree of curvature. 



34. Longitudinal Forces. The stresses produced in the bracing of the trestle towers, in any 

 members of the trusses, or in the attachments of the girders or trusses to their bearings, by sud- 

 denly stopping the maximum electric car trains on any part of the work must be provided for; 

 the coefficient of friction of the wheels on the rails being assumed as 0.20. 



35. All parts shall be so designed that the stresses coming upon them can be accurately 

 calculated. 



