Dimensions 



for 



Calculations 



Loads 



Dead Load 



Live Load 



Eccentricity 



Impact 



LOADS AND STRESSES 



25. The dimensions for calculation of stresses shall be as follows: 



Span Lengths 



For trusses and girders, the distance centre to centre of end bearings. 

 For floor beams, the distance centre to centre of trusses or girders. 

 For stringers, the distance centre to centre of floor beams. 



Depths 



For riveted trusses, the distance between centres of gravity of chord sections. 

 For pin connected trusses, the distance centre to centre of chord pins. 

 For plate girders, floor beams and stringers, the distance between centres of gravity 

 of flanges, but shall not exceed the distance back to back of flange angles. 



26. The structure shall be proportioned for the following loads: 



(a) Dead load. 



(b) Live load. 



(c) Impact or dynamic effect of live load. 



(d) Lateral force, without impact. 



(e) Centrifugal force, without impact. 

 (/) Longitudinal force, without impact. 

 (g) Temperature stresses. 



Stresses due to each of the above loads and forces shall be shown separately on the stress 

 sheet. 



27. The dead load shall consist of the weight of the entire supported structure, 

 computed in accordance with the following unit weights: 



Steel ....................................... 490 Ibs. per cu. ft. 



Concrete .................................... 150 " " 



Sand and Gravel (ballast) ..................... 120 " " 



Asphalt mastic .............................. 150 " 



Paving brick ............................... . 150 " " 



Oak, birch or maple .......................... 5 Ibs. per ft. B.M. 



Creosoted pine or fir .......................... 5 " " " " 



All other soft woods ....... ................... 4" "" " 



Rails and fastenings .......................... 150 Ibs. per lin. ft. per track 



28. The live load for each track shall consist of two typical engines followed by a 

 uniform train load, or of the alternative loading on two axles 7 feet apart, both as shown 

 in Appendix IV; and that producing the greater stress shall be used. The class of live 

 load shall be as specified by the Engineer. 



29. In designing either deck or through bridges provision shall be made for the in- 

 creased load carried by any member due to the eccentricity of the load, arising from curved 

 track or other cause. Stringers shall follow the curvature of the track as nearly as 

 practicable. 



30A. Impact shall be computed by the formula 



r = 



Lateral 

 Forces 



L+D 



in which /= Impact stress. 



L = computed maximum live load stress. 



D = computed dead load stress which in combination with live load stress and 

 impact gives the maximum total stress (See Par. No. 41). 



31. The lateral (or wind) force shall consist of a moving load equal to 30 pounds 

 per square foot on 1^ times the area of the structure projected on a vertical plane 

 parallel to the longitudinal axis (but not less than 200 pounds per lineal foot at the 

 loaded chord and 150 pounds per lineal foot at the unloaded chord), and a moving load 

 of 600 pounds per lineal foot applied 8 feet above base of rail. 



