Centrifugal 

 Forces 



Longitudinal 

 Force 



Temperature 

 Stresses 



Combined 

 Stresses 



Reversal 

 of Stress 



32. If greater stresses are provided by a moving load of 50 pounds per square foot 

 acting on lyZ times the area of the unloaded structure projected on a vertical plane 

 parallel to the longitudinal axis, such stresses shall be provided for. 



33. For Viaduct Towers, the lateral forces to be considered shall be as above, but 

 provision shall be made for a horizontal force on the towers of not less than 225 pounds 

 per lineal foot of height on each bent from top of pedestal to under side of girder for the 

 structure when loaded, and 375 pounds per lineal foot of height for the structure un- 

 loaded. In calculating the uplift the viaduct shall be considered as loaded on any 

 one track with empty cars assumed to weigh 1200 pounds per lineal foot, if this load 

 ing gives a greater result than that specified above for the unloaded condition. 



34. On curves, the centrifugal force (assumed to act 6 feet above base of rail) shall 

 be taken as equal to a percentage of the live load, according to the following table: 

 Degree of Curve ...... 20' 40' 1 2 3 4 5 6 7 8 9 10 11 12 



Percentage. .......... 2^ 5 7^10 10 10 10 10 10 10 10 10 10 10 



Speed, Miles per hour. .80 80 80 65 53 46 41 38 35 33 31 29 28 27 



35. The longitudinal force produced either by the traction of the engines, or by 

 the application of the brakes when applied to a moving train, shall be assumed to act 

 6 feet above the base of rail. For loaded lengths up to 400 feet this longitudinal force 

 shall be computed by the formula: 



in which T = longitudinal force, in percentage of the live load. 

 L = loaded length, in units of 100 feet. 



36. For loaded lengths of 400 feet and over the longitudinal force shall be assumed 

 equal to 10 per cent, of the live load. 



37. Provision shall be made for temperature stresses due to an extreme variation of 

 160 degrees Fahrenheit, with a normal temperature of 60 degrees, and a range from 40 

 degrees below zero to 120 degrees above. 



38. In proportioning the various parts of the structure, provision shall be made 

 for each of the following groups of stresses : 



Group A Dead load, 

 Live load, 

 Impact, 



Centrifugal force, 

 at the Unit Stresses herein specified. 



Group B Lateral force, 



Longitudinal force, 

 Temperature stress, 

 jointly or separately, at the Unit Stresses herein specified. 



Group C Any one or more of Group A combined with any one or more of 

 Group B, at Unit Stresses 25 per cent, greater than those hereinafter 

 specified. 



39. Members in which the static live load stress may have two maximum values, 

 one of similar sign to the dead load stress, and one of opposite sign, shall be proportioned 

 for either 



(a) The full dead load stress combined with the maximum live load stress of the 



same sign with its corresponding impact, 

 (6) The maximum live load stress of the opposite sign with its corresponding im- 



pact, less two-thirds of the dead load stress, 



each combination being further increased by an amount equal to one-half the smaller of 

 the two combination^. 



40. Where the live load stress is always of opposite sign to the dead load stress only 

 combination (6) will apply, and no further increase shall be made. 



