206 STEEL RAILWAY BRIDGES. CHAP. IV. 



14. Unit Stresses. For a comparison of the tensile unit stresses see Table XVI. 



22. Alternate Stresses. Four of the sixteen specifications use the same specification as in 

 section 22. Six specifications use Cooper's specification. "All members and their connections 

 shall be designed to resist each kind of stress. Both of the stresses shall, however, be considered 

 as increased by 0.8 of the least of the two stresses." One specification increases each stress by 

 0.60 of the lesser stress, one by 0.70, and two by 0.75. One specification uses Weyrauch's formula, 



P = S ( I m ' ) . where P = allowable unit stress for alternate stresses, and 5 



\ 2 max. stress / 

 = allowable unit stress for live loads alone. 



26. Net Sections. Section 26 is standard. In addition the method of calculating the 

 net area of a riveted tension member is given in several specifications. 



Cooper requires that "The rupture of a riveted tension member is to be considered as equally 

 probable, either through a transverse line of rivet holes or through a zigzag line of rivet holes, where 

 the net section does not exceed by 30 per cent the net section along a transverse line." 



The Baltimore & Ohio R. R. requires that "The greatest number of rivet holes that can be 

 cut by a transverse plane, or come within one inch of the plane is to be deducted in calculating 

 the net section." 



The New York Central Lines require that "The net section of riveted members shall be the 

 least area which can be obtained by deducting from the gross sectional area the areas of holes cut 

 by any plane perpendicular to the axis of the member and parts of the areas of other holes on one 

 side of the plane, within a distance of 4 inches, and which are on other gage lines than those of the 

 holes cut by the plane, the parts being determined by the formula: A (i pi\), in which A = the 

 area of the hole, and p = the distance in inches of the center of the hole from the plane." 



The Canadian Society of Civil Engineers requires "There shall be deducted from each member 

 as many rivets as there are gage lines, unless the distance center to center of rivets measured in 

 the diagonal direction is 40 per cent greater than their distance center to center of gage lines." 



29. Plate Girders. Seven of the sixteen specifications require that plate girders be pro- 

 portioned either by the moment of inertia of their net section; or by assuming that the flanges 

 are concentrated at their centers of gravity; in which case one-eighth of the gross section of the 

 web, if properly spliced, may be used as flange section. Six specifications require that the bending 

 moment all be taken by the flanges. Two specifications require that the bending moment be 

 taken by the flanges and that one-eighth of the gross section of the web be taken as flange area. 

 One specification requires that plate girders with stiffeners be designed on the assumption that 

 the flanges take all the bending moment, and that for plate girders without stiffeners one-eighth of 

 the web may be considered as flange area. 



30. Compression Flanges. Two specifications require that the flange angles shall contain 

 at least one-half of the area of the flange. The specifications uniformly require that the com- 

 pression flange shall have the same gross area as the tension flange. 



36. Counters. Eight specifications require that counters be stiff members. Eight speci- 

 fications permit adjustable counters and laterals. 



45. Minimum Angles. Five specifications give 3!" X 3" X I" as the minimum angle. 

 Two specifications give 3" X 2|" X f " as the minimum angle. One specification requires that 

 the vertical leg be not less than 3!". One specification requires that connection angles for stringers 

 and floorbeams be not less than 4" X 4" X f"; one specification 3!" X 35" X f ", and one 

 specification 6" X 4" X f ". 



59. Expansion. Six specifications require that provision be made for an expansion of | in. 

 for each 10 ft. of span. Five specifications require that provision be made for a range in tempera- 

 ture of 150 degrees F. ; one for 180 degrees F. Three specifications require that provision be 

 made for an expansion of I in. in 100 ft.; one for an expansion of i in. in 70 ft. 



62. Rollers. Six specifications require that rollers be at least 6 in. in diameter. Five 

 specifications permit rollers 4 in. in diameter. One specification permits rollers 3 in. in diameter. 

 Cooper requires that rollers for spans up to 100 ft. be 4! in., and that the diameter be increased 

 i in. for each 10 ft. increase in span over 100 ft. The New York Central R. R. requires that rollers 

 shall not have a less diameter in inches than 3 + 0.03 (span in feet). 



68. Stringer Connection Angles. One specification requires that connection angles of 

 stringers and floorbeams be not less than 4" X 4" X f"; one specification 3?" X 3?" X ", 

 and one specification 6" X 4" X f ". 



77. Camber of Plate Girders. Four specifications require that plate girders more than 

 50 ft. long be cambered ^ in. per 10 ft. of length. Two specifications require full camber. Two 

 specifications require a camber of T^ the span. Two specifications require a camber of y^ the 

 span. One specification requires a camber of | in. per 10 ft. of length, one specification requires 

 a camber of j^ in. per 15 ft. of length. Four specifications do not require that plate girders be 

 cambered. 



