142 STEEL HIGHWAY BRIDGES. CHAP. III. 



48. Stress Due to Weight and Eccentric Loading. If the fiber stress due to weight and 

 eccentric loading on any member exceeds 10 per cent of the allowable unit stress on the member 

 such excess must be considered in proportioning the member. See 46. 



49. Counters. Counters in bridges carrying electric cars shall be designed so that an increase 

 of the live load of 25 per cent will not increase the stress in the counters more than 25 per cent. 



50. Design of Plate Girders. Plate girders shall be proportioned 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. The thickness of web plates shall be not less than & in., nor less than 1/160 of 

 the unsupported distance between flange angles. 



Compression Flanges. In beams and plate girders the compression flanges shall have the 

 same gross section as the tension flanges. Through plate girders shall have their top flanges 

 stayed at each end of every floorbeam, or in case of solid floors, at distances not exceeding 12 ft., by 

 knee braces or gusset plates. The stress per sq. in. in compression flange of any beam or girder 

 shall not exceed 16,000 2oo-l/b, when flange consists of angles only or if cover consists of flat 

 plates, or 16,000 150 l/b if cover consists of a channel section, where / = unsupported distance 

 and b = width of flange. 



51. Web Stiffeners. There shall be web stiff eners, generally in pairs, over bearings, at points 

 of concentrated loading, and at other points where the thickness of the web is less than ^ of the 

 unsupported distance between flange angles. The distance between Stiffeners shall not exceed 

 that given by the following formula, with a maximum limit of six feet (and not greater than the 

 clear depth of the web): d = t (12,000 5)740. 



Where d = clear distance, between Stiffeners of flange angles ; t = thickness of web ; 5 = shear 

 per sq. in. 



The Stiffeners at ends and at points of concentrated loads shall be proportioned by the formula 

 of paragraph 38, the effective length being assumed as one-half the depth of girders. End Stiffeners 

 and those under concentrated loads shall be on fillers and have their outstanding legs as wide as 

 the flange angles will allow and shall fit tightly against them. Intermediate Stiffeners may be 

 offset or on fillers, and their outstanding legs shall be not less than one-thirtieth of the depth of 

 girder, plus 2 in. 



52. Flange Rivets. The flanges of plate girders shall be connected to the web with a sufficient 

 number of rivets to transfer the total shear at any point in a distance equal to the effective depth 

 of the girder at that point combined with any load that is applied directly on the flange. The 

 wheel loads, where the ties rest on the flanges, shall be assumed to be distributed over three ties. 



53. Depth Ratios. Trusses shall preferably have a depth of not less than one-tenth of the 

 span. Plate girders and rolled beams, used as girders, shall preferably have a depth of not less 

 than one-twelfth of the span. If shallower trusses, girders or beams are used, the section shall be 

 increased so that the maximum deflection will not be greater than if the above limiting ratios had 

 not been exceeded. For steel joists and track stringers, see 19. 



54. Low Trusses. Riveted low trusses shall have top chords composed of a double web mem- 

 ber with cover plate. The top chords shall be stayed against lateral bending by means of brackets 

 or knee braces rigidly connected to the floorbeam at intervals not greater than twelve times the 

 width of the cover plate. The posts shall be solid web members. The floorbeams shall be riveted, 

 preferably above the lower chord. Pin-connected low truss bridges shall not be used. 



55. Rolled Beams. Rolled beams shall be designed by using their moments of inertia. The 

 webs of rolled beams and plate girders shall be assumed to take all the shear. 



PART IV. DETAILS OF DESIGN. 

 GENERAL REQUIREMENTS. 



56. Open Sections. Structures shall be so designed that all parts will be accessible for in- 

 spection, cleaning and painting. 



57. Water Pockets. Pockets or depressions which would hold water shall have drain holes, 

 or be filled with waterproof material. 



58. Symmetrical Sections. Main members shall be so designed that the neutral axis will be 

 as nearly as practicable in the center of section, and the neutral axes of intersecting main members 

 of trusses shall meet at a common point. 



59. Counters. Rigid counters are preferred; and where subject to reversal of stress shall 

 preferably have riveted connections to the chords. Adjustable counters shall have open turn- 

 buckles. 



60. Strength of Connections. The strength of connections shall be sufficient to develop the 

 full strength of the member, even though the computed stress is less, the kind of stress to which 

 the member is subjected being considered. 



61. Minimum Thickness. The minimum thickness of rnetal shall be ^ in. in classes A, B, 

 C, Ei, E 2 and E 3 , except for fillers; and j in. in classes DI and D 2 , except for fillers and webs of chan- 

 nels. Webs of channels for classes DI and D 2 may have a minimum thickness of 0.20 in. The 

 minimum angle shall be 2 in. x 2 in. x j in. The minimum rod shall have an area of at least 

 I sq. in., in all classes except DI and D 2 , which shall have no rods less than f in. in diameter. Webs 

 of plate girders shall not be less than ^ in. 



62. Pitch of Rivets. The minimum distance between centers of rivet holes shall be three 

 diameters of the rivet; but the distance shall preferably be. not less than 3 in. for |-in. rivets, 



