SPECIFICATIONS. 141 



PART III. UNIT STRESSES AND PROPORTION OF PARTS. 



^6. Unit Stresses. All parts of the structure shall be proportioned so that the sum of the 

 maximum stresses shall not exceed the following amounts in Ib. per sq. in., except as modified by 

 45 and 48. 



Impact. The dynamic increment of the live load stress shall be added to the maximum live 

 lo.ul Musses as follows: 



For the floor and its supports including floor slabs, floor joist, floorbeams and hangers, 30 

 per cent. 



For all truss members other than the floor and its supports, the impact increment shall be 

 / = loo/(L + 300), where L = length of span for simple highway spans (for trestle bents, towers, 

 movable bridges, arch and cantilever bridges, and for bridges carrying electric trains, L shall be 

 taken as the loaded length of the bridge in feet producing maximum stress in the member). 



Impact shall not be added to the stresses produced by longitudinal, centrifugal and lateral or 

 wind forces. 



37. Tension. Axial tension on net section 16,000 



The lengths of riveted tension members in horizontal or inclined positions shall not exceed 



200 times their radius of gyration about the horizontal axis. The horizontal projection of the 

 unsupported portion of the member is to be considered as the effective length. 



38. Compression. Axial compression on gross section 16,000 7O'//r 



with a maximum of 14,000 Ib.; where "/" is the length of member in inches and "r" is the least 

 radius of gyration in inches. 



No compression member, however, shall have a length exceeding 100 times its least radius of 

 gyration for main members or 120 times for laterals for classes A, B, C, Ei, Ej, and E s ; or 125 times 

 its least radius of gyration for main members or 150 times for laterals for classes DI and D z . 



39. Bending. Bending: on extreme fibers of rolled shapes, built sections and girders; 



net section 16,000 



on extreme fibers of pins 24,000 



40. Shearing. Shearing: shop driven rivets and pins 12,000 



field driven rivets and turned bolts 10,000 



plate girder webs; gross section 10,000 



41. Bearing. Bearing: shop driven rivets and pins .". 24,000 



field driven rivets and turned bolts 20,000 



granite masonry and Portland cement concrete 600 



sandstone and limestone 400 



expansion rollers; per linear inch 6ood 



where "d" is the diameter of the roller in inches. 



Rivets shall not be used in direct tension, except for lateral bracing where unavoidable; in 

 irhich case the value for direct tension on the rivet shall be taken the same as for single shear. 



. 42. Alternate Stresses. Members subject to alternate stresses of tension and compression 

 lall be proportioned for the stresses giving the largest section. If the alternate stresses occur 

 succession during the passage of one train, as in stiff counters, each stress shall be increased by 

 per cent of the smaller. The connections shall in all cases be proportioned for the sum of the 

 resses. 



43. Angles in Tension. When single-angle members subject to direct tension are fastened by 

 leg, only seventy-five per cent of the net area shall be considered effective. Angles with lug 



igle connections shall not be considered as fastened by both legs. 



44. Net Section. In members subject to tensile stresses full allowance shall be^made for 

 uction of section by rivet-holes, screw-threads, etc. In calculating net area the rivet-holes 



aall be taken as having a diameter | in. greater than the normal size of rivet. 



45. Long Span Bridges. For long span bridges, where the ratio of the length to width of 

 in is such that it makes the top chords acting as a whole, a longer.column than the segments of 



ic chords, the chord shall be proportioned for the greater length. 



46. Wind Stresses. The stresses in truss members or trestle posts from assumed wind forces 

 not be considered except as follows: 



1. When the direct wind stresses per square inch in any member exceed 25 per cent of the 

 stresses due to dead and live loads in the same member. The section shall then be increased 

 until the total unit stress shall not exceed by more than 25 per cent the maximum allowable 

 stress for dead and live loads. 



2. When the wind stress alone or in combination with a possible temperature stress can 

 neutralize or reverse the stresses in the member. 



When both direct and flexural stresses due to wind are considered 50 per cent may be added 

 to allowable stresses for dead and live loads, provided the area thus obtained is not less than re- 

 quired for dead and live loads alone, or for dead, live and direct wind loads designed as in 46. 



47. Combined Stresses. Members subjected to direct and bending stresses shall be designed 

 so that the greatest fiber stress shall not exceed the allowable unit stress on the member. 





