Provision for 41. Where a bridge is designed for a future heavy deck, but a lighter deck is to be 



Heavy Decks used temporarily, that dead load stress shall be used, which in combination with the live 

 load stress will require the maximum section. 



Axial and 42. Members subject to both axial and bending stresses shall be proportioned so 



Bending Stresses that the combined fibre stress will not exceed the allowed axial Unit Stress; except such 



Combined members as may be subject to bendin-g moments from their own weight, when the total 



fibre stress shall not exceed the allowed axial Unit Stress by more than 10 per cent.; but 



in no case shall the section be less than that required for the axial stresses alone. 



43. For members continuous over panel points and subject to transverse loading, 

 the bending moments, both at the panel points and at the centre, shall be taken as three- 

 fourths of those computed as for a simple beam of span equal to one panel length. 



44. Designing and detailing shall be carried out so as to minimize secondary stresses. 



Secondary I n ordinary trusses without sub-panelling, no account usually need be taken of the 



Stresses secondary stresses in any member whose width measured in the plane of the truss is less 



than one-tenth of its length. Where this ratio is exceeded, or where sub-panelling is 

 used, secondary stresses due to deflection of the truss shall be computed. The allowable 

 Unit Stresses specified may be increased one-third for a combination of the secondary 

 stresses with the axial stresses. 



UNIT STRESSES 



45. The several parts of the structure shall be so proportioned that the Unit Stresses 

 will not exceed the following, except as modified elsewhere in this Specification. Unless 

 otherwise noted, the Unit Stresses herewith are for "Structural Steel for Bridges", and 

 are given in pounds per square inch. 



Tension 46. Axial tension on net section 16 000 



Compression 47. Axial compression on gross section of columns 15 000 50(_ i_) 



but not greater than 12 500 



in which 



/ = the unsupported length, centre to centre of intersections. 

 r = the least radius of gyration applicable to this length. 



Direct compression on steel castings 14 000 



Direct compression on iron castings 10 000 



Bending 48. Bending on extreme fibres of: 



Rolled shapes, built-up sections and girders (net section) 16 000 



Pins 24 000 



Steel castings 12 000 



Iron castings 3 000 



White oak, Douglas fir and Southern long-leaf pine 1 800 



White and red pine, and Spruce 1 200 



Shearing 49. Shearing on: 



Pins and power driven shop rivets 12 000 



Power driven field rivets 10 000 



Hand driven field rivets and turned bolts 8 000 



Plate girder webs (gross section) 10 000 



Bearing 50. Bearing on: 



Pins, power driven shop rivets, outstanding legs of stiffener angles, 



and other steel parts in contact 24 000 



Power driven field rivets 20 000 



Hand driven field rivets and turned bolts 16 000 



Hard bronze sliding expansion bearings 1 000 



Expansion rollers, per lineal inch * 60Qd 



in which d = diameter of roller in inches. 



White oak Perpendicular to grain 1 100 



Across grain 500 



