112p 



STEEL HIGHWAY BRIDGES. 



CHAP. III. 



The distribution of the moving concentrated load to the joists as specified by different highway 

 commissions and others, and by the author have already been given. 



Steel Stringers. The sizes of steel I-beams of minimum weights required for stringers with 

 different spacings to carry a dead load of 100 Ib. per sq. ft. and a 2o-ton auto truck with 30 per cent 

 impact or a live load of 125 Ib. per sq. ft. with 30 per cent impact are given in Fig. 9; and to carry 

 a dead load of 100 Ib. per sq. ft. and a 1 5-ton auto truck with 30 per cent impact or a live load of 

 loo Ib. per sq. ft. with 30 per cent impact are given in Fig. 10. The sizes of steel I-beams of mini- 

 mum weights required to carry a dead load of 100 Ib. per sq. ft. and a 1 5-ton auto truck without 

 impact or a live load of 100 Ib. per sq. ft. without impact are given in Fig. 1 1. The steel stringers 

 used by the Wisconsin Highway Commission to carry a 15-ton road roller without impact, and the 

 steel stringers used by the Iowa Highway Commission to carry a 1 5-ton traction engine without 

 impact are practically the same as those given in Fig. u. 



Timber Joists. The sizes of timber stringers or joists for different spacings and spans to 

 carry a 2O-ton auto truck are given in Table VI ; to carry a 1 5-ton auto truck in Table VII, and to 

 carry a lO-ton auto truck in Table VIII. The timber joists were designed for the following unit 

 stresses, to be used without impact: Allowable bending stress, 1,500 Ib. per sq. in.; allowable 

 bearing across the grain, 400 Ib. per sq. in.; allowable longitudinal shear in beams, 140 Ib. per sq. in. 

 The maximum spacings of timber joists for short spans are determined by the longitudinal shear. 



TABLE VI. 



SPACING OF TIMBER STRINGERS OR JOISTS. 

 Calculated for 2O-ton Auto Truck, Without Impact. 



DESIGN OF FLOORBEAMS. The floor loads may be carried to the floorbeams by means 

 of stringers or joists, or the loads may be carried to the floorbeams directly by the floor slabs. 

 The loads carried by the floorbeams consist of (i) the dead load which is the weight of the floor 

 system; (2) a uniform live load; or a concentrated moving load. The uniform live loads are the 

 same as the uniform live loads used in designing the floor slabs an9 stringers, but the distribution 

 of the concentrated moving load is not the same as for either the floor slabs or the stringers. The 

 distribution of the moving concentrated load to floorbeams as specified by different highway com- 

 missions and others, and by the author have already been given. 



