112f 



STEEL HIGHWAY BRIDGES. 



CHAP. III. 



Floor Stringers and Floorbeams. The Illinois Highway Commission specifies that longi- 

 tudinal stringers be spaced not more than 2%-ft. centers, and that each stringer be designed for 20 

 per cent of the rear axle load concentrated at the center of the span when a concrete sub-floor is 

 used, and 25 per cent of the rear axle load when a plank floor is used. Transverse stringers or 

 floorbeams, spaced not more than 2|-ft. centers, shall be designed to carry 40 per cent of the rear 

 axle load distributed over the middle 10 ft. of the stringer. Floorbeams shall be designed for 

 maximum stresses due to concentrated load. 



The Iowa Highway Commission specifies that one-third of a wheel load be assumed as carried 

 by one joist, when a concrete floor slab is used, and that one-half of a wheel load be assumed as 

 carried by one joist, when a plank floor is used. 



The Massachusetts Railway Commission specifies that the wheel load on plank floors be dis- 

 tributed over a width in feet equal to the thickness of the floor in inches, with a maximum distri- 

 bution of 6 ft. With solid floors each wheel load is assumed as distributed over a width of 6 ft. 



Watson's "General Specifications for Concrete Bridges," third edition, 1916, specifies that 

 the part of the concentrated load carried by one stringer shall be found by dividing the stringer 

 spacing by the gage distance of the concentrated load. With a gage distance of 6 ft. this gives 

 one-third the total load for a stringer spacing of 2 ft. ; one-half the total load for a stringer spac- 

 ing of 3 ft. ; the total load for a stringer spacing of 6 ft. 



Ketchum's Specifications for Distribution of Concentrated Loads. From a study of the 

 various tests and specifications, the author has adopted the following rules for calculating the 

 stresses in slabs, stringers and floorbeams: 



_i 



FiG.6. 



FIG. 7. 



(a) The distribution of concentrated wheel loads for bending moments in reinforced concrete 

 slabs with longitudinal girders shall be calculated by the formula, 



e = f (/ + c) 



(37) 



with a maximum limit of 6 ft. for e, where e = effective width (distance that the load may be con- 

 sidered as uniformly distributed on a line down the middle of the slab parallel to the supports), 

 / = span, and c = width of tire of wheel, all distances in feet. See Fig. 6. 



(b) The distribution of concentrated wheel loads for bending moments in reinforced concrete 

 slabs with transverse girders shall be calculated by the formula 



e = 2//3 + c 



(38) 



with a maximum limit of 6 ft. for e, where e = effective width, / = span, and c = width of tire of 

 wheel as defined in paragraph (a). See Fig. 7. 



(c) The distribution of concentrated wheel loads for bending moments in slabs of girder 

 bridges in which the span of the bridge is not less than the width of bridge center to center of 

 girders, shall be calculated for spans of 9 ft. or over by the formula 



e = 2// 3 



(39) 



with a maximum limit of e = 12 ft., where e = effective width, and / = span as defined in para- 

 graph (a). 



