CONCENTRATED LIVE LOADS. 112e 



Motor trucks have narrower tires and are driven at greater speeds than traction engines, and 

 thrn-fore not only produce greater static stresses in the floor, but should have a greater impact 

 allowance. In view of the above, it would not appear to be necessary to consider any road rollers 

 or traction engines now in use in addition to the above motor-truck loadings. 



DISTRIBUTION OF CONCENTRATED LOADS. In designing floor slabs, floor stringers 

 and floorbeams it is necessary to know the distribution of the concentrated loads. 



Concrete Floor Slabs. Tests of the distribution of concentrated loads on concrete floor slabs 

 have been made by the Ohio Highway Commission, the results of which are given in Bulletin No. 

 28, published by the Commission; by Mr. W. A. Slater at the University of Illinois and described 

 in Proceedings of American Society for Testing Materials, Vol. XIII, 1913, and by A. T. Goldbeck 

 and E. B. Smith, described in Journal of Agricultural Research, Vol. VI, No. 6, Department of 

 Agriculture, Washington, D. C., May 8, 1916. 



Ohio Tests. The following conclusions drawn from the Ohio tests are of interest: 



" The percentage of reinforcement has little or no effect upon the distribution to the joists, so 

 long as safe loads on the slabs are not exceeded. 



"The outside joists should be designed for the same total live load as the intermediate joists. 

 " The axle load of a truck may be considered as distributed over 12 ft. in width of roadway. 

 "The safe value for ' effective width 'of a slab, where the total width of slab is greater than 

 1.33 L + 4 ft. is given by the formula, e = O.6L + 1.7 ft., where e = effective width (width over 

 which a single concentrated load may be considered as uniformly distributed on a line down the 

 middle of the slab parallel to the supports) and L = span in feet. ' 



Slater Tests. It was recommended that where the total width of slab is greater than twice 

 the span, the effective width be taken as e = 42/3 + d, where x is the distance from the concen- 

 ited load to the nearest support, and d is the width at right angles to the support over which the 

 id is applied. While the depth of slab and the amount of longitudinal reinforcement had little 

 Feet on the distribution, it was recommended that the latter be limited to I percent. 



Goldbeck and Smith Tests. Tests were made on three slabs, each slab being 32 ft. wide, 16 ft. 

 in, and with effective depths of 10.5 in., 8.5 in. and 6 in., respectively. All slabs were made of 

 1-2-4 Portland cement concrete, and were reinforced with 0.75 per cent of mild steel. 

 The following conclusions were drawn from these tests: 



(1) The effective width decreases as the effective depth increases; the effective width for safe 

 ids being 75.7 percent; 81.1 percent, and 109.3 percent of the span, for the slabs having effective 

 jpths of 10.5 in., 8.5 in. and 6 in., respectively. 



(2) For slabs in which the ratio of the width of the slab is not less than twice the span length, 

 ic effective width may be taken as 



e = 0.7 L (34) 



icre e is the effective width and L is the span length. 



(Additional tests by Goldbeck, Proceedings American Concrete Institute, 1917, show that 

 lula (34) may be used when the width of the slab is not less than the span.) 

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

 itrated loads on reinforced concrete slabs may be assumed as distributed over a distance of 4 ft. 

 right angles to the supports, and a distance parallel to the supports equal to 2 ft. plus three- 

 iths of the span of the slab. 



The State Highway Department of Ohio uses the following distribution of concentrated loads 

 floor slabs. 



For spans less than 6 ft. the percentage, p, of the wheel load carried by one foot in width of 

 ib for a span in feet, /, is given by the formula 



p = 42 - 47 (35) 



vhile for spans greater than 6 ft. the percentage, p', of the wheel load carried by one foot in width 

 Df slab for a span in feet, /, is given by the formula 



p' = 20 - o.4/ (36) 



For a span of 5$ ft., from formula (35), p = 20 per cent, and the concentrated load is assumed 

 as carried by a slab 5 ft. wide, applied on a line parallel to the supports. 



For a span of 10 ft., from formula (36), p' = 16 per cent, and the concentrated load is assumed 

 as carried by a slab 6.67 ft. wide, applied on a line parallel to the supports. 



! 



