SLABS, CROSS-BEAMS, AND GIRDERS 141 



article. Assumptions of this load being either uniformly dis- 

 tributed, or varying as the ordinates of a parabola, give a lower 

 resulting moment than the triangle method and for this reason 

 are not so much in use. 



Let w be the uniform load per unit of area on the slab, and w 2 

 and w l the parts of this unit load that go to the shorter and longer 

 beams respectively. Applying the loads in the form of a triangle 

 having its apex at the middle of the beam, the maximum moment 

 will be for the longer beam, and, if this beam is considered simply 

 supported and as carrying the load from one panel only, 



and for the shorter beam M=~- w 2 b z l 



w 



If the slab is square, w t is and M= 1/24 wl 3 . For beams made 



J!i 



continuous, the bending moment may be multiplied by the 

 proper coefficient as already described. (Art. 54.) With beams 

 or girders common to two panels, the bending moment should be 

 multiplied by 2. 



Unless the panel is nearly square, floor slabs should not be 

 reinforced in two directions, as it is evident that no economy 

 results from double reinforcement when the ratio of length to 

 breadth of panel is greater than about 1.2. If the length of the 

 slab exceeds 1.5 times its breadth, the entire load should surely 

 be carried by the transverse reinforcement. 



57. Distribution of Beam and Slab Loads to Girders. Fig. 66 

 shows cross-beams running into girders, with the one-way type 

 of reinforcement. The load upon the girders in such construc- 

 tion consists of the concentrated live and dead loads from the 

 cross-beams acting at their points of intersection with the girder, 

 the uniformly distributed weight of the girder itself, and the 

 unsymmetrically distributed weight of a small portion of the 

 floor slab with its live load which bears directly upon the girder. 

 (Fig. 72.) In order to avoid computing several moments, it has 

 been found 1 that the maximum bending moment on a girder sup- 

 porting cross-beams may be obtained without appreciable error 

 by considering, as a uniformly distributed load, the weight of the 

 stems of the girder and cross-beams plus the weight of the slab 



i Taylor and Thompson's "Concrete, Plain and Reinforced," 2nd edition, page 432. 



