SLABS, CROSS-BEAMS, AND GIRDERS 137 



When placing slab reinforcement in long spans, a top reinforce- 

 ment at least to the third point will be desirable. In ordinary 

 spans the steel should at least be lapped a sufficient distance over 

 supports to provide adequate bond strength. The use of a 

 webbing for reinforcing the slab is to be preferred, since the 

 spacing of the bars is not likely to be deranged, as occurs when 

 separate rods are used. 





a 





FIG. 68. 



A slab should be figured in the same manner as a rectangular 

 beam, and the depth and percentage of steel are therefore 

 obtained by the formulas given in Art. 33 the bending 

 moment being figured for a width of slab equal to one foot. For 

 short uniformly loaded spans, fully continuous over two or more 

 intermediate supports, a moment of 1/12 wl 2 may be used both 

 in the centers of all spans and also over all supports, for both 





Fig. 69. 



dead and live loads. For slabs of long span, and for all slabs 

 over two bays only, the moments should be taken as for continu- 

 ous beams. (Art. 54.) The ratio of steel in a slab is most 

 readily found by dividing the cross-section of one bar by the 

 area between two bars, this area being the spacing of the bars 

 multiplied by the depth of steel below the top of slab. 



Slab bars should not be spaced too far apart to properly 

 take stress directly nor yet should they be spaced so close that 



