138 REINFORCED CONCRETE CONSTRUCTION 



the concrete cannot properly be placed between them. The 

 reinforced concrete regulations of New York City adopted Dec. 

 28, 19 11, require that the reinforcement shall not be spaced farther 

 apart than two and one-half times the thickness of the slab. 

 The minimum limit should be about the same as in beams. 



When a floor panel is square, or nearly so, the slab may 

 advantageously be reinforced in both directions. Exact analy- 

 sis of stresses in such a case is impossible, but some important 

 facts have been brought out by approximate solutions for uniform 

 loading. The theory Applied in such an analysis depends upon 

 the fact that the load at any point on the slab is distributed to 

 the two systems of reinforcing bars at that point, in proportion to 

 the stiffness of the beam elements lying in those directions. The 

 A b d B 



FIG. 70. 



distribution of load on each system of reinforcement of a square 

 slab, either along aa' or bb' , Fig. 70, has been found to approxi- 

 mate the curve of a parabola aOa' '. The center bending moment 

 along aa' or bb' will be found by this assumption to have a slightly 

 greater value than a slab reinforced in only one direction and 

 carrying one-half the uniform load. Also the distribution of 

 load along cc' (or dd') is shown by the curve aGa' , and when AB 

 (or BD) is reached by an element parallel to aa' (or bb') , the 

 load supported by the reinforcement becomes zero. 



The assumption usually made is that one-half the load is car- 

 ried uniformly by each system of reinforcement, and the rods 

 consequently for such an assumption have an equal spacing 

 throughout the slab. The center bending moment resulting 

 from an analysis, as above described, is so near the center bending 

 moment for uniform distribution of load to each system, that 



1 From Turneaure and Maurer's "Principles of Reinforced Concrete Construction," 2nd 

 edition, page 309. 



