170 RESISTANCE OF MATERIALS 



Wl 



amount, or . Assuming that each of the four sets of slab rods 

 ^4 



carries one fourth the total load, the bending moment from which 

 to determine the thickness of the slab and the amount of reenf orce- 

 ment in the head becomes 



5* 



M 4 Wl 



= == ' 



where W denotes the total load on the panel, and I is the distance 

 center to center of columns. The formula determined by experi- 



Wl 7 



ment and used in practice is M = For a top of diameter D = Z, 



50 ID 



the moment per foot of width, say M ., is therefore M l = -* /, 



oO lo 



Wl 7 . W ,. ., 

 =" Z=ft - lb - 



102. Thickness of slab. Let 



p s = unit working tensile stress in reenf orcement, 

 p c = unit working compressive stress in concrete, 



J7I 



n = = ratio of elastic moduli, 

 M l = bending moment in foot-pounds per foot of width. 



Then the thickness of slab h from the outer fiber in compres- 

 sion to the center of the reenf orcement is given by the formula 

 (article 95) 



6 M 1 



For working values of p s = 16,000 lb./in. 2 , p c = 600 lb./hi' 2 > and 



n = 15 this becomes . 



h = 0.1026 VHF^ 



W 

 Since M l = , as explained above, this may also be written 



22 



h = 0.02187 VPF, 



where h is expressed in inches and W denotes the total load on the 

 panel in pounds. 



