8 BULLETIN 789, U. S. DEPARTMENT OF AGRICULTURE. 



For any effective depth to steel the maximum resisting moment, 

 which is developed when the steel and concrete are simultaneously 

 stressed to the assumed allowable values for /g and /c, is given by 

 the equation: 



For selected values of /s, /c, and n and for h = 12, the product 

 fsjpi is a constant and 



M= Cd? 



The area of steel required is given by the equation 



a = 'pbd 

 The area of steel may also be figured from the equation 



M 



LxPxU 



mXfsXJXd 



For selected values of fs, fo, and n and for different values of m 

 and d the product m x/s xjxd is a constant which may be designated 



12 

 hjR. Then 



LxV 



L = 



B 



axR 



P 



The thickness of the wall is determined by the bending moment at 

 the bottom, and, as this thickness is maintained for the full height, 

 it will be greater in the upper portion of the wall than required to 

 develop the assumed stresses. The stress in the concrete will de- 

 crease from bottom to top and, as the thickness of the wall is usually 

 selected in even inches, the stress at the bottom will generally be less 

 than the assumed allowable value. In some cases, however, a wall 

 thickness may be chosen slightly less than actually required for the 

 moment at the bottom, in which case the stress in the concrete at that 

 level will exceed the assumed value. 



With each change from the bottom to the top in the spacing or 

 size of the reinforcing bars, there is a decrease in the value for the 

 percentage of steel, and, assuming n to be constant, a corresponding 

 increase in the value for j. Therefore, the results obtained by use 



of the formula X = — p—i forwhichi? has been computed for a constant 



value of j, are not absolutely correct. In any case, however, the 

 error is small and, moreover, is on the safe side, because, if the 

 increased value for j was used, slightly larger values for L, the allow- 

 able pressure for a given steel area, would be obtained. 



