292 CONCRETE DESIGN 



First assume values for F s , F c , and n. These values are 

 usually controlled by building ordinances. The following 

 values are recommended as safe working values by the Joint 

 Committee and are used throughout the text to serve as 

 examples in working out problems used as illustrations: 

 F s = 16,000; F c = 650; = 15for concrete capable of devel- 

 oping an average compressive stress of 2,000 pounds in 

 28 days when tested in cylinders of specified shape. 



After values of F Sj F c , and n are decided on, substitute 

 them in the following formula and solve for p. This formula 

 gives the value of p that makes F s and F c reach their full 

 values under the same load. 



1 

 *> = *X^r 



F c 

 Substituting the values mentioned above 



1 



f X 16,000 / 16,000 N 

 650 \15X650 / 

 The value of k is now found by the formula 



k= \2pn -\- (pn) 2 pn 

 Substituting the values for p and n gives 



k = V2 X .00769 X 15 + (.00769 X 15)2 _ .Q0769 X 15 = .379 

 From this value, j is found by the following formula: 



Substituting the value of k just found, 



/ = 1 i X .379 == .874, or approximately J. 



For any value of F s , F c , and n employed, the preceding 

 formulas must be solved to obtain p, k, and / before the 

 problem proper can be attacked. For example, the values 

 of k and j may be taken as $ and , respectively, because 

 these values are close to the values found from the values 

 of F s , F c , and , assumed. 



The resisting moment M may then be found by transposing 

 in either of the following formulas: 



MM 2M 



or F c = - 



