Question

Determine the value of PM, and verify code compliance for the beam shown below. b = 18 inch h = 32 inch f'c = 5,500 psi fy = 60,000 psi 6#8

Answer 1

Given data:- f'c = 5.5 ksi , fy =60 ksi, b= 18 in , D =32 in, d= 32-1.5-0.5-0.5 = 29.5 in As (6 #8 )= 4.74 sqin 0.85 x f'c x Ac = As x fy 0.85 x 5.5 x Ac = 4.74 x 60 Ac = 60.83 a= 60.83 sqin / 18 = 3.38 in z = d-a/2 = 29.5- (3.38/2) = 27.81 in Checking minimum reinforcing As min = 3 x sqrt (fc) xbxd/fy = 3 xsqrt ( 5500 ) x 18 x 29.5/60000 =1.96 sqin but not less than As min = 200 xbxd / fy = 200 x 18 x 29.5 /600000 1.77 sqin OK Computing c, Et and phi c=a/ beta1 = 3.38 /.85 = 3.98 in Et= (d-c)x 0.003/c (29.5-3.98 ) x 0.003 / 3.98 0.019> 0.005 so phi = 0.9 ( as assumed ) phi Mn = phi x As x fy x (d-al 2) = 0.9 x 4.74 x 60 x ( 29.5-3.38/2) = 7118.25 in K 593.19 ft - K If we take bigger section, then also our criteria will be fulfilled but we should recheck the section with min As value and max As value. the next section can be 19 inch x 33 inch.