Question

Design a cross section and reinforcement to supports Pu = 450 kips, Mux = 100 k-ft, and Muy = 130 k-ft. Use and fy = 60000 psi

f'c = 4000psi

ACI code

Answer 1

here is a solution of this question

lets consider that we have concrete section to take load of Pu= 450kips

Now maximum compressive stress concrete can carry = 0.67*4000psi = 2680psi

so Pu/ cross sectional area= 2680psi

so A(area)= 168 in2

Now assume breadth of section is 9 in

so depth of the section is 18 in

Now here Moment about X axis is 100 k-ft = 1200 k-in

now section modulus Z = db^2/6 = 18*81/6= 243

stress in the section = M/Z= 1200/243 = 4.94 kn/in2 = 1110.5 psi

here Mux is torsional moment

so as per ACI moment considered for longitudnal reinforcement = Mux[{1+(d/b)}/1.7]

                                                                                       Mt = 2117.64 k-in

now furthure calculation is given in image