Problem 2: For the hollow reinforced concrete beam section shown below, determine the design strength Mn permitted by ACI. For your calculations assume a concrete compressive strength f = 3,000psi and flexural reinforcement yield strength fy = 60,000psi. d=30" bw=4.5" h₁ =4.5" 7 - #9 be=32" Figure 2: Problem 2 - Hollow rectangular beam with tension reinforcement.