EXAMPLE $4\mathrm{.}4$ Using the equivalent rectangular stress distribution, directly calculate the nominal strength of the beam previously analyzed in Example $4\mathrm{.}3.$ Recall that $b=10in.,d=23in.,A_s=2\mathrm{.}37i{n}^2.,f_c^{\text{'}}=4000,f_y=60,000,{}_y=0\mathrm{.}002,$ and ${}_1=0\mathrm{.}85.$

Calculate the design moment capacity $M_n$ for the beam analyzed earlier in Example $4\mathrm{.}4.$