Problem $13-$ A singly reinforced rectangular beam section $($Fig$.6)$ has $f_c^{\text{'}}=6,000$ psi and $f_y=60,000$ psi. The

design moment strength, $M_n,$ of the beam is most nearly:

$($A$)255$ kip$-$ft

$($B$)270$kip$-ft$

$($C$)285$kip$-ft$

$($D$)318$kip$-ft$

Fig. $6-$RC beam section for Problem $13$