There is a RC rectangular beam section. Determine the maximum design bending moment $(M_u)$ and reduction factor $().(15$ Points$)$

$f^{\text{'}}\text{'}c=280kg\frac{f}{c}{m}^2$

$f_y=4200k\frac{g}{c}{m}^2$

$b=30cm$

$h=60cm$

$d=53\mathrm{.}5cm$

$As=30\mathrm{.}21c{m}^2$

$a=x=0\mathrm{.}85x$