$2\mathrm{.}4-8$ The fixed$-$end bar ABCD consists of three prismatic segments, as shown in the figure. The end segments have cross$-$sectional area $A_3=840m{m}^2$ and length $L_1=200mm.$ The middle segment has cross$-$sectional area $A_2=1260m{m}^2$ and length $L_2=250mm.$ Loads $P_s$ and $P_c$ are equal to $25\mathrm{.}5kN$ and $17\mathrm{.}0kN,$ reupectively.

$($a$)$ Determine the reactions $R_A$ and $R_B$ at the fixed supports.

$($b$)$ Determine the compressive axial force $F_{sc}$ in the middle segment of the bar