$($a$)$ Explain material compatibility between steel and concrete for composite columns.

$(5$ Marks$)$

$($b$)$ Figure Q$4($a$)$ shows the cross$-$section profile of a concrete$-$encased I$-$section column under combined compression and bending moment about the major principal axis $($ y $-$axis$).$ Assuming the neutral axis h$\_($n$)$ is within the flange of the

l$-$section, derive the expression for h$\_($n$).$

$(8$ Marks$)$