$($a$)$ What is a composite structure? State three reasons why concrete structures are normally reinforced with steel rods.

$($b$)$ A rectangular singly reinforced concrete beam is $120mm$ wide and has an overall depth of $240mm.$ It is reinforced in the bottom face by two $15mm$ diameter steel bars located at a depth of $210mm.$ Calculate the maximum stress in both the concrete and steel if the beam is subjected to a moment of $25kNm.$ Take $\frac{E_{\sqrt[\phantom{2}]{?}}}{E_5}=10.$