Design of singly and doubly reinforced rectangular beams

Example $8\mathrm{.}2$ Bending reinforcement for a singly reinforced beam $($EC $2)$

Determine the area of main steel, $A_s,$ required for the beam assuming the following material strengths: $f_k=25Nm{m}^{-2}$ and $f_k=500Nm{m}^{-2}$