Question 12.P.9: Repeat P.12.6 using ultimate load theory. Assume σcu = 25.5 ...

From S.12.6, M_{\max } = 189 kN m. Assuming the neutral axis to be at the base of the flange

189 \times 10^{6}=0.87 \times 280 A_{\mathrm{s}}\left(550-\frac{125}{2}\right)

from which

A_{s} = 1592 mm²

Then, since the compressive force in the concrete is equal to the tensile force in the steel

0.4 × 25.5 × 125b=0.87 × 280 × 1592

which gives

b = 304.2 mm

Say a flange width of 304 mm.