When $X_u=X_{umax}$ the section is a balanced one and at the time of limit state of collapse, strain in the concrete is 0.0035 and strain is steal is 0.87 $\frac{f_y}{E_s}+0.002$

The moment of resistance will be maximum on limiting.

W.K.T

$M_{umax}=C_1{\times}L_1 \\ \ \ \ \ \ \ \ \ \ \ \ =(0.36f_kX_{umax})(d-0.416X_{umax})$

Rearranging, [Take d common 2 x 2 by d]

$M_{umax}=0.36\frac{X_{umax}}{d}\Big[1-\frac{0.416X_{umax}}{d}\Big]ld^2f_ck$

Case 1 :-

For $F_{e250},f_y=250 NM/mm^2, X_{umax}=0.531 \\ M_{umax}=0.36{\times}0.531{\times}(1-0.416{\times}0.531)ld^2f_ck \\ M_{umax}=0.149f_ckld^2...................(1)$

Case 2 :-

For $F_{e415}, f_y=415W/mm^2,X_{umax}=0.48d \\ M_{umax}=0.36{\times}0.48{\times}[1-0.416{\times}0.48]f_ckld^2 \\ M_{umax}=0.138f_ckld^2...................(2)$

Case 3 :-

For $F_{e500},f_y=500N/mm^2,X_{umax}=0.46d \\ M_{umax}=0.36{\times}0.46{\times}(1-0.416{\times}0.16)f_ckld^2 \\ M_{umax}=0.133f_ckld^2..................(3)$