[ P_u = 0.80 \phi [0.85 f' c (A_g - A {st}) + f_y A_{st}] ] [ 850 \times 10^3 = 0.80 \times 0.65 \left[0.85 \times 21 (A_g - 0.015A_g) + 420 \times 0.015 A_g \right] ]

[ h = \sqrt{A_g} = \sqrt{68492} \approx 262 , \text{mm} ] Use 300 mm × 300 mm (common practical size).

[ \frac{1}{P_{n,bi}} = \frac{1}{P_{nx}} + \frac{1}{P_{ny}} - \frac{1}{P_{n0}} ] [ \frac{1}{P_{n,bi}} = \frac{1}{2200} + \frac{1}{2300} - \frac{1}{6886} ] [ = 0.0004545 + 0.0004348 - 0.0001452 = 0.0007441 ] [ P_{n,bi} = 1344 , \text{kN} ]