The strength of a column under truly concentric axial load is

P_n,o = 0.85 * f'_c * (A_g - A_st) + f_y * A_st

For nonprestressed compression members with tie reinforcement,

φ * P_n,max = 0.80 * φ * [0.85 * f’_c * (A_g - A_st) + f_y * A_st]