Following column design philosophy is used in STAAD.Pro:
Ast_min= Minimum area of concrete required should be Pu/0.446fck
Pst_min = 0.8 % on Ag_min
Following design steps are followed.
1. Calculate initial % of steel based on gross area. i.e. Pst_gross = Ast_min /Ag, where Ag = Gross area of concrete
2. With given Pst_gross and axial load Pu, the moment capacity of column sections (Muz1 and Muy1) are calculated about both major and minor axis. You can get this using SP-16 also for hand calculation.
3. Calculate interaction ratio (IR)
4. If IR is not satisfied increase Pst_gross. Repeat steps 2 and 3.
5. If IR is satisfied select bar diameter, arrange bars and calculate Pst_gross_provided.
6.Perform following checks :
a) If % of steel based on final Pst_gross_provided >= 0.8 % (i.e. net concrete area is same as gross area), area of concrete required Ac_reqd = Ag – Ast_gross, where Ast_gross = (Pst_gross /100) *Ag
b) If % of steel based on final Pst_gross_provided < 0.8 % (i.e. net concrete area is less than gross area), area of concrete required Ac_reqd = Ast_gross *100 / 0.8 (i.e. 0.8 % of Ac_reqd = Ast_gross)