The legs of storage racks are usually made of perforated steel columns. Shall I use minimum cross sectional area and section modulus when calculating it's capacity?
Sounds reasonable to me - its probably controlled by buckling. is it thin walled? AISI or AISC would give you the design equations possibly. Unless they have a design standard of their own?
If you're referring to products such as Unistrut or Kindorf their product catalogs contain design values, including those for their perforated sections.