The allowable compressive stress on the gross section of axially loaded compression members is calculated based on the formula 3.2.2-1 in the API Code when the largest effective slenderness ratio, Kl/r is less than or equal to C_c. If Kl/r exceeds C_c, then the allowable compressive stress is increased as per formula (3.2.2-2) of the Code.

Where:

For D/t > 60, the lesser of F_xe or F_xc is substituted for F_xy.

Where:

• F_xe = the elastic local buckling stress calculated with C, the critical elastic buckling coefficient = 0.3 (3.2.2-3)
• F_xc = the inelastic local buckling stress. (3.2.2-4)