Column design is typically based on the calculation of effective lengths factors for the columns through some form of “elastic” buckling analysis, whether it be at the member, storey, or system (frame) level. The elastic effective lengths so calculated are then used to determine slenderness for the column which, in conjunction with a column strength curve, facilitates the calculation of the column design strength. While this approach has been used for many years, it does comprise one inconsistency which is that an “elastic” column buckling curve is used to determine the slenderness of a member which is then employed in an “inelastic” column strength curve.

An alternative and more consistent approach to column design is the calculate the effective length factors using the same “inelastic” buckling strength curve that is used for column strength. In other words the elastic buckling curve is replaced by an inelastic buckling curve for effective length factor determination. This so-called “inelastic” buckling approach often leads to lower effective length factors for “critical” columns, and hence greater economy in design, compared to the traditional elastic buckling approach.

The results of this inelastic buckling approach can be employed to evaluate the true inelastic compression buckling behaviour. The inelastic buckling modeshape stresses can then be amplified and superimposed onto the original static stresses used for the buckling load case such that a simple stress check can evaluate compliance to the relevant design standard for combined biaxial bending and axial stresses accounting for inelastic compression behaviour.