Many of the principal concepts that underpin current metallic structural design codes were developed on the basis of bilinear (elastic, perfectly-plastic) material behaviour; such material behaviour lends itself to the concept of section classification. Resistance based on the assignment of cross-sections into this discrete classification system is a useful, but artificial, simplification.

The resistance of structural cross-sections is, in reality, a continuous function of the slenderness of the constituent plate elements. Although not explicitly included in the determination of resistance, strain hardening is an essential component of the section classification system, and is required, for example, to enable the attainment of the plastic moment at finite strains.

The Continuous Strength Method (CSM) represents an alternative treatment to cross-section classification, which is based on a continuous relationship between slenderness and (inelastic) local buckling and a rational exploitation of strain hardening. .

Materials that exhibit a high degree of non-linearity and strain hardening, such as aluminium, stainless steel and some high-strength steels, fit less appropriately into the framework of cross-section classification, and generally benefit to a greater extent from the Continuous Strength Method. The method provides better agreement with test results in comparison to existing design codes, and offers increases in member resistance and a reduction in scatter of the prediction.

An additional benefit of the proposed approach is that cross-section deformation capacity is explicitly determined in the calculations, thus enabling a more sophisticated and informed assessment of ductility supply and demand. Further developments to the method are underway.

Further details can be found in the following paper:

Gardner, L., Yun, X. and Walport, F., 2023. The Continuous Strength Method - Review and outlook. Engineering Structures, 275, 114924.