Calculation of the strain hardening behaviour of steel structures based on mill tests

The influence of strain hardening on elastic-plastic frame instability has become more critical since the introduction of modern design codes (e.g. Eurocode 3: part 1.1), which require a much more rigorous treatment of frame instability than was the case with earlier codes. At the same time, it has become apparent that portal frames are much more sensitive to second-order effects than was previously believed. Both the simplicity and economy of plastic design are endangered and there is renewed interest in defining the extent to which advantage can be taken of strain hardening with modern steel qualities and section shapes. This paper provides an up to date assessment of the strain hardening factor that may be incorporated into computer programs for second-order elastic-plastic analysis. A survey of tensile tests (mill tests) is presented which shows that the strain hardening behaviour is independent of material thickness and steel grade. This allows a generalisation of the stress-strain relationship for hot rolled steel to be defined. A modelling technique is presented that allows the non-linear moment-deformation behaviour of beams to be accurately predicted using mill test data. Finally, results from the survey of mill tests are combined with the moment-deformation model to estimate values of the strain hardening factor to be used during elastic-plastic design.