A deformation based approach to structural steel design

Current structural steel design codes, such as EN 1993-1-1, were developed on the basis of a bi-linear (elastic, perfectly-plastic) material model, which lends itself to the idea of cross-section classification. This step-wise design concept is a useful, but somewhat artificial simplification of the true behaviour of structural steel since the relationship between the resistance of a structural cross-section and its slenderness is, in reality, continuous. The aim of this study is therefore to develop a more efficient structural steel design method recognising this relationship and rationally exploiting strain-hardening, whilst maintaining, where possible, consistency with current design approaches. As part of the present study, laboratory tests were carried out on cold-formed and hot-rolled steel hollow sections. A total of 6 simple beams and 12 continuous beams (with two configurations) and corresponding material coupon tests were conducted. These experimental results were added to existing collected test data to develop and calibrate a new structural steel design method. The test results indicated that capacities beyond the yield load in compression and the plastic moment capacity in bending could be achieved due to strain-hardening. The new design approach, termed the continuous strength method (CSM), enables this extra capacity to be harnessed. The developed deformation based steel design method employs a continuous ‘base curve’ to provide a relationship between cross-section slenderness and deformation capacity in conjunction with a strain-hardening material model. The material model is elastic, linear-hardening and has been calibrated on the basis of collected stress-strain data from a range of structural sections. The CSM has been developed for both statically determinate and indeterminate structures utilising both experimental data and that generated through sophisticated numerical modelling. Comparisons between test results and predictions according to EN 1993-1-1 and the proposed method were made. The results revealed that the CSM provides a more accurate prediction of test response and enhanced structural capacity over current design methods.