Structural behaviour of stainless steel elements subjected to combined loading

Abstract

Stainless steel has been gaining increasing use in a variety of engineering applications due to its unique combination of mechanical properties, durability and aesthetics. Significant progress in the development of structural design guidance has been made in recent years. However, an area that has remained relatively unexplored is that of combined loading. Testing and analysis of structural stainless steel elements subjected to combined axial load and bending is therefore the subject of this thesis.

A comprehensive experimental programme was firstly carried out. At the cross-sectional level, a total of 15 stub column tests, 10 four-point bending beam tests, and 58 combined loading tests were conducted on different cross-section profiles (square, rectangular and circular hollow sections) from different material grades (austenitic, duplex and ferritic stainless steels). At the member level, 48 beam-columns were tested under both equal and unequal end moments to investigate the buckling behaviour of beam-column members subjected to constant bending and various moment gradients, respectively. The test results were then employed in a parallel finite element (FE) study. The FE models were first validated against the test results and then used to conduct a series of parametric studies to extend the available results over a wider range of cross-section sizes, member slendernesses, loading eccentricities, and moment gradients. The experimentally and numerically derived data were used to study the structural performance of stainless steel elements subjected to combined axial load and bending moment, to assess the accuracy of current codified design provisions, and to establish new, more accurate, design rules.

Following the comparisons between the test and FE results and the existing design provisions, it was found that all existing design codes lead to unduly conservative strength predictions for stainless steel cross-sections under combined loading, mainly due to the neglect of strain hardening. For stainless steel beam column members, there was a high degree of scatter in the strength predictions, with both conservative and unsafe results. This was attributed mainly to inaccurate predictions of the column buckling and bending end points of the design interaction curves and inaccurate interaction factors. Improved design expressions were sought through extension of the deformation-based continuous strength method (CSM) to the case of combined loading at both cross-section and member levels. Comparisons of the predicted capacities with over 6000 test and FE results indicated that the CSM proposals yield accurate and consistent strength predictions for both stainless steel cross-sections and members under combined loading. The reliability of the proposals has been confirmed by means of statistical analyses, demonstrating their suitability for incorporation into future revisions of international design codes for stainless steel structures.