Effect of strain hardening on the rotation capacity of welded I-section high-strength steel beams

High-strength steel beams are known to have less plastic rotation capacity than beams with lower yield strengths. This has been related to the decreased strain-hardening ability of high-strength steels, and various rules and standards for steel structures stipulate maximum limits on the allowable yield-to-tensile strength ratio (s(y)/s(u)), which indirectly acts as a measure of strain hardening. While the literature suggests that there is an interdependence between strain hardening ability, yield strength, cross-sectional slenderness and rotation capacity, the presently prescribed limits on s(y)/s(u) (e.g. 0.91, 0.94, 0.95) are typically constant for a given material regardless of the other parameters mentioned. This computational study hence investigates how the rotation capacity is simultaneously dependent on yield strength, strain hardening ability and cross-sectional slenderness, and how each parameter affects the relationship between the others. The results show that, with the geometrical aspect kept constant through the use of normalised slenderness parameters, a higher yield strength leads to higher rotation capacity for a given s(y)/s(u), while the well-known decrease of rotation capacity with higher s(y)/s(u) is confirmed. This suggests the possibility of more efficient use of high-strength steels with high s(y)/s(u) when the interdependence of all the variables are accounted for. The results also suggest the importance of accounting for the relative slendernesses of the web and the flange and whether the buckling behaviour is web-or flange-dominated, since a switch between a web-and flange-dominated buckling response could lead to a reverse in the trend between the rotation capacity and the overall cross-sectional slenderness.