Stability and plasticity interaction in building‐size steel sway frames

Accurate design rules are crucial for the safety, sustainability and economy of steel structures. To predict the load‐bearing capacity of sway structures, Eurocode 3, Part 1‐1, uses individual member unity checks, which are assumed to include the interaction between members, related to their stability and plastic behaviour. To verify this assumption, basic (one‐bay single‐storey) 2D and 3D sway structures, and building‐size 3D structures are simulated by nonlinear finite element simulations, and predictions are compared to those according to Eurocode 3. The results show that as long as basic 2D and 3D structures are loaded such that all their members fail due to instability, the load‐bearing capacity is predicted accurately by Eurocode 3. If, however, they are loaded to have members failing due to stability and plasticity, Eurocode 3 predicts the load‐bearing capacity to be too low, i.e., conservatively. For building‐size 3D structures, it is shown that system eigenvalues, which may range from single‐member buckling, via coupled multi‐member instabilities, to overall buckling, are important for ultimate load predictions; however, they are not taken into account in all Eurocode 3 methods. In conclusion, for the investigated cases, the design rules of Eurocode 3 are able to capture stability and plasticity interaction effects safely. However, for more economic design, design rules should take system stability into account, e.g., by predicting column buckling via system eigenvalues, and by considering plastic redistribution and formation of subsequent plastic mechanisms. Finally, for the assessment of a building structure in general, also for other materials than steel, it is advised to perform a linear buckling analysis on the complete building, and to study all relevant local, intermediate and system eigenmodes for their impact on the stability, strength and safety of the building structure.