Modelling and slenderness design of high-capacity hybrid beam–columns under lateral cyclic loading

The ‘dual-steel’ concept involves combining High-Strength Steel (HSS) and mild steel to control the energy dissipation and overall failure of structures under seismic loads. In this study, the performance and design of a hybrid hollow beam–column were investigated with a focus on the seismic resistance. The hollow section was fabricated with mild steel plates (grade 250) and HSS tubes (grade 750) and can be used as a lightweight replacement for conventional composite members. A finite-element (FE) model was developed and validated against available experimental data. To consider the cyclic softening of HSS tubes, a combined kinematic and isotropic plasticity model was incorporated into the FE model. The cyclic performance of the hybrid hollow sections was compared with that of equivalent concrete-filled square composite members, and the strength and weight reduction benefits were assessed. Furthermore, considering the current and modified slenderness limits, a sensitivity study was performed on the section geometry. An optimum design for the section geometry was proposed, in which the HSS tube elements significantly increase the axial and lateral capacities of the hybrid section and the mild steel plates provide the required ductility to mitigate seismic deformations. The overall failure mechanism of this design is governed by mild steel plate elements, rather than a sudden failure initiated in the HSS tubes. © 2021