Improving the seismic capacity of steel–concrete composite frames with spiral-confined slabs

The seismic performance analysis of steel–concrete composite frames involves, as known, the interaction of several load-bearing components that should be properly designed, with multiple geometrical and mechanical parameters to account. Practical recommendations are given by the Eurocode 8 (EC8) – Annex C for the optimal detailing of transverse rebars, so as to ensure the activation of conventional resisting mechanisms. In this paper, the attention is focused on the analysis of effects and benefits due to a novel confinement solution for the reinforced concrete (RC) slab. The intervention is based on the use of diagonal steel spirals, that are expected to enforce the overall compressive response of the RC slab, thanks to the activation of an optimized strut-and-tie resisting mechanism. The final expectation is to first increase the resistance capacity of the slab that can thus transfer higher compressive actions under seismic loads. Further, as shown, the same resisting mechanism can be beneficial for the yielding of steel rebars, depending on the final detailing of components, and thus possibly improve the ductility of the system. To this aim, a refined finite element (FE) numerical analysis is carried out for several configurations of technical interest. The in-plane compressive behaviour and the activation of resisting mechanisms are explored for several spiral-confined slabs, based on various arrangements. Major advantage is taken from literature experimental data on RC slabs that are further investigated by introducing the examined confinement technique. The attention is hence given to the local and the global structural effects due to different arrangements for the proposed steel spirals. As shown, once the spirals are optimally placed into the slab, the strength and ductility parameters of the concrete struts can be efficiently improved, with marked benefits for the overall resisting mechanisms of the slab, and thus for the steel–composite frames as a whole.