Confinement effects within the seismic design of reinforced concrete frames: reliability assessment and comparison

The current Italian code “NTC2018” and Eurocode “EC8” provide specific design requirements in terms of flexural overstrength factors for reinforced concrete (RC) buildings in seismic areas. This study proposes to improve the design methodology by including explicitly the confinement effects within the seismic design of a new regular RC frame due to their influence on the capacity design principles (at structural and member level). Specifically, the seismic performance of a RC frame designed according to codes is compared, in reliability terms, with the one of the same frame designed including the concrete confinement effects. Moreover, a comparison between three constitutive models for confined concrete is performed. Selecting L'Aquila (Italy) as reference site, the two design methodologies combined with the three confinement models are numerically implemented. Scaling 30 non-frequent natural ground motions to increasing seismic intensity measures, according to the site seismic hazard, non-linear incremental dynamic analyses (IDAs) have been developed. Assuming the interstory drift indices as engineering demand parameters, the peak responses have been fitted through log-normal distributions to define IDA curves. Subsequently, appropriate limit state thresholds have been adopted to define the seismic fragility curves. Finally, through the convolution integral and Poisson model, the seismic reliability curves have been derived showing the importance of the proposed design methodology. In fact, for the frame under investigation, the seismic performance strongly improves in terms of ductility respecting all the reliability objective levels if the confinement effects are explicitly considered in the seismic design. Furthermore, the different confinement models provide similar results when the proposed seismic design is adopted, implying a reduction of the model uncertainty.