Seismic performance of non-invasive single brace made of steel and shape memory alloy for retrofit of gravity load designed sub-assemblages

A large stock of existing reinforced concrete structures, even those located in seismic zones, is poorly designed or in-adequately detailed. Such structures are extremely vulnerable during earthquakes and therefore, those structures require immediate intervention in terms of providing retrofit strategies which are adequate, easy to implement and cause minimum disruption. Single bracing system is found to be a promising strategy possessing the qualities mentioned above. In the present study, the load transfer mechanism in the beam-column sub-assemblage due to adoption of single brace system is established. Analytical studies are carried out to understand the influence of geometry and disposition of the single bracing on the reduction of demand in weak joint rezone. Many parameters such as hysteretic behaviour, energy dissipation, material non-linearity, local stiffness distribution etc. which are important to check the efficacy of any retrofit strategy under seismic loading, can not be accommodated in the analytical model. In view of this, experimentally validated non-linear finite element models are developed in the present study. Numerical investigations are carried out using the validated FE models with different types of steel bracing. The study reveals that a properly designed steel bracing can provide significant improvement in seismic performance, for example, 4 times in energy dissipation and 60-80% improvement in strength degradation with respect to the gravity load designed structure. It also indicates that a super-elastic material like shape memory alloy (SMA) with extraordinary properties would be an excellent candidate for usage as bracing. Hence, the effectiveness of nickel-titanium SMA for seismic retrofit of the beam-column sub-assemblage is also investigated and found to be very efficient and better than that of steel bracing.