Alternative retrofit strategies for seismic risk-reduction: studying the attractiveness of low-damage external exoskeletons.

Recent earthquakes that occurred worldwide have further highlighted the high vulnerability of existing Reinforced Concrete (RC) buildings designed prior to the enforcement of modern seismic codes. These types of structures are expected to be affected by critical structural weaknesses, mainly related to the absence of the “hierarchy of strength” principles, potentially leading to a “no-ductile” global behaviour. As a result, in the last decades, a significant research effort has been undertaken to develop and implement retrofit strategies able to enhance seismic safety and community resilience. Several retrofit strategies and techniques are nowadays available for improving the seismic performance of existing RC buildings, involving both local (e.g., Fibre Reinforced Polymers FRP, metallic haunches, selective weakening, concrete or steel jacketing) and global interventions (e.g., elastic or dissipative braces, external exoskeletons consisting of walls or frames). Among the others, exoskeletons are deemed a promising solution, since they can be implemented entirely from outside, significantly reducing the invasiveness of the intervention (i.e., owners’ disruption), yet providing the possibility of a holistic refurbishment of the building system based on the concept of a high-performance “double-skin”. Further advantages of exoskeletons can be achieved by implementing low-damage technologies, enhancing the seismic performance of the retrofitted structure. Therefore, this paper aims to investigate the use of external exoskeletons based on the low-damage PREcast Seismic Structural System (PRESSS) technology [1]. Specifically, this advanced seismic-resistant system is based on “jointed ductile” connections, replacing the traditional “plastic hinge” in monolithic systems with a rocking and dissipative mechanism at the interface of structural members. To demonstrate the benefits of adopting low-damage exoskeletons rather than traditional retrofit techniques, an illustrative application is herein presented. Specifically, a pre-1970 existing RC building is considered, and alternative retrofit strategies are implemented targeting different performance levels. The seismic behaviour of the as-built and the alternative retrofitted structures is evaluated in terms of probability of collapse through non-linear dynamic analyses. This allows to provide a correlation between the “Safety Index” (i.e., the ratio between the capacity of the structure to the demand of a newly designed building on the same site) and the expected annual probability of collapse, thus highlighting the advantages of adopting high-preforming low-damage exoskeletons. The seismic residual capacity of the structure in its as-built and retrofitted configuration is also evaluated through a scenario-based framework. Specifically, the variation of the Safety Index and Expected Annual Losses (EAL) index is assessed considering ground-motion sequences. The concepts shown in this research work, if supported by experimental data and ad-hoc design/implementation guidelines, can represent a significant step toward the seismic risk reduction together with the improvement of the community resilience at the national level.