Performance-based optimization of steel exoskeletons: An alternative approach to standard regulations

Among the various seismic retrofitting techniques, steel exoskeletons are distinguished as a valuable retrofitting approach to mitigate structural vulnerability under lateral loads while simultaneously preserving the buildings’ functionality and activities’ full operability. However, they are not a commonly selected option by designers, as several standard regulations recommend a design approach based on the relative stiffness between the exoskeletons and the building. They establish a restrictive limit for this ratio, resulting in costly and heavy designs. This paper proposes a paradigm shift in exoskeleton design, moving from the control of the stiffness ratio to a performance-based design approach. Different inter-story drift thresholds are adopted as performance constraints of an innovative optimized design procedure where the number, position, and sizing of the exoskeletons are assumed as design variables. Based on the outcomes of the optimization processes conducted on three real-world inspired case studies, a sensitivity analysis is performed. In all scenarios, the results demonstrate that the performance-based approach allows for greater utilization of the building's capacity in the elastic field to resist horizontal actions while preserving structural safety. Consequently, in contrast with the conservative designs obtained following standard regulations, the proposed approach leads to lighter and more economically efficient designs, which make the exoskeletons a more attractive alternative.