Seismic fragility assessment of a reinforced concrete building retrofitted with a rocking exoskeleton system

Exoskeleton systems have emerged as a highly effective seismic retrofit solution, enhancing a building’s earthquake resistance while allowing it to remain in use during installation. To validate the performance of this retrofit strategy, seismic fragility curves provide a probabilistic means of estimating the likelihood of damage under different earthquake intensities. In this study, a self-centering, rocking steel-braced exoskeleton was designed to prevent major structural damage and minimize residual drift. The fragility curves were derived via Incremental Dynamic Analysis (IDA), which subjects a structural model to a suite of ground-motion records scaled through increasing intensity levels. IDA-based curves capture both the median capacity and the record-to-record variability, offering detailed probabilities of exceeding damage states from minor nonstructural cracks to total collapse. Results show that using IDA refines fragility estimates by incorporating nonlinear behavior and seismic record variability, and enables a more nuanced comparison of retrofit options. A real case study demonstrates that this exoskeleton shifts the fragility curves toward higher intensity thresholds, confirming its effectiveness in strengthening existing structures. By quantifying expected reductions in damage, the IDA-based approach facilitates performance-based decision-making for seismic retrofitting.