Gutiérrez Urzúa, Luis Fernando; (2023) Performance-based seismic assessment of existing frames and their retrofit with buckling-restrained braces. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In the aftermath of the 1994 Northridge earthquake, many steel Moment Resisting Frames (MRFs) were revealed to have severe deficiencies that compromised their future seismic performance. Since then, many seismic design codes have rapidly evolved to address these deficiencies; nonetheless, several existing steel frames were built before the introduction of modern seismic design codes and are in need of reliable assessment methodologies which include provisions on the modelling and evaluation of structural components that are often overlooked, such as panel zones, and non-structural components that may have a contribution on the strength and stiffness of the structure, such as masonry infills. Many modern assessment codes also include recommendations for the strengthening of deficient buildings. For the retrofitting of steel MRFs, Buckling-Restrained Braces (BRBs) represent an advantageous solution, as these devices can provide strength, stiffness and energy dissipation capacity. Nonetheless, the European assessment regulation lacks guidelines for these devices, while the USA regulations include design procedures which tend to focus only on a few characteristics of the BRBs, neglecting aspects such as the residual drifts induced by the low post-yielding stiffness or their propensity for low-cycle fatigue rupture as a consequence of cumulative ductility, particularly in multiple earthquake scenarios. Thus, the present thesis focuses on addressing some of these deficiencies and omissions in assessing steel MRFs and retrofitting these structures by means of BRBs. The document starts with a comprehensive critical comparison between modern assessment codes to establish the limitations of the current assessment regulations. Then, it focuses on the strengthening aspects by reflecting on the impact of the design decisions on the performance of the BRB-retrofitted structure and provides a probabilistic framework to evaluate the path-dependent failure modes of the BRBs, including the cumulative ductility and residual drifts. In addition to the analytical and numerical work, the present thesis uses experimental data to calibrate a masonry strut model that allows evaluation of the impact of these elements on the steel frame's performance and their interaction with the BRB retrofitting system.

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