PhD Defense - Bryam Xavier Astudillo Carpio, "Assessing practical design methods for strongback braced frames using hazard-consistent collapse risk analysis"

Abstract:

Improving community resilience requires structural systems that limit structural damage and reduce downtime after an earthquake. Strongback braced frames (SBFs) with buckling-restrained braces (BRBs) offer an alternative to more traditional braced frames by reducing drift concentrations. Particularly, SBFs use a stiff, essentially elastic vertical truss (i.e., “strongback”) that redistributes seismic demands, enables a more uniform drift distribution, and mitigates story mechanisms. However, challenges in linking practical design methods to collapse performance, while accounting for substantial higher-mode force demands in the strongback, hinder SBF adoption in practice. This dissertation develops design and collapse risk assessment methods able to account for first- and higher-mode demands in SBFs. First, the proposed Multi-Modal Capacity Design (MMCD) method uses free-body diagrams to gain intuition on load paths, providing practical estimates of strongback force demands while accounting for nonlinear behavior in the BRBs and other energy dissipators. Second, the proposed Spectral Hazard Consistency Assessment (SHCA) method measures how well ground-motion records can represent the underlying probabilistic seismic hazard across a range of periods and shaking intensities, supporting more robust SBF collapse risk estimates with reduced dependence on the period used to define seismic demands. Third, the collapse capacity resulting from several design methods with varying levels of modeling complexity was assessed, including Modal Pushover Analysis (MPA) and Multi-R Response Spectrum Analysis (MRRSA), which account for first- and higher-mode force demands using nonlinear and linear elastic models, respectively. Hazard-consistent collapse assessment showed that SBF archetypes with strongbacks designed without accounting for higher-mode force demands result in unacceptably high collapse probabilities, exceeding the target of 1% in 50 years. In contrast, designs using MPA and MRRSA to account for higher-mode force demands meet this collapse risk threshold. Comparisons with FEMA P-695 highlight the influence of spectral shape and ground-motion record selection on assessed collapse performance. Together, these contributions improve understanding of SBF force demands, support hazard-consistent collapse risk assessment of SBF, and demonstrate the importance of accounting for higher modes in practical SBF design, informing future seismic design guidelines and building-code provisions.