UBC Theses and Dissertations
Evaluation of seismic fragilities of special concentrically braced frame equipped with innovative cross-laminated timber-steel floor diaphragms Zhang, Chaoyue
Floor diaphragms are structural elements mainly responsible for transmitting lateral loads to the adjoining vertical members of the Seismic-Force-Resisting System (SFRS). Under significant seismic events, the actual in-plane stiffness of floor diaphragms affects patterns of load distribution in-between horizontal and vertical elements and, as such, contributes to the local nonlinear response and the global dynamic behavior. However, for SFRS with mass timber and hybrid timber-based floor diaphragms, neither comprehensive code design provisions nor accurate procedures exist to account for its in-plane flexibility. This knowledge deficit becomes more apparent if compared to the standard reinforced concrete flooring system. To gauge influences of the actual in-plane stiffness of diaphragms on a timber-steel hybrid building system, this research analyzes its seismic performance via a series of nonlinear dynamic analyses. Three finite-element building models were developed by adopting the OpenSees framework and were characterized by (i) rigid diaphragms, (ii) actual stiffness of the CLT diaphragm subassemblies, including the panel-to-panel slab connections, and (iii) stiffness of the CLT diaphragm subassemblies without accounting the panel-to-panel slab connections, respectively. In addition, the SFRS of the building entails a Special Concentrically Braced Frame whose nonlinear behaviors were explicitly simulated, including global buckling, tensile yielding, and post-buckling behaviors. Numerical models of steel struts and hybrid CLT diaphragms were calibrated and validated upon experimental datasets. Specifically, this paper outlines the global collapse capacity under varying ground motion intensities among the three building models. The collapse fragility analysis reveals that the conditional probability of exceeding all three considered limit states given any level of ground motion intensity for the SFRS equipped with the CLT-steel floor diaphragm is close to that of the SFRS with rigid diaphragm behavior. Removal of CLT panel-to-panel connections attracts disproportionate and excessive lateral deformation demands towards the unbraced frames due to the increased in-plane diaphragm flexibility.
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