New Methods for Seismic Performance Evaluation and Retrofit of Nonductile Concentrically Braced Frames

Thesis (Ph.D.)--University of Washington, 2018Concentrically braced frames (CBFs) have been used in steel construction as seismic-force-resisting systems for many decades and constitute a substantial proportion of existing building infrastructure. Until about 1990, CBFs were designed without the codified capacity-based and other ductile design provisions that ensure safety in today's special CBFs (SCBFs) used in regions with high seismic risk. Thousands of these older and potentially nonductile CBFs (NCBFs) remain in service in the high-seismicity areas of the west coast of the US and other more moderately seismically vulnerable regions. These NCBFs utilize a wide variety of connections, components, and frame configurations with deficiencies expected to lead to significant damage and potential collapse in earthquakes. Seismic retrofit of NCBFs may be necessary to ensure occupant safety and building functionality, but current engineering guidance for determining retrofit need and type is limited. The state of practice evaluates the seismic vulnerability of CBFs using simplistic models for braces, beams, and columns, and the nonlinear behavior of connections is typically not considered; it is clear that the vulnerability depends on more complex component behavior. To develop more comprehensive engineering methods that can accurately estimate the vulnerability of NCBFs and the improved performance of retrofitted NCBFs, integrated experimental and computational research programs were conducted. First, two series of large-scale experiments of existing and retrofitted NCBF subassemblages were performed to investigate brace, connection, and beam deficiencies common to NCBFs. The experiments identified critical deficiencies but also beneficial yielding mechanisms (e.g., bolt-hole elongation, beam yielding in the chevron configuration, etc.) which could be retained in retrofit. Experimentally validated, nonlinear modeling approaches capable of simulating brace fracture, connection fracture, weak frame elements, and post-fracture response of components with secondary yielding mechanisms were then developed to advance numerical simulation capabilities. These models were used to enable system-level response-history analysis for seismic performance evaluation. Specifically, the seismic performance (including collapse) of three- and nine-story buildings were investigated at multiple (5) hazard levels. The models were also used to evaluate retrofit strategies; these results combined with the experimental work were used to develop a cost-effective seismic retrofit methodology based on balancing yielding mechanisms and suppressing severe failure modes