Steel concentrically braced frames (CBFs) are stiff and strong, making them efficient seismic load resisting systems. Modern CBF seismic design was initiated with significant code changes in 1988 and is differentiated from older CBFs (classified as non-seismic CBFs or NCBFs) because NCBFs were designed without consideration of the brace behavior or capacity-design principles. In regions of high seismicity, thousands of NCBFs remain in service. In regions of low and moderate seismicity, these systems are still being constructed. A cursory review of the NCBF inventory indicates that braces and connections vary, and there is a large potential for unwanted and brittle response modes. Research into these systems is largely absent from the literature. Taken together, these systems are both widespread and unpredictable, making this category of lateral force resisting system a critical seismic hazard. This research project will advance evaluation and rehabilitation of NCBFs. Large-scale experimentation will be conducted using the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES) facility at the University of California, Berkeley, to simulate existing and rehabilitated multi-story NCBFs. The experimental results will provide high-resolution data to support nonlinear simulation of NCBFs. The integration of the experiments and simulations will provide key information about the system performance, identify vulnerabilities, and inform the retrofit requirements. The identified deficiencies will be addressed through retrofit schemes based on innovative rehabilitation strategies investigated in prior NSF-supported NEES research projects, whose data is available through the NEES Project Warehouse/data repository (www.nees.org). Retrofit categories slated for study include use of innovative braces and bracing systems, column uplift and rocking, and connection designs balanced with the replaced and existing components. These strategies will be integrated with the existing frames in the second phase of the large-scale, integrated multi-story testing, with the design informed from nonlinear analytical simulation and archived NEES Project Warehouse data. The final phase of the experimentation will study a full building using advanced hybrid simulation. In older construction, the seismic response may depend more heavily on the gravity frames. This culminating experiment will study a partially retrofitted NCBF building which may be an economical option to meet a collapse prevention performance objective in many cases; the non-retrofitted NCBFs are simulated in the computer and the retrofitted NCBFs and gravity frames are simulated experimentally. The research will result in unique data to support the evaluation and retrofit of these seismically vulnerable systems. This award is a collaboration among researchers from the University of Washington, University of California, Berkeley, National Taiwan University, and the American Institute of Steel Construction. Data from this project will be archived and made available to the public through the NEES Project Warehouse/data repository.
As a whole, the research results will transform the understanding of and facilitate rehabilitation strategies for NCBFs. The results will provide important findings to validate and advance engineering approaches for NCBF evaluation and rehabilitation. Specific tools that will be developed using the research data include acceptance criteria, backbone curves, and collapse margin ratios for both NCBFs and retrofitted NCBFs. The project will also advance student learning and exposure through the research process. High school students from underrepresented groups will be selected for summer research internships to aid in the experimental research in collaboration with the Seattle Mathematics, Engineering and Science Achievement program. The research results will also be integrated into the senior level curriculum, using a senior-level cross-class project on seismic evaluation and retrofit of a NCBF building as the central learning tool. Analysis of the building will be conducted in the senior-level structural analysis class. Analysis of the individual components will be the focus of the project in the structural steel design class. The retrofitted system will be designed in the capstone design class, taught in the spring of the students' senior year. Technology transfer will be achieved through webinars and a diverse industry coalition who will help disseminate results. This award is part of the National Earthquake Hazards Reduction Program (NEHRP).
