Seismic assessment and dynamic testing of reinforced concrete frames retrofitted with a shape-memory alloy brace system
Vega Behar, Pablo
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Seismic risk assessments have estimated the economic and life-safety impact of a large magnitude earthquake in the US New Madrid Seismic Zone at over $300 billion and close to 100,000 casualties, respectively. Seismic rehabilitation of buildings has been a research priority for the past 3 decades. Non-ductile reinforced concrete (RC) frame structures have been a focus of this research due to their prevalence in the Central and Eastern US. Retrofits have been used to address local (component-level) and global (system-level) deficiencies in these structures, based on past performance during earthquakes and testing at both component and system levels. However, prior research and existing solutions have limitations. Experimental research has focused on single components – which fail to capture the interaction of multiple components in a full system; or reduced scale systems – which do not appropriately replicate resistance mechanisms such as the bond between concrete and reinforcing bars. Existing retrofit solutions have been effective in increasing life-safety for occupants of seismically deficient structures but have shown two main limitations. The first is that while preventing structural collapse during a seismic event, the retrofits withstand levels of damage that render the full structure irreparable; which does not improve the economic impact. The second limitation is that their construction is typically invasive, which may disrupt building occupants and operations. This study addresses these limitations by proposing a retrofit solution that has a minimally invasive construction procedure, is reusable, and limits overall damage through self-centering. The retrofit was tested dynamically in a full-scale system to overcome the limitations of prior, reduced-scale research. The retrofit is a bracing system with shape memory alloy (SMA) components designed to dissipate energy and limit residual drifts. The brace was tested independently and as the main component of a retrofit scheme in a full-scale dynamic test of a prototype RC moment frame representative of 1950s-1970s low-rise buildings in the Central US. A two-story, two-bay specimen was retrofitted with the bracing system and tested using a linear inertial shaker to assess seismic performance. The brace was also tested independently, in quasi-static cycles, to quantify the full hysteretic behavior. Using results from both sets of tests, numerical models were calibrated to assess the fragility of the two-story two-bay frame. A comparison to an identical as-built (i.e. non-retrofitted) frame shows that the bracing device reduces peak and residual inter-story drifts. While the as-built structure developed a global sway mechanism at drifts above 1.5%, the retrofitted structure showed no visible damage at similar drift magnitudes. The retrofitted structure also withstood multiple tests at larger input accelerations than those of the as-built frame. Numerically, the retrofit reduced the probability of exceeding all damage states. The design and assembly steps suggest that this retrofit can be beneficial in practical applications where disruptions to building occupants are a concern. The design procedure, its behavior and response to all test loads, a qualitative evaluation of the design method, seismic risk performance assessment, and implications on future research are discussed.