Collaborative Research: Resilient Seismic Retrofit by Integrating Selective Weakening and Self-Centering

The occurrence of a major seismic event, although low in probability, carries high risk. Many reinforced concrete buildings in the United States and around the world were built before the implementation of modern seismic design codes. Based on seismic events of recent history outside the United States, significant economic losses can be expected in the United States if similar events occur near densely populated cities. Reducing economic losses, shortening time to recovery, and returning to normal operation levels are essential and define resiliency against seismic hazard. Existing seismic retrofit methods achieve life safety through permanent damage. This project will investigate a new seismic retrofit method which focuses on not only public safety but also on resiliency by minimizing damage. The new retrofit system will be applicable to buildings with sub-standard reinforced concrete shear walls as the lateral load resisting system. Low damage ensures that buildings can remain operational and habitable within a reasonable amount of time following an earthquake, while keeping the socio-economic losses related to the building stock to a minimum compared to systems retrofitted with traditional methods. The retrofit method can also be architecturally advantageous for cases where the scope or budget for additional lateral load resisting systems is not available. Education, outreach, and technology transfer activities will focus on raising awareness about resiliency, multiple hazards, and new building technologies, as well as increasing participation of underrepresented groups in STEM. In this project, a new seismic retrofit method, integrating the concepts of selective weakening, hinged walls, and self-centering, will be investigated to achieve low seismic damage. The method will be targeted toward improving the performance of reinforced concrete shear walls that have design deficiencies related to confinement, transverse reinforcement, and reinforcement detailing, which have been associated with slender wall failures in past earthquakes. The retrofit method will involve converting traditional and deficient shear walls into self-centering walls by creating a cold joint near the shear wall-foundation interface (weakening) and adding unbonded external post-tensioning. The objective of this research is to understand the change in fundamental response of deficient reinforced concrete buildings when retrofitted with this method. Research methods to achieve this objective will include element and system level structural testing and analysis. Element level studies will include nonlinear analytical modeling and testing of retrofitted shear walls under reverse cyclic loading. These studies will identify viable retrofit details and target shear wall deficiencies. System level investigations will include finite element and time-history analyses to understand the interaction of retrofitted shear walls with other structural elements to control damage to these elements. Performance enhancement that can be achieved by this retrofit method will be quantified using reliability concepts. This work will create a fundamental understanding of building systems retrofitted with this method in terms of capacity, hysteretic response, and self-centering capability through nonlinear analyses, testing, and fragility studies. The findings of this research also will have the potential to advance the state-of-the-art of self-centering structures that are newly constructed. Project outcomes will produce new basic knowledge needed to retrofit deficient concrete walls and develop more resilient buildings in seismic regions and thus contribute to seismic hazard mitigation in the United States and around the world. Project data will be made available in the NHERI Data Depot at https://www.designsafe-ci.org/.