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Low Income Senior Housing
One of the larger hurdles for this project was the significant amount of seismic/life safety improvements needed to address the concerns of ownership, the city and the lender. The Tower, an 18-story, non-ductile concrete building, was constructed in the late 1960's and prone to damage during an earthquake. The building used a formed reinforced concrete one way slab gravity system, coupled with corridor and exterior spandrels for gravity support and shear walls in the corridor and transverse end walls for lateral support. Typical of the era, the seismic resisting system was marked by weak lateral resisting elements and with low ductility detailing of the reinforcing steel. Together, this left a building that was both relatively weak and brittle by today's standards and expected to perform poorly in a design level seismic event. An initial probabilistic seismic loss study suggested the expected loss would significantly exceed the thresholds placed by most lenders, making the building difficult to refinance or sell. In order to maintain the viability of the building from both a life safety and economic perspective, a seismic retrofit would have to be undertaken. Complicating matters, the Owner wanted to develop a retrofitting scheme that would allow the building to remain occupied during the course of the work. This meant that the new and existing (strengthened) elements would have to be located in areas that would minimally disrupt the occupants and the structure; the life safety systems of the building could not be significantly compromised during the construction process. This meant that the seismic retrofit design had to be imagined in such a way that the work would progressively improve the building at each phase of construction.
Elements showing high levels of risk included existing column with inadequate confinement, discontinuous shear walls at the lobby level, and generally weak poorly detailed shear walls. Retrofit of these items included carbon fiber wrapping the columns, adding new concrete walls in the lobby to provide an uninterrupted load path, and strengthening of existing walls with both shotcrete and FRP. The foundation retrofit included high capacity micropiles with a mat pile cap and grade beam pile caps running the interior corridor, where it could be segmented around egress requirements and allow the building to remain occupied. New piles and pile caps were carefully laid out to target areas of high demand, while allowing the building lobby to remain open throughout the construction.
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