Dihong Shao, M.S., S.E., Senior Project Engineer, Andersen Bjornstad Kane Jacobs, Inc., Seattle, Washington
Kit Miyamoto, M.S., S.E., President Miyamoto, International, Sacramento, California
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This paper describes a preliminary-assessment case study for seismic performance improvement of a stiffness and strength asymmetric concrete shear wall building using toggle-brace-dampers (TBD).
The structure is a 16-story major regional phone switch building located in downtown Seattle. Its bottom 7-story including the basement was constructed in the mid 1950’s. The additional 9-story vertical expansion was completed in the early 1970’s. It is a rectangular concrete shear wall building with a complete steel beam-column building gravity system encased in the concrete. The building has solid exterior concrete shear walls at its South and East sides and exterior concrete shear walls with regular punched window/louver openings at its North and West sides. This creates a stiffness and strength asymmetrical or torsional irregular building lateral system. TBD were used since they amplify damper strokes under relatively small story drift of a concrete shear wall building and reduce damper sizes while providing efficient effective damping. These dampers were arranged at the opposite sides of the solid concrete walls to balance and reduce the building stiffness and strength eccentricity.
Three-dimensional computer models were constructed. Nonlinear-static-procedure of FEMA356/ATC40 was utilized to determine the building performance-points under site specific 50%50-year, 10%50-year, and 2%50-year seismic events. A series of site-specific nonlinear time history analyses were performed to evaluate the effectiveness of TBD. The analyses indicate that TBD assist the building existing lateral system to achieve the Enhanced Rehabilitation Objective of FEMA356 and provide a cost-effective solution for building seismic performance improvement in comparison with conventional rehabilitation methods.