An innovative seismic system allows an economical, high-performance renovation.
This six-story medical office building, with its distinctive exterior masonry cladding, was originally constructed in 1970, and, like many buildings of similar age and concrete construction, its nonductile frame would have been highly vulnerable in the event of a large earthquake.
To protect their real estate investment and improve the building's leasability, the owners undertook voluntary seismic improvements. Because of the building's proximity to the University of California—regarded as a likely tenant for the building—the improvements were designed to comply with the university’s stringent seismic safety criteria, while meeting a fixed construction budget that balanced costs against the projected return. Finally, the structural solution had to avoid any exterior modifications, which would have triggered a lengthy city review process.
We developed a cost-effective retrofit strategy that incorporated a system of hybrid concrete walls, which are especially notable for their self-healing properties that come into play after an earthquake. The hybrid system blends the inherent advantages of reinforced-concrete walls, including energy dissipation and stiffness, with the strength and elasticity of vertically arranged post-tensioning tendons to provide increased performance. More importantly, the unbonded post-tensioning allows the walls to re-center after an earthquake, reducing seismic damage and minimizing permanent deformation.
A specially developed shotcrete mix—equal parts of slag and concrete—provided the high compressive strength of 8,000 psi and allowed the concrete to be pneumatically applied. This labor- and cost-saving application was possible because more than half of the vertical steel reinforcement was replaced with lower-volume post-tensioning, thus reducing the reinforcement congestion within the wall.
Carbon-reinforced polymer strips applied to the existing floor slabs collect and transfer the inertial forces of the floors to the post-tensioned walls. Applying the strips directly to the top of each slab (rather than underneath, as is typical) eliminated the laborious process of removing and replacing the mechanical systems and ceilings.
These components work together to achieve significantly enhanced performance with minimal intrusion into the building’s leasable space and at a feasible cost.
Executed as Tipping Mar
2009 ATC & SEI Top U.S. Seismic Strengthening Project Awards, Top Project of the Last Decade Involving the Seismic Retrofit of an Existing Concrete Structure
2008 Post-Tensioning Institute's PTI Awards, Award of Merit: Repair, Rehabilitation, and Strengthening
2006 SEAONC Excellence in Structural Engineering, Award of Excellence: Best Use of Conventional Technologies in a Building Retrofit
2006 SEAOC Excellence in Structural Engineering, Certificate of Merit: Best Use of Conventional Technologies in a Building Retrofit