2150 Shattuck Avenue
Multidisciplinary collaboration and advanced computer modeling reveals an innovative and cost-effective retrofit solution, for a structure that itself was once considered state-of-the-art.
When it opened in 1970, this fourteen-story office building in downtown Berkeley was distinguished by its height, which made it the city’s tallest building, and its unique structural system. Unlike typical building structures in which columns support the floors, the building's floors are suspended from two reinforced-concrete cores by a system of steel trusses and tension hangers. While this innovative solution allowed for desirable, column-free lease spaces, the two cores were brittle and weak by today’s standards. They had insufficient shear capacity, inadequate lap splices for the vertical reinforcement, and insufficient foundation strength, given the building’s location less than a mile from the Hayward Fault.
Developing a cost-effective, broad-based, and state-of-the-art upgrade of this unusual structure required the collaboration and integration of expertise from different disciplines. The client’s design mandate was a high level of seismic performance within a tight budget. Specifically, the goals were to achieve a PML rating of less than twenty percent (required by the lender) and a “good” rating according to U.C. Berkeley’s Seismic Performance Criteria in order to satisfy the University’s leasing requirements. To achieve resilient structural behavior, the retrofit scheme focused on adding toughness by mitigating critical weaknesses rather than by adding strength in a conventional and expensive brute-force approach.
Advanced 3-D nonlinear computer modeling revealed how the behavior could be tuned by adding a unidirectional carbon-fiber fabric to the cores’ bases (in the plastic hinge region) to overcome the shear strength deficit without increasing the flexural demand. Consequently, the brittle shear failure mode was eliminated, enabling tough and ductile flexural behavior. Additionally, we posited that the deficiencies in the lap splices and concrete toughness could be corrected by using headed through-pins to externally confine the concrete for much lower costs than conventional strategies. The performance benefits of this innovation were validated for this project via an academic-commercial research partnership between Tipping, McGill University, and Headed Research Corporation (the maker of the pins). Finally, new micro-piles and buttress walls were used to strengthen the foundation.
Executed as Tipping Mar