Tipping Building | 1908 Shattuck

Seismic isolation on display

Once the basic geometry of the building was set and the planning and development issues resolved, we turned to designing the seismic isolation system, which became the most compelling feature of 1908 Shattuck. Isolation bearings are typically installed below grade, hidden from view, requiring intensive—and expensive—foundation work. But at our site, previous construction had left retaining walls on three sides of the lot to accommodate a grade change of about eight feet. We used this unique opportunity to create a building that would not only be at the forefront of seismic resiliency, but would educate the public about seismic isolation. Spliced into the ground-floor columns ten feet above grade, the isolator bearings are visible to restaurant patrons, while the façade design features one column viewable from the street.

While traditional base isolation systems require a ground-level moat that allows the isolated building freedom to move during an earthquake, splicing the isolation bearings into the columns above grade resulted in an atypical isolation plane that required a different solution: ground-floor walls cantilever from the foundation, and an isolation moat was designed at the second story. In the end, this unique isolation solution ended up being more cost-efficient than traditional below-grade installations.

The site’s proximity to the Hayward fault dictated isolation bearings with an allowable horizontal movement of roughly 36 inches, which would have resulted in a maximum building width of 33 feet. Tipping originally planned for a 35-foot-wide building, and given the already small lot size and the architectural program, Tipping wanted to avoid losing this additional space. We worked with Earthquake Protection Systems (EPS) in Vallejo, CA to customize six stock triple-pendulum bearings with a smaller allowable movement of 24 inches to meet seismic requirements and preserve floor space.

Advanced analysis + response

To ensure the custom triple pendulum bearings would meet seismic resiliency requirements, Tipping ran nonlinear response history analyses using ground motions at maximum-displacement earthquake (MCE) and design-basis earthquake (DBE) hazard levels. 

Two of the 14 analyses caused the isolator to “lock up,” or reach the 24-inch allowable displacement capacity. To account for this slight probability of lock-up, we designed the cantilever columns with a larger diameter, allowing them to remain elastic at lock-up. 

Then, we modeled every wood wall to analyze the effect that locking up would have on the superstructure. Based on those results, we concluded that to achieve the desired elastic behavior, all walls had to be sheathed with half-inch plywood, in some cases on both faces. 

Finally, to allow the footings to yield, displace, and then self-center in the event of an earthquake force larger than the lock-up, the foundation’s grade beams were reinforced with post-tensioned high-strength steel rods. All these design moves added cost to the project, but the gain in programmable square footage was a tradeoff well worth the expense.

Exposed triple-pendulum friction bearings, highlighted in yellow for public awareness and education, allow the floors above to move 24 inches in any horizontal direction.