In seismic territory, something as simple as adding two floors to an existing building, even if it was designed to accept more load, can be complicated. But for structural engineer John Sumnicht and his colleagues in the San Francisco office of Simpson Gumpertz & Heger Inc., complication is seen as opportunity. That is how they arrived at a unique application for seismic isolators for a San Francisco building expansion. Typically, base isolators let a structure above it ride out a temblor by gliding back and forth. For 185 Berry Street, Sumnicht put the isolators on the roof of the 1980s concrete structure to isolate a two-story addition above it. Without the approach, the $44-million project would not have gotten off the ground.
The isolators allowed an economically viable 150,000-sq-ft rooftop expansion instead of the unworkable 50,000 sq ft envisioned when the old building was designed. And the solution avoided triggering a major seismic retrofit of the old building, required by stricter codes. The isolated addition, like a mother hen nesting, acts as a giant mass damper, reducing earthquake forces and displacement demand on the old building, says Sumnicht, an SGH senior principal.