Immunex Helix Facilities
Seattle, Washington

Seattle's waterfront was selected by the Immunex to house their corporate headquarters and some of their research and development facilities. The first phase of their development included construction of 7 buildings over ground that had been previously reclaimed along the waterfront. Because of concerns for the earthquake performance of the buildings and the potential impacts upon company operations, Immunex decided to use a base isolation system in the building design to reduce earthquake forces on the structure.
PanGEO was retained to provide earthquake time histories suitable for the design of the base isolators. The development of the earthquake ground motions was complicated in that the subsurface soil conditions were variable across the site. To simplify the design, ground motions were developed for two generalized site conditions. Probabilistic procedures were used to develop response spectra at the foundation level of the buildings corresponding to earthquake events having a 475, 975 and 2,475-year return interval. Next, seven earthquake time history pairs were selected to match the general characteristics of the design events and the time histories were subsequently modified to match the probabilistic site-specific response spectra. The time histories were further adjusted to avoid falling below UBC minimum threshold motions. This resulted in the generation of 42 time histories for the building evaluation for one generalized soil profile. Because of the unique nature of the project, the City of Seattle requested an out-of-state third party review of the seismic design of the facility. This reviewer concurred with the design procedure and results.