Seismic isolation design of the San Bernardino County Medical Center replacement project

The San Bernardino County Medical Center Replacement Project, located in Colton, California, consists of six separate building structures containing a total of approximately 920 000 ft2 of floor space for hospital and support services. Five of the six buildings will be base isolated. Each of the five base isolated buildings is of different size and configuration, ranging from a six story, 360 000 ft2 curved front Nursing Tower to a two story, 24 000 ft2 rectangular shaped Central Plant building. The building structures will be framed with structural steel, utilizing concentric braced frames as the lateral force resisting system. The design ground motion for the site, which is located 3 km and 15 km from the San Jacinto and San Andreas faults respectively, is very severe. A base isolation system has been designed for this facility which will provide an essentially elastic building response to the design strong ground motion. The base isolation system is a hybrid passive energy dissipation system consisting of both linear and nonlinear and high damping rubber bearings along with viscous damping devices located at the base of the structure. The high damping rubber bearings provide both lateral stiffness which governs the fundamental period of vibration of the system, and hysteretic damping, while the viscous damping devices provide velocity dependent damping which serves to control overall building displacements. This will maximize the probability that this essential facility will remain fully operational after a major earthquake. Design criteria, structural analysis and design methodologies, and construction details are presented and discussed. The response of one of the base isolated structures is calculated utilizing actual recorded ground motions from the 1994 Northridge earthquake.