Design of a Seismic Isolation System with Supplemental Viscous Damping for a Near-Fault Essential Services Facility

Abstract eng:
The subject building is the L.A. Regional Transportation Management Center. This steel building encloses five floors comprising about 88,000 square-feet. As an essential service facility it meets the Seismic Performance criteria of "Immediate Occupancy" after a Maximum Considered Earthquake (MCE). Located within proximity of large active faults (closest at 400 feet), the facility can be subjected to high-velocity pulse-type ground motion. Conventional Base-Isolation utilizing different types of isolators would need to accommodate an impractical 40” of lateral displacement. A supplemental viscous fluid damping (VFD) system was selected combined with Natural Rubber Bearings isolators selected for their reliability, consistency of properties over time, cost effectiveness, wide availability and ease of modeling. Sixteen 320 kip nonlinear VFDs reduce the maximum lateral displacement to 24” without adding elastic stiffness to the system. Due to the low initial stiffness of the natural rubber isolators and low initial sticking force in the VFDs, an innovative wind-brake ring with adjustable tension bolts was added to the dampers to achieve the code required wind load resistance. The considerably lower tension capacity of the isolators compared to compression capacities created challenges when resisting seismic over turning forces. Double end bay spans were created and braced frames utilized as transfer trusses to concentrate dead loads at corners to minimize the upward displacement and isolator tension. Modeling this behavior for isolators was also a challenge. A creative technique was devised utilizing a combination of three finite elements, namely; a bilinear biaxial (horizontal) shear hysteretic element with linear axial (vertical) stiffness combined with a uniaxial gap element connected by a rigid link. Four elements were used at damper locations.

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