Virtual Base Isolation by Building Softening with Drift Control Provided by Fluid Viscous Dampers

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Michael Gemmill, Kurt Lindorfer, H. Kit Miyamoto

Large mass and high story heights are common requirements for many
data storage and collocation facilities. These building characteristics, which are typically considered design obstacles, actually provide a unique opportunity for reducing seismic response through behavior similar to that of a base isolated building.

In accordance with the 1999 SEAOC Blue Book (SEAOC, 1999) recommendations for passive energy dissipation, the building’s Lateral Force Resisting System (LFRS) is designed for strength requirements only, resulting in a relatively flexible LFRS, while Fluid Viscous Dampers (FVD) are incorporated to limit story drifts to acceptable levels. Due to the high building mass, large story heights of 18’-0” (5.5m), and long period LFRS, the building exhibits a fundamental period of 1.45 seconds, compared to 0.5 seconds for a typical two story moment frame building. The long period LFRS emulates the response of a traditional base isolated system by reducing the acceleration on the building and its contents, while story drifts are controlled by FVD.

There are many benefits to this “virtual isolation” system and
incorporation of the SEAOC Blue Book recommendations. With the elimination
of the maximum drift requirements, the moment frames are substantially lighter
than a traditionally framed building, thus lowering the structural steel cost of the LFRS. The long period structure also produces significantly reduced forces in the foundation elements. Velocity and displacement are reduced significantly through the use of the FVDs, which protects the sensitive contents of the building. These benefits lead to a reduced response resulting in an enhanced performance level during a major seismic event.