Retrofit of seismically isolated buildings for near field ground motion using additional damping

Technical Abstract

Recently it has been observed that near field earthquakes containing forward directivity ground motion often causes a response in structures, particularly flexible structures, considerably larger than the design level response. As seismically isolated structures are designed with a flexible isolation layer, a number of these existing structures are now considered vulnerable to near field earthquakes. Such earthquakes are likely to cause the isolation systems of the seismically isolated buildings to exceed their maximum allowable displacements, resulting in failure at the bases or a large amount of damage in their superstructures.

In this thesis a number of generic seismically isolated buildings, designed using the requirements set out in the 1997 Uniform Building Code, have been retrofitted with additional damping. It was found that to control deformations at each level, in response to near field earthquakes, additional damping was required at the isolation layer and between at least the base and first floor of the superstructure.

Using concepts derived from analyses of the generic structures, a model of the seismically isolated William Clayton building located in Wellington was retrofitted. It was found that isolator deformations could be limited to maximum allowable levels in response to near field earthquakes using viscous dampers. Unlike the generic structures no additional damping was required in the superstructure to control inter-storey deformations, thus damping was only required at the isolation layer.

The impact of additional damping in the William Clayton building, on the design level response and the accelerations at each floor, was found to be minimal.