| Abstract: | The recorded earthquake response of a base-isolated building—the Foothill Communities Law and Justice Center in Rancho Cucamonga—shaken by the 1985 Redlands earthquake (ML 4–8) is discussed and analysed by employing system identification techniques. The calculated response of one-dimensional and three-dimensional linear structural models is fitted to the recorded motions of the superstructure using the ‘modal minimization method’ for structural identification, in order to determine optimal estimates of the parameters of the dominant modes of the building. Simple one-dimensional analyses are used to identify also the effective values of key parameters (e.g. damping) of the isolation system. Furthermore, the recorded motions obtained from the densely instrumented foundation (i.e. below the isolation bearings) of the structure and from the free-field station located 330 ft 1 1 ft =0.3048 m; 1 mile=1.609 km. from the building show how the presence of the structure affects the incoming seismic waves. It is observed that the transverse component of motion (i.e. the component which is perpendicular to the long dimension of the plan of the building) is affected by the presence of the structure considerably more than the longitudinal component. Factors contributing to this effect are the extreme length of the structure (414 ft) and the rotational motions of the superstructure caused by the spatial variability of ground motion. It is pointed out that, despite the fact that the shift in the effective frequency of the structure induced by the isolation was very small, the elastomeric bearings were very effective in reducing the accelerations transmitted to the structure. This is attributed to the damping capacity of the isolation. Based on the observed response of the building to this small earthquake it can be stated with confidence that the structure performed according to expectations. |
