Use of vibration monitoring data to track structural changes in a retrofitted building

Structural health monitoring (SHM) approaches for condition assessment and damage detection based on vibration signature of target infrastructure systems have received much attention for a long time. However, there is a paucity of studies that evaluate methodologies based on actual vibration measurements, obtained under realistic field conditions. Particularly lacking is the availability of experimental data from physical structures, where quantifiable changes are made in the structure while structural health monitoring studies are being performed. That is precisely the focus of this paper. As a result of the 1994 Northridge earthquake, a critical facility in the metropolitan Los Angeles region was found to need significant seismic mitigation measures. The facility consisted of a six-storey building that housed essential emergency services. The building was instrumented with 14 state-of-the-art strong-motion accelerometers that were placed at various locations and in different orientations throughout the building. The instrumentation network was used to acquire extensive data sets at regular intervals that covered the whole construction phase, during which the building evolved from its original condition to the retrofitted status. This paper presents an overview of the ambient vibration data collected before, during, and after the structural retrofit. A finite-element model of the building was developed and used to estimate the dominant frequencies and mode shapes before and after the retrofit. The experimental data was used to determine the spectral characteristics of the ambient response and to compare the experimental measurements with the computational model. Changes in the identified structural frequencies are correlated with the time that specific structural changes were made. It is shown that this unique collection of data scan be extremely useful in calibrating the accuracy and sensitivity of various SHM schemes. Copyright (c) 2005 John Wiley & Sons. Ltd.