Detection thresholds in structural health monitoring

Abstract

Most health-monitoring methods for analysis of full-scale structures detect changes in the soil-system response by comparison of the identified response parameters “before” and “after” large excitations. However, because the deformations of the foundation soil, which are almost always present, are nonlinear, and because permanent deformations and changes in the soil depend differently on the excitation histories of different sites, it is difficult at best to (1) make use of classical concepts, which are based on signal-to-noise ratio, and (2) the vibrational description of the response in terms of transfer functions. This situation then necessitates a selection of methods and metrics that detect changes in the system parameters that (1) can be evaluated in almost real-time conditions, and (2) are as insensitive as possible to the contributions to the response of the soil–structure interaction. This, in turn, requires case-by-case, specific, and complex methods of analysis, which can only occasionally be generalized to other buildings and which can also be quite different from one event to the next. In this paper, we present examples of what has been done thus far to avoid the associated complexities for a group of full-scale structures, two of which experienced damaging response during earthquake shaking. In the examples presented, we emphasize those features of the analyses that are related to detection thresholds and to the time- and case-dependent amplitudes of the detection “noise”.