This document presents guidelines for a practical assessment of the seismic performance of existing structures. Performance is measured in terms of the likelihood that a building, given a level of ground motion, can be in any one of five structural limit states after an earthquake: onset-of-damage, green tag, yellow tag, red tag, and collapse. The green-, yellow-, and red-tag states are directly related to structural post-earthquake functionality. The link between the patterns of structural damage that could be observed after a mainshock and the assignment of the appropriate limit state to each damage pattern is made using loss of capacity as the quantitative measure of performance degradation. The objective criterion used by the procedure to assign the appropriate tagging condition to a given damage pattern is the likelihood that an aftershock ground motion will exceed the specific (reduced) capacity. The loss of capacity for each damage pattern induced by a mainshock is evaluated using state-of-practice engineering analyses performed before an earthquake.
The results of these guidelines are twofold. First and foremost, after appropriate consideration of uncertainty in building response and capacity, the guidelines provide a rational procedure for developing fragility curves for green, yellow, and red tags, and for collapse of an intact building. A convolution of such fragility curves with the seismic hazard at the building site provides an estimate of the frequency of future building access restrictions and collapse. Such information is valuable when estimating the likelihood of loss of functionality of a critical facility, or downtime of a building for purposes of estimating expected loss. Secondly, this procedure provides the engineer inspecting the facility after an earthquake a rational criterion for deciding whether and when to permit
re-occupancy. These guidelines, although developed for buildings that are common in the building inventory of major electric utilities, such as Pacific Gas and Electric, are of more general applicability.
Keywords: seismic fragility curve, post-earthquake building functionality, aftershock hazard, push-over analysis, post-earthquake building tagging, post-earthquake structural capacity loss.
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