| In 2004, Switzerland started to apply an approach based on risk for the seismic assessment of existing buildings. In this innovative approach, the concept is to guarantee an acceptably low individual risk coupled with an evaluation of the efficiency of the retrofitting measures. It includes a relationship derived from empirical data between the compliance factor and the individual risk. The compliance factor can be deterministically computed from usual analytical seismic assessment methods, as the ratio between the capacity and the requirement of the current codes for new buildings. However, it is also possible to proceed probabilistically by computing the individual risk using probabilistic seismic hazard assessment, site response, fragility curves of the structure and probability of death depending on the damage grade. Comparisons between these computations on a typical Swiss building and the values proposed in the Swiss approach show significant differences and raise important issues for earthquake engineering. The first issue is the real meaning of the compliance factor when computed with force-based or displacement- based methods. Moreover, the collapse probability of a building having a compliance factor of 1 for the design ground motion is difficult to determine. The second issue concerns the degree of conservatism in the different parameters used, hazard and vulnerability. In conclusion, the probabilistic analysis of existing buildings, even with all the currently available tools is not straightforward. |
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