Two different way of assessing seismic vulnerability are available nowadays: observed or empirical and calculated vulnerability assessment methods. The first methods are based on observed damage after earthquakes correlated with the structural properties of buildings, whereas the second methods are based on numerical models more or less representing the buildings. In both cases, the trouble is the imperfect knowledge of existing buildings. We propose here a new method for estimating the vulnerability based on experimental modal parameters (resonance frequencies, modal shapes and damping ratio) estimated under ambient vibrations. They allow to build up a simplified numerical model of the elastic building behaviour. The motion produced by numerous earthquakes leads to determine its first damage level and therefore its vulnerability. An inter-story drift threshold based on HAZUS values defines the first damage level of the building. This method is applied to the Grenoble (France) city in which 60 buildings have been instrumented.
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