  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.
