An empirical method to assess the seismic vulnerability of existing buildings using the HVSR technique

The seismic vulnerability of existing buildings is usually estimated according to procedures based on checklists of main structural features. The relationship with damage is then assessed using experience from past events. An approach used in seismology for the evaluation of site amplification, based on horizontal-to-vertical ratio of weak motion and microtremors, has been applied to the structural field. This methodology provides an alternative, promising tool towards a quick and reliable estimate of seismic vulnerability. The advantages are: o The measurements are quick, simple and stable. They are non-invasive and do not affect at all, even temporarily, the functions housed in the buildings studied. o The site effect and the soil structure interaction are explicitly accounted for in the vulnerability estimate, when they are excluded in the traditional approaches. o The relationship with damage is established using meaningful physical parameters related to the construction technology, instead of adimensional, normalised indexes. The procedure has been applied to several case histories of buildings damaged in the recent Umbria-Marche earthquake which occurred in Italy in 1997. The same model has been applied to different structures (brick/stone masonry and infilled r.c. frames), on different geological conditions and under very different seismic loads. Using this combined site/building approach, it was possible to explain very sharp variations in the damage pattern.