SEISMIC RISK ANALYSIS OF HISTORICAL TOWN CENTERS: THE ROLE OF LOCAL EFFECTS AND VULNERABILITY OF INTERACTING BUILDINGS

摘要

The seismic protection of historical town centers is a critical issue in many European countries. These centers are often clustered on steep slopes and ridges, with old, traditional masonry buildings often aligned and interconnected to form blocks and street rows. Seismic risk is generally defined as the convolution of seismic hazard, vulnerability, and value: an effective methodology for its assessment must rely on a multidisciplinary approach. In this perspective, the work presented here focuses on the two key-aspects highlighted above: the evaluation of local effects and the acknowledgement of structural interaction in the assessment of vulnerability. For the first aspect, a practical procedure is demonstrated through an example, corresponding to an application carried out in pre-earthquake conditions in an area of Umbria (Italy). After a series of geologic and geomorphologic analyses (1:5,000 scale) as well as geotechnical investigations in small town centers, and after having defined the expected seismic input, amplification factors have been evaluated in terms of pseudo-acceleration response spectra and coefficients calculated as ratio of spectral output and input intensities. These factors, defined for homogeneous areas and, as such, applicable at the level of local planning, are intended for use in new design as well as in the retrofitting of existing buildings. For the second point, methodologies developed to date for the assessment of structural vulnerability generally refer to independent buildings, rather than continuous structures. For these last, the most frequent situations, in terms of effectiveness of connection, as well as of geometry in plan, in elevation, and in differential foundation level due to ground morphology, have been classified.