A POSSIBLE REVISION OF THE CURRENT SEISMIC DESIGN PROCESS

We present a possible Performance Based Seismic Design (PBSD) process grounded in a scenario based definition of the seismic input. The proposed procedure aims to address the following considerations, arisen from the analysis of seismic phenomena: a) any structure at a given location, regardless of its importance, is subject to the same shaking as a result of a given earthquake, b) it is impossible to determine with precision when a future earthquake of a given intensity/magnitude will occur, c) insufficient data are available to develop reliable statistics for earthquakes. Following these considerations, the seismic input at a given site - determined on the basis of the seismic history and the seismogenic zones, nodes and faults - is defined using the Neo Deterministic Seismic Hazard Assessment. Two different analyses are carried out at different levels of detail. The first one (Regional Scale Analysis) provides the “Maximum Deterministic Seismic Input” as a response spectrum at the bedrock (MDSIBD), similarly to what is proposed by the codes. The second one (Site Specific Analysis) takes the site effects into account, providing a site specific seismic input (MDSISS); This approach envelops uncertainties by means of a wide range of NDSHA simulations rather than quantifying them probabilistically and provides realistic site specific seismograms that could be used to run time history analysis even where no registrations are available. Reviewing the standard PBSD procedure, MDSISS is always associated with the worst structural performance acceptable for a building (Target Performance Level). Thus the importance of the structure (risk category) is taken into account by changing the structural performance level to check, rather than to change the seismic input. The procedure is a generalized enhancement of what has already been applied for the seismic upgrading of several critical and essential buildings (e.g. schools) of the Trieste Province (Italy).