A proposal for the calibration of the Italian seismic action in accordance with a reliability-based design procedure

In Italy the current seismic design standards for new and existing constructions relies on Uniform-Hazard Approach (UHA), assigning fixed hazard-exceedance probabilities to different Limit States (LSs). However, this approach leads to non-uniform seismic risk, since failure probabilities vary across a territory depending on local seismic hazard conditions. To cover this gap, this study proposes a reliability-based procedure for calibrating the seismic action of constructions, consistent with the probabilistic framework of SAC FEMA and the Risk-Targeted Approach (RTA). In accordance with this procedure, target reliability indexes and the corresponding reliability factors are proposed by referring to the current MPS-04 seismic hazard model. To this scope exceedance probabilities for different LSs and Consequence Classes (CCs) are defined, and the seismic hazard function parameters are optimized in a log–log space. Assuming the annual failure probability of (Formula presented) provided by FEMA, average target reliability factors of 2.11 for the Ultimate Limit State (ULS) and 1.0 for the Serviceability Limit State (SLS) are obtained for the Italian territory. Based on these values, mean target reliability indexes are then derived as functions of the LS and CC. The results obtained clearly show that, coherently with the UHA, the Expected Annual Losses (EALs) and reliability indexes vary significantly across Italy, indicating a non-uniform risk distribution. Conversely, the reliability-based procedure achieves uniform seismic risk by introducing a site-specific modification factor to the MPS-04 seismic action. The reliability factors derived according to the proposed design procedure may be readily implemented in the design code to calibrate a seismic action ensuring a uniform seismic risk across the Italian territory.