Calibration of partial factors for wind action: an application to the Italian wind climate

Abstract

The design of structures to wind actions requires: (1) a wind hazard map providing the characteristic value of the wind speed (or velocity pressure), usually associated with a return period T = 50 yrs; (2) an equation accounting for return periods different from 50 yrs; and (3) a set of partial factors for the evaluation of the design value of the wind action. Partial factors from EN1990 prove to lead to an average reliability level that not always matches the target value, and in particular in the case of wind actions. This is the case of Italy, whose extreme wind map dates back more than 30 years, and whose wind climate appears to be different from that of other European countries, with larger coefficient of variation of the annual maximum wind speed and velocity pressure.

The main purpose of current work, is to assess the actual reliability level of structures subject to wind action for the Italian wind climate, according to EN1990 and EN1991-1-4 (CEN, 2002; CEN, 2005). To this aim, the stochastic models and the characteristic values for the variables involved in the calibration will be mainly based on the Probabilistic Model Code, PMC (JCSS, 2001). In the assessment of the reliability level, three different structural materials will be considered: masonry, reinforced concrete, and steel. They will be analyzed against axial loads and bending moments. Regarding the wind action, the time-variant component will be modelled by considering the work of Ballio et al. (1991a, 1991b, 1999) which is the basis of the current Italian extreme wind map. It provides the probabilistic models for the annual maximum wind speed at different locations throughout Italy. Instead, the time-invariant component of the wind action will be modelled according to PMC.