Seismic fragility of circular piers in simply supported RC bridges: a proposal for capacity assessment

Bridges are crucial structures of a roadway system, always exposed to both natural and anthropic hazards. Several approaches have been investigated in the past two decades to evaluate the seismic vulnerability of bridges; the development of fragility curves represents a sound tool to evaluate their performances at a large scale. This study aims at the estimation of seismic fragility curves for a structural element of a very common bridge typology in Italy, that is the single circular cross-section pier of a simply supported Reinforced Concrete (RC) bridge. Two analytical methodologies for deriving fragility curves are applied to a case study model. The first one is the Capacity Spectrum Method (CSM). This method accounts for the non-linearities and the seismic energy dissipation, in fact it is based on a non-linear static analysis and the definition of Damage States (DSs) on the capacity curve of the piers. By contrast, the second one is the Response Spectrum Analysis (RSA), that is a methodology based on a linear method. By using both methods, a parametric study is carried out, varying the bridge's geometrical and mechanical properties within standard ranges, to evaluate their effect on the seismic vulnerability of the pier. This study refers to four DSs and proposes a multilinear regression model to directly evaluate the pier's seismic performances based on the geometrical and mechanical properties of the bridge to which it belongs.