RETRO' project, acronym of "assessment of the seismic vulnerability of an old Reinforce concrete viaduct with portal frame piers and study of the Effectiveness of different isolation systems through Pseudo-dynamic Test on a laRge scale mOdel", funded by the European commission within the project SERIES (http://www.series.upatras.gr/), aims at studying the seismic behaviour of existing r.c. bridges and the effectiveness of innovative retrofitting systems. The research focuses on old bridges, not properly designed for seismic action. In particular the seismic vulnerability of an old Italian viaduct with portal frame piers realized with plain steel bars (Rio Torto Viaduct) is seismically evaluated and a proper isolation system is designed using both Lead Rubber Bearings and Slide Spherical Bearings.

Some results of predictive numerical analyses, both for "as-built" and "isolated" configuration are here illustrated and discussed. The work is subdivided in two parts. Part I° is dedicated to the seismic analysis of the as-built configuration, whereas the present paper is devoted to the analysis of  the isolated case.

The retrofitting strategy employed base isolation systems. In so doing, two different types of isolators were employed, i.e. friction pendulum systems (FPSs) and lead-rubber bearings (LRBs). Comparative analyses were carried out to determine the optimal retrofitting scheme in order to minimize the maximum displacement demands on the expansion joints located at the abutments.

The outcomes of the present studies prove that the seismic assessment of the existing RC bridges poses a number of problems for the selection of the earthquake input motion, the modelling of the local behaviour, e.g. bond effects, and adequate definition of structural response parameters. Additionally, the selection of the type of seismic retrofitting scheme is not straightforward. Base isolation is an effective means to reduce the maximum seismic demands, however the choice of the devices should comply with existing structural details, e.g. expansion joints. It may result of paramount importance to modify the boundary conditions of some viaducts, for instance modifying them into a continuous system, before base isolation is applied.