RETRO' project, the 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 constructed 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.

Results of predictive numerical analyses, both for "as-built" and "isolated" configurations are here illustrated and discussed. The work is subdivided in two parts: the present paper is devoted to the analysis of the "as-built" configuration whereas the second part shows the results of the isolated case.

The as-built structure was assessed with nonlinear time history dynamic analyses. To these end, the Italian guidelines for the seismic assessment of existing bridges, proposed during the Reluis research program (2005-2008), were used.  As far as the definition of the model is concerned, unlike the deck which is modelled as an elastic element, the model includes the possibility of occurrence of important plastic deformations and strain-penetration effect of the bars. Progressive collapse using natural records, selected according to the coherence conditions with the design spectrum, was estimated and failure modes of the system components were accurately analyzed.  The assessed systems is found sub-standard with respect both to the current Italian regulation and the Seismic European code (EC8), suggesting the necessity of a proper retrofitting system such as seismic isolation.