The RETRO project aims at studying the seismic behaviour of existing reinforced concrete bridges (RC) and the effectiveness of innovative retrofitting systems. Emphasis has been paid on old bridges generally not properly designed for seismic action. On the basis of a previous experimental campaign consisting of cyclically imposed displacements on 1:4 reduced scale models of a frame pier, belonging to a typical old RC highway viaduct, a new experimental activity has been proposed. Two specimens (scale 1:2.5), a frame pier of 2 levels (height 5.8 m) and a frame pier of 3 levels (height 10.3 m), have been built and tested using the continuous Pseudo-dynamic (PsD) method with non-linear substructuring technique, including the modelling of the remaining entire viaduct to which they belong. Two test configurations have been considered: 1) retrofitted viaduct using Friction Pendulm Isolators, and an 2) ?as-built? configuration imposing a Serviceability and Ultimate limit state conditions. For each phase of the experimental campaign model identification has been performed. The aim of the campaign was twofold: 1) increasing the knowledge on the non-linear behaviour of RC frame piers in presence of plain steel bars and detailing of the late 1950?s for which few studies have been carried out, and 2) study the effectiveness of seismic isolation systems for the seismic response mitigation.