Seismic assessment of as-built and isolated cases of an existing RC bridge through experimental PsD testing

Abstract

In Italy most of Infrastructures and highway bridge systems have been built before the issue of modern seismic design codes. Chiefly, the existing transportation systems that built in the late 60s and early 70s were designed primarily for gravity loads. As a result, such systems do not employ seismic details and hence their structural performance is inadequate under earthquake ground motions. To decide for any retrofitting operation, the seismic behaviour of existing bridges must be assessed. Because the nonlinear procedures are becoming very popular in the frameworks of structural assessment, there is a need to accurately simulate authentic model to provide reliable indications about the performance required to this kind of structures. Based on these observations, this thesis is devoted to the seismic assessment of existing RC bridges. In particular, the vulnerability of an old RC viaduct with portal frame piers equipped with plain steel bars has been numerically and experimentally investigated. Subsequently, the retrofitting has been evaluated using Friction Pendulum System isolation “FPS”. The nonlinear FE modelling has been executed using OpenSEES software framework. The local behaviour of the bridge has been taking into account, mainly, the bond-slip, strain penetration effect of the plain steel bars, bar buckling and the shear behaviour effect of the transverse beams of the portal frame piers. Depending on a previous experimental campaign consisting of cyclically imposed displacements on 1:4 reduced scale models of a frame pier, belonging to the same RC highway viaduct, a new experimental activity has been proposed within the RETRO project funded by the European Union. In particular, Two specimens (scale 1:2.5), 2 floors (total height is 5.8 m) and 3 floors (total height is 10.3 m) one-bay reinforced concrete frame respectively, were built and tested using the PsD technique with sub-structuring; the modelling of the entire viaduct is considered along with the non-linear behaviour of each pier. During the test campaign, two different configurations for the analysed viaduct have been considered; “As-built” and “Isolated with Friction Bearings”. The response of both cases has been compared to evaluate the effectiveness of the isolation system. To this end, two natural accelerograms were selected based on studying the seismiogenic zone of the bridge to represent, according to current seismic codes, the Serviceability and Ultimate limit states. Analysis and interpretation of the experimental were achieved throughout the data recorded by the instrumentation used in the test campaign. The results have been used to calibrate numerical models and to provide a detailed analysis of the performance of the as-built configuration and to show the effectiveness of friction pendulum systems for seismic protection, which will ultimately lead to guidelines for the assessment and design of similar structures as part of pre-Normative research in support to the Eurocodes.