PUSH-OVER ANALYSIS OF FRP-RETROFITTED EXISTING RC FRAME STRUCTURES

FRP–wrapping of columns is considered one of the most effective retrofitting techniques for RC structures in seismic regions. Plastic hinge confinement by FRP-wrapping enables the development of large displacement or chord rotation ductility factors, as many experimental studies confirmed (e.g. [1]). Moreover, in some cases, it may avoid to activation of brittle failure modes, such as soft-floor mechanisms [2]. Nonlinear dynamic analysis is the most complete method to describe structural response under seismic action. Nevertheless, such technique is a very time-consuming and complex process, inadequate for general design applications. As such, recent years have witnessed an increased focus on the development of design/assessment procedures based on nonlinear static analysis (or pushover analysis). In the present study, nonlinear static and dynamic analyses of frame structures retrofitted by FRP are performed. Existing RC structures, not designed structures with seismic criteria are considered. Nonlinear analyses are performed by a fibre finite element model, considering appropriate cyclic constitutive laws for FRP confined-concrete, recently proposed in Ref. [3]. Different pushover procedures are adopted. For non-adaptive analyses, two different force distributions are considered, uniform and proportional to the first modal shape. For adaptive pushover procedures, Displacement-based (DAP) technique [4] is employed. In order to validate pushover procedures, Incremental Dynamic Analyses (IDA) [5] are carried out using a set of artificial timehistories derived to fit the Eurocode response spectra [6]. The limit state condition is defined as the attainment of ultimate strain in confined concrete or, alternatively, of a limit value of interstorey drift. Comparison of static against dynamic results, in terms of both capacity curves as well as interstorey drift profiles, leads to the conclusion that displacement-based adaptive pushover features the highest potential to better reproduce results of incremental dynamic analysis. The effect of wrapping in increasing ductility against seismic actions is studied. It is shown that FRP wrapping strongly increases structural ductility. Moreover, by adopting FRP-wrapping, the effect of local deficiencies of strength in some columns of the frame is strongly reduced, so increasing overall reliability of the structure. This aspect in particularly significant for existing RC structures, where often concrete strength in some columns is lower than average due to incorrect casting processes.