CYCLIC RESPONSE AND NONLINAR MODELING OF EXTERIOR UNREINFORCED RC BEAM-COLUMN JOINTS WITH PLAIN LONGITUDINAL BARS

Abstract eng:
The seismic performance of existing Reinforced Concrete (RC) frames is significantly affected by the behaviour of beam-column intersections involved in the collapse mechanism, especially in non-conforming buildings with poor structural detailing or completely unreinforced joints. Even if a quite significant amount of studies on seismic performance of unreinforced joints has been carried out in last years, a very limited number of them deals with specimens reinforced with plain hook-ended longitudinal bars, widespread in Italian and Mediterranean building stock, or with the analysis of local aspects, such as the evaluation of joint shear strains. The majority of the models proposed in literature to simulate the cyclic behaviour of RC joints were developed and calibrated by means of tests performed on elements with deformed bars. Thus, these models may be not adequate for elements with hookended plain bars, especially due to the peculiarities in the interaction mechanisms between concrete and steel for this bar typology. This study first analyses the cyclic experimental tests of four full-scale exterior unreinforced RC beam-column joints with longitudinal plain bars in beams and columns carried out by the authors, in continuity with a previous experimental campaign. The specimens mainly differ for joint aspect ratio and beam longitudinal reinforcement ratio. Global and local responses of such tests are analyzed. The joint deformation mechanisms – rotation at the interface between beam/columns and joint, and shear deformation of the joint panel – are also analyzed. Then, a numerical modelling approach is carried out by OpenSees software to reproduce the experimental cyclic response. Bond-slip is particularly taken into account by introducing a slip spring whose properties are calculated using a bond-slip model properly proposed in literature for plain bars. The numerical results are presented and finally compared with the experimental results highlighting the influence of nonlinear response of joint panel and bondslip mechanism in the response of the sub-assemblage.

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