Detailed seismic modelling of portal RC frames with precast beams connected by pocket joints at the upper column ends

Past research works and post-earthquake field missions have revealed the vulnerability of a number of portal RC frames made of precast RC members using pocket connections to join beams and columns. Typically the beam-column joint in these frames is assumed for analysis purposes as a simple pinned joint so that relative translations between beam and column ends are neglected while allowing the free rotation of these ends; however this assumption cannot always be justified, particularly when slippage at the beam seat can occur or when the beam ends make contact with the back walls of the pocket connection as a result of slippage and relative rotations of the beam with respect to the column. Furthermore, modelling the above connections as pinned joints does not allow the assessment of beam seat loss as a possible cause of collapse. This article introduces a new approach to model the nonlinear response of the above joints accounting for the kinematics of the problem as well as material nonlinearities. To that effect the joint is modelled by an assembly of gap elements, frictional joints, rigid links and nonlinear fibre elements. A series of nonlinear inelastic time-history seismic analyses of precast RC portal frames of existing industrial buildings are included in the study. Results demonstrate the ability of the new model to predict both the frictional sliding at the beam seat region and the pounding between the beam and the back wall of the pocket connections.