| Reinforced concrete(RC) moment frames comprise a significant portion of the built environment in areas with seismic hazards. Investigation of laboratory and post-earthquake reconnaissance suggest that joint stiffness and strength loss can have a great influence on frame structural response, but the inelastic response of component is rarely considered in analysis or design of structure. To achieve accurate prediction of the full-frame numerical simulation of the reinforced concrete structures with models of the under reinforced beam-column joints, a new way, building joint element model, is invented and developed, the full-frame numerical simulation is performed using the joint element and common beam element.Based on Timoshenko beam element, the multilayered beam element is used for analysis of columns and beams. OpenSees concrete01 model is used for concrete, and Vecchio model is used for steel bars. The concrete UMAT and steel bar UMAT are built on the commercial platform of ABAQUS. To analyze nonlinear behavior of RC structure due to rebar bulking, based on the above Vecchio model, a formulation of a cyclic stress-strain relationship of reinforcing bars is presented. For the sake of easy use in engineering design, a finite element program based on ABAQUS has been developed. The model is comprised of a tension/compression envelope and cyclic loops. The tension envelope contains an elastic range and a linear hardening zone. The compression envelope also includes a linear elastic range followed by a nonlinear buckling model. The cyclic loops follow Ramberg-Osgood formulations with some modifications to account for the effect of buckling. A complete path-dependent cyclic constitutive model is then obtained by combining the equations representing the two monotonic envelopes and the cyclic loops. The numerical results compared with the experimental results demonstrate that the multilayered element depict the cyclic behavior of concrete elements accurately.To analyze the nonlinear hysteretic behaviors of RC frame structures, based on four-node plane stress element, a new element called super DOF element is developed to represent the response of reinforced concrete beam-column joints model under reserved cyclic loading.The super DOF element edges are divided into “joint plane” and “beam-column plane” at the joint-column interface and joint-beam interface. The inelastic mechanism of joint core is represented by the four-node plane stress element. The anchorage failures of beam and column longitudinal reinforcement embedded in the joint are determined by eight springs between “joint plane” and “beam-column plane”. The element has four exterior nodes and four interior nodes. There are two degrees of freedom on each interior node. There are three degrees of freedom on each exterior node, coinciding with ones of typical beam element, so that the element is appropriate for use with typical hysteretic beam-column line elements in two-dimensional nonlinear analysis of reinforced concrete structures. The element is implemented as a four-node twenty-degree-of-freedom element, according to moving the degrees of freedom on the interior nodes to ones on the exterior nodes. The proposed bean-column joint model is implemented in ABAQUS by using UEL.Correlation studies between the experimental response of several reinforced concrete subassemblies from previous experimental investigation of joints and the numerical simulation results show the ability of super DOF element model to describe the hysteretic behavior of RC joints. Moreover, the model is validated by series of full-frames. The analytical results are in good agreement with the experimental data in terms of strength and pinch. |
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