New Multiaxial Concrete Column Element with Adaptive Calibrated Constitutive Models

Abstract

A new calibrated multiaxial material model coupled to a multiaxial zero-length element is proposed for simulating the seismic behavior of concrete columns. The simulation tools are intended for use within a two-dimensional lumped-plasticity framework and are aimed at capturing the lateral cyclic behavior of nonretrofitted and retrofitted reinforced concrete columns subjected to seismic motions, up to complete loss of lateral strength. The material model was calibrated using a newly compiled database of 689 cyclic column test and only requires the input of column geometric and material properties to define envelope and cyclic behaviors, including identifying the appropriate strength-degradation modes, and the rate of strength loss due to cyclic loading. The proposed multiaxial material model houses several capacity models to capture salient lateral strength-degradation modes including flexure, shear, flexure-shear, splice, and flexure-splice modes. The proposed multiaxial material model allows each degree-of-freedom (DOF) to have seamless access to information such as force and deformation from other DOFs. Furthermore, it is capable of dynamically changing the sequence of execution of axial/shear/rotation DOFs based on their dependencies on other DOFs during the analysis. Such features allow the implemented capacity models to be adaptive in that they adjust their parameters during analysis to varying boundary conditions. Moreover, they make the proposed tools capable of further accounting for coupling between axial/shear/flexure actions and automatically adjusting the limit surfaces during analyses. The model takes a different approach to simulating cyclic behavior from existing models by utilizing an energy dissipation term as the central parameter to adjust the level of pinching in a cyclic response. A cubic spline curve is implemented to simulate the gradual stiffness changes and pinching effects during cycling, while cyclic damage functions capture the stiffness and strength damage rate observed in test data, even due to nonsymmetric cyclic loading. The performance of the proposed computational tools is compared with results from shake table tests. The coupled multiaxial material and element were implemented in the open-source simulation platform OpenSees as CMAC2D and CMAZ2D respectively.