Triaxial Concrete Constitutive Model for Simulation of Composite Plate Shear Wall-Concrete Encased: THUC3

A new triaxial constitutive model [Tsinghua University Concrete Three-dimensional (THUC3)] of concrete was developed for high-fidelity finite-element (FE) simulation of composite plate shear wall-concrete encased (C-PSW/CE), which has been adopted as the lateral force-resisting system in numerous ultrahigh-rise buildings. In this formulation, a uniaxial constitutive model of concrete was introduced. The following five key features of concrete were included in the proposed model: compressive softening of unconfined and confined concrete, tension softening, pinching effect, shear softening, and strength degradation due to principal tensile strain. A triaxial constitutive law was also illustrated based on a fixed crack assumption. By assuming that the stress of the concrete can be decoupled in the crack coordinate system upon initial cracking, the triaxial constitutive law was assembled from the aforementioned uniaxial stress-strain relations. A new formulation is proposed to achieve strain decomposition and stress assembly in three-dimensional stress space. The numerical implementation of the stress update algorithm in an ABAQUS user material (UMAT) subroutine was illustrated. The modeling scheme and material constitutive laws of reinforcing bars, steel plates, and studs were also introduced. Subsequently, the developed triaxial constitutive model was validated at the material level by simulating the cyclic behavior of concrete and reinforced concrete (RC) specimens. The developed FE model showed good accuracy in simulating the hysteretic behavior, ultimate capacity, and residual strain in cyclic tensile loading. Two flexural critical C-PSW/CE tests, Specimens C30-5 and C40-5, were also completed and reported in this research, and 18 shear critical C-PSW/CE tests were collected from the existing literature. These tests were simulated to further validate the developed model at the structural level. Comparisons indicated that the proposed model showed a reasonable level of accuracy in terms of ultimate capacity, energy consumption, and failure mode. In addition, the modeling of studs by a nonlinear connector element is recommended based on comparisons between the test and FE results. The current AISC provisions for shear capacity of C-PSW/CE specimens were also evaluated relative to the computational and experimental results.