Numerical Modeling of Circular Concrete-Filled Double Steel Tubular Columns Under Fire Incorporating a New Concrete Confinement Model

The internal circular steel tube provides considerable confinement to the core concrete in a Circular Concrete-Filled Double Steel Tubular (CCFDST) column exposed to fire. However, no constitutive model has been developed for the core concrete at elevated temperatures considering confinement. This paper develops a numerical modeling method for fire resistance of CCFDST columns exposed to fire, which incorporates a confinement-dependent model for concrete at elevated temperatures. The confinement-dependent model is implemented through a MATLAB algorithm. The algorithm incorporates several segments: an average temperature model is proposed to address the temperature-gradient problem within a CCFDST column; the temperature-dependent failure criterion is formulated for determining the compressive strength of confined concrete; the temperature-dependent axial-to-lateral strain relation is established; equations are derived for calculating the instantaneous confining stress on the core concrete exerted by the inner steel tube; and an automatic error-control algorithm is designed to enforce the accuracy of the proposed stress–strain model for the core concrete. To verify the proposed model, the finite element model developed using ABAQUS is presented, which incorporates the proposed constitutive model for the core concrete. The comparative analysis shows that the developed finite element model predicts well with the existing test results. This suggests that the proposed model is reliable and can be implemented in numerical models for the fire performance simulations of CCFDST columns.