RESEARCH ON THE CREEP OF CONCRETE FILLED STEEL TUBULAR COLUMNS BASED ON THE GENERALIZED KELVIN CHAIN

Concrete creep is an inherent time-dependent behavior of concrete and an important composition of structural responses. Its calculating method is usually established on the basis of uniaxial test and theory. To explore the creep behavior of concrete-filled steel tubular columns, the creep of circular concrete columns and concrete filled circular steel tubular (CFST) columns were measured by a self-made and self-balanced loading device. The results show that the creep deformation of CFST columns is smaller than that of concrete columns. In the present concrete creep tests, the maximum difference between the two aforementioned creep deformations was close to 50%. It is attributed to the fact that the core concrete cannot exchange moisture with its external atmosphere environment so that no dry shrinkage and creep of the core concrete occurred. It is also attributed to the confining pressure of the steel tube. According to the theory of viscoelasticity, a Volterra integral equation for solving the concrete creep under uniaxial stress condition was established by introducing a model consisting of a multi-parameter Kelvin element chain. The model parameters were approximately expressed as a successive retardation spectrum. Therefore, the constitutive relationship of concrete creep stress and strain increments under uniaxial stress condition was derived by discretizing the time variable t and integration by parts. Based on the principle of creep superposition and the Poisson effects, the constitutive model of the concrete creep stress and strain increments under uniaxial stress condition state was extended to a multiaxial stress state to analyze the CFST creep. The commercial finite element analysis software Ansys was secondarily-developed. The constitutive equation under multiaxial stress state for concrete creep performance was compiled with the Fortran language and introduced into the user subroutine Usermat of Ansys. Finally, the finite element numerical analysis for the long-term creep performance of CFST columns was carried out. Comparing the numerical solutions with the test results, it is found that the proposed method provides another effective approach to the analysis of concrete creep.