Behavior of Concrete-Encased CFST Members under Axial Tension

This paper studies the behavior of concrete-encased concrete-filled steel tube (CFST) members under axial tension. Experimental results of thirteen specimens were reported. The variables in the test were the diameter of the steel tube, bond condition between the inner CFST and outer reinforced concrete (RC) components. Push-out tests were conducted to investigate the bond performance between the inner CFST and outer RC components. A finite element model (FEM) was then developed to perform further analysis on the tensile behavior of the concrete-encased CFST member. The internal load distribution between the inner CFST and outer RC components, the interaction between the core concrete and steel tube, and the interaction between the outer concrete and steel tube were investigated. The core concrete and steel tube worked together well, and the tensile strength of the steel tube was enhanced because of the "composite effect,"while the outer RC component was separated from the inner CFST and thus worked independently. In addition, the bond strength between the steel tube and outer concrete is larger than that between the steel tube and core concrete. Based on the full range analysis and parametric analysis, a simplified model was proposed to predict the tensile strength of concrete-encased CFST members, and generally good agreement was achieved between the predictions and test results. (C) 2015 American Society of Civil Engineers.