Influence of slenderness on confining effect of axially loaded circular CFST columns with high-strength materials

Abstract

To investigate the influence of slenderness on the confining effect of circular concrete-filled steel tube (CFST) columns with high-strength materials, 20 tests on CFST slender columns with varying slenderness were carried out. As the increase in slenderness, the confining pressure at the mid-height section decreases and exhibits an uneven distribution across the section, which is attributed to the increasing bending moment introduced by second-order effect. To further investigate the effects of slenderness, concrete strength, and yield strength of steel tubes on the load-bearing capacity of the CFST slender columns, a parametric analysis was conducted. A three-dimensional constitutive model was adopted in the finite element model to simulate the distribution of confining pressure and the axial stress of concrete and steel tubes more precisely. The results indicate that with the increase in slenderness, the establishment of confining pressure is delayed with the magnitude reduced. When the relative slenderness is less than 0.5, the confining pressure is still effective with the strength enhancement index no less than 1. However, while the relative slenderness exceeds 1.0, the confining effect becomes negligible. The reduction factor, defined as the ratio of the load-bearing capacity of the slender column to that of the short column, is derived from the analysis. When comparing the CFST columns infilled with different grades of concrete, the reduction factor for CFST columns filled with normal-strength concrete (NSC) is lower than that for CFST columns filled with ultra-high-performance concrete (UHPC) because the former is more sensitive to confining pressure. The increasing yield strength of steel tubes contributes to enhancing the load-bearing capacity of CFST columns, however, the efficiency decreases with the increase in slenderness. Finally, a reduction factor accounting for the confining pressure is introduced, providing a comprehensive consideration of the confining effect related to slenderness on the load-bearing capacity of CFST slender columns. This analysis applies to steel tubes with a grade of up to Q890 and concrete with a compressive strength of up to 150 MPa.