Experimental, numerical and design investigations on the performance of axially-loaded ECFT stub columns with spherical-cap gaps

Abstract

Elliptical concrete-filled steel tubular (ECFT) columns have been increasingly used in engineering practice in recent years due to their aesthetic appeal together with structural advantages. The spherical-cap gaps were generally found in the actual CFT structures based on the latest survey. However, related literatures on the ECFT stub columns with spherical-cap gaps (ECFT-SG) under axial load are rarely reported. Thus, this paper aims to reveal the effects of gaps on the performance of axially-loaded ECFT stub columns. In total, ten axially-loaded ECFT-SG specimens were tested to explore the effects of gap ratio (χsg) and steel yield strength (fy) on the failure modes, axial fore (N) – longitudinal shortening (Δ), strain responses, strength and ductility indexes, etc. Following this, a series of numerical models using a modified equivalent concrete stress-strain relationship were established to expand the parametric scope and further study the steel-concrete interaction stresses and the concrete longitudinal stress distribution. Finally, two strength prediction methods of ECFT-SG stub columns were presented. The results indicated that the initial concrete gaps would weaken the confinement effect provided by steel tube, thus decrease the initial stiffness, ultimate strength and ductility of the columns. With the χsg increasing, the overall bending and inward buckling phenomena would be observed because of the uneven damage of infill concrete. The two design methods proposed herein are expected to provide a reference to evaluate the safety of the existing CFT columns with initial gap imperfection.