Development of uniaxial interaction diagram for circular concrete filled steel tube columns using finite element analysis

Abstract

In recent times, concrete filled steel tubes have gained worldwide acceptance in the construction of high rise and large span structures due to their structural efficiency. Steel tube serves as both reinforcement and formwork, eliminating the need for both, and provides large tensile and compressive capacities. The concrete fill restrains buckling of the steel tube, which increases the strength and stiffness of the section. However, their designs involve tiresome calculations and interaction charts development procedures. This study involves numerical investigations on the performance of concrete-filled-steeltube (CFST) finite element models subjected to axial loading. Nonlinear material models for confined concrete and steel tubes were used. The results obtained from the finite element analysis were verified against prior experimental results by comparing its results and axial load-displacement curves. Finally, outputs from the finite element analysis were used to develop uniaxial interaction charts for circular CFST columns. Axial load eccentricity due to lateral mid height deflection of column, P-Δ or second order effect was considered to determine moment capacity. The study shows confinement of concrete core provided by the steel tube increases strength and ductility of concrete, which considerably maximizes axial load and bending moment capacity of the CFST column by about 25% and 17% respectively. It has been noted that increase in diameter of circular CFSTs enhanced load and moment capacity. It was also observed that eccentric axial loading conditions have notable effects on load resistance of a column. This numerical analysis perceived that column length also have a remarkable effect in which mid height deflections were pronounced by 82% for longer columns. The proposed interaction diagram was compared with that predicted by Eurocode4. The comparison confirmsthat strength of CFST column computed by EC4 is more conservative.