Behavior and general design method of concrete-filled high-strength steel tube (CFHST) columns

Abstract

Concrete-filled steel tubes (CFST) incorporating high-strength steel (HSS) could produce more efficient structural system with lighter weight and higher capacity. However, the design methods for applying concrete-filled high-strength steel tubes (CFHST) are not available yet and limited investigations had been reported. In this research, 8 full scale mid-slenderness circular CFHST specimens, made of innovative high-performance Q460qENH structural steel with actual yielding stress fy as high as 530 MPa, were tested subject to axial and eccentric compression. The main parameters considered in the experimental program included: (a) infilled concrete strength fc' = 39.8–75.3 MPa and (b) loading eccentricity ratio e/D = 0–0.3. The numerical model for CFHST was established and validated with load–displacement curves, failure modes and neutral axis locations obtained from the 8 experiments. The numerical models were further validated in capacity predictions with 232 axial compressions and compression-bending CFHST experimental data collected from the literature, proved to be generally applicable with fy = 435–835 MPa, ξ = 0.5–8.5 and λn = 0.07–1.90 in both circular and square sections. Based on the validated numerical models, total 526 simulations were performed to investigate the influence of: (a) higher yielding strength fy and thinner-walled steel tubes; (b) confinement factor ξ and (c) normalized slenderness ratio λn, on the composite strength fsc and compression-bending (N-M) interaction behavior. Experimental and numerical investigations showed that high strength steel could further improve the CFST capacity with basically no reduction in safety margin and ductility performance, but was in need of design method modifications due to changes in composite mechanism. On this basis, an analytical refined plastic-section equilibrium model (RPE model) was proposed to derive practical N-M interaction design curve with consideration of strength enhancement due to the composition and actual stress distribution at ultimate state. The further proposed general design method for compression-bending CFHST included: (a) formulas of fsc-ξ relationship; (b) validated formulas of pure-bending capacity and overall stability inherited from GB 50936; (c) practical design curves of N-M interaction considering the effect of ξ and λn. By comparing with GB 50936, AISC 360, EC 4 and CECS 28 provisions, the proposed method provided more accurate solutions in capacity predictions of CFHST members.