Optimum reinforcement ratio of axially loaded reinforced concrete-filled square steel tube column

Abstract

For the purpose of clarifying the influencing mechanisms of the longitudinal reinforcement on loading performances of Reinforced Concrete-Filled Square Steel Tube (R-CFSST) short columns under axial load, experimental and numerical studies were performed. Setting the longitudinal reinforcement ratio as parameter, ten R-CFSSTs with five different ratios and three Concrete-Filled Square Steel Tube (CFSST) specimens for comparison were fabricated and tested. Based on the test, the influencing law of longitudinal reinforcement on failure, loading behavior, stiffness, peak load, composite efficiency, ductility and stress state of steel tube of R-CFSSTs were theoretically investigated. A novel material model and simulation method for concrete core was proposed and verified against the test, and thereafter 32 model specimens with four wall-thicknesses of steel tube coupling with eight reinforcement ratios were simulated. The strength prediction equations from ANSI/AISC A360–16, Eurocode 4 and AS/NZS 5100.6 and GB 50936–2014 were studied, modified and compared each other, and based on the test and simulation results, novel equations were proposed and validated against the experiment. The results demonstrate followings: longitudinal reinforcement significantly impacts the behavior of R-CFSSTs as the steel tube does, and thus it is recommended to configure reinforcements in CFSST for the improved performances; proper ratios of longitudinal reinforcement enable the R-CFSSTs to acquire better balance between performance and construction cost, and a range for the optimum ratios is recommended between 1.0% and 3.2%, regardless of wall-thickness of steel tube; the proposed equations are recommended for more accurate and stable prediction of load bearing capacity of R-CFSSTs.