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• Low temperatures improve compressive resistances and elastic stiffness of CFST stub columns. • Low temperature reduces the confinement effect of steel tube and thus the ductility of CFST column. • Increasing the D/B generally improves the compressive behaviour of S-CFST columns. • The compressive stress of C-CFST is higher than S-CFST at low temperature. • Developed equations predict well compressive resistances of CFST at low temperature. This paper studied the axial compressive behaviours of both circular concrete-filled-steel-tube (CFST) stub columns and square CFST stub columns at low temperatures through a 23-test program. In the testing program, the low temperatures of 20 °C, −30 °C, −60 °C, and −80 °C, D/t ratios of 41, 29, and 23, different grades of normal weight concrete of C30, C40, and C50, D/B ratios of 1.0 and 1.5, and different circular and square shape of cross section were studied in this test program. The test results reported different failure modes of square and circular CFST columns at low temperatures. The effects of these studied parameters on ultimate compressive behaviours of CFST columns at low temperatures were discussed and analysed that include load-shortening behaviours, ultimate compressive resistance, initial stiffness, and ductility ratio. The studies revealed that the low temperatures generally improves the compressive behaviour of stub CFST columns. Including the experimental studies, prediction equations in different code provisions, e.g., ACI318, AISC360, AIJ, Eurocode 4, and GB50936 were modified to estimate the ultimate compressive resistance of CFST columns at low temperatures. Predictions by these code provisions were validated by 23 reported tests to check their accuracy and reliability. Finally, modified Eurocode 4 prediction equations were recommended for evaluation on the ultimate compressive resistance of CFST columns at low temperatures. [ABSTRACT FROM AUTHOR]

This paper developed the circular ultra-high performance fiber-reinforced concrete (UHPFRC)-filled steel tubular stub columns (CUFTSCs) for buildings and bridge piers. Eccentric-compression behaviours of CUFTSCs were studied through a 16-test program, and the studied key influencing parameters were eccentricity ratio (e/r = 0.0, 0.30, 0.68, and 0.98) and thickness of steel tube (t s). Under eccentric compression, the CUFTSCs failed in flexure-governed mode and exhibited linear, nonlinear, and recession working stages. The nonlinearities of CUFTSCs under eccentric compression were mainly contributed by nonlinear mechanical behaviours of compression steel tube and UHPFRC and minor steel-tube local buckling. UHPFRC crushing and major steel-tube local buckling occurred to the peak load point and recession branch of N-Δ / N-δ curves of CUFTSCs, respectively. As the eccentricity ratio increases the failure mode of CUFTSCs gradually changed from the compression-governed mode to flexural-bending-governed mode. The ductility of CUFTSCs is significantly improved through using thick steel tube. The code equations in EC4, AISC and GB50936 were conservative on predicting N u - M u interaction curves of CUFTSCs due to the ignorance of tensile strength of UHPFRC. The developed analytical models offer the most accurate estimations on N u - M u interaction curves of CUFTSCs. Image 1 • N-M interaction behaviours of stub CFST columns with UHPC were studied. • CUFTSCs at eccentric loads fail in UHPC crushing, yielding and local buckling of steel tube. • The ductility of CUFTSCs is significantly improved through using thick steel tube. • Developed analytical models predict well N-M interaction behaviours of CUFTSCs. [ABSTRACT FROM AUTHOR]

Applications of ultra-high performance fiber-reinforced concrete (UHPFRC) in composite columns for high-rise buildings or bridges keep increasing to achieve improved structural performances. A testing program with 16 specimens was carried out to study the behaviour of UHPFRC-filled square steel tubular stub columns (UFST-SCs) under eccentric compression. Besides, influences of eccentric ratio (e/r) and thickness of steel tube (t s) on the behaviours of UFST-SCs were studied. The UFST-SCs under eccentric compression exhibited three working stages categorized as linear, nonlinear, and recession. Compression-flange yielding of steel tube and crushing of UHPFRC occurred at the elastic limit and peak load point of load-displacement curves, respectively. Despite the increasing e/r ratio decreased the stiffness and ultimate resistance of the UFST-SCs, its ductility was improved. Moreover, increasing thickness of steel tube enhanced the structural resistance of UFST-SCs under eccentric compression. The validations showed that EC4, AISC and GB50936 were conservative on estimating the N u - M u interaction relationships of UFST-SCs under eccentric compression. Considering the accuracy and reliability, EC4 was recommended to predict the N u - M u interaction curves of UFST-SCs. • Tests on behavior of CFST columns with UHPC under eccentric compression are performed. • UFST-SCs under eccentric compression failed in plastic buckling of steel tube and crushing of UHPFRC. • M-N interaction behaviors of CFST with UHPC under eccentric compression are reported. • Code equations were checked on M − N interaction curves of CFST columns with UHPC. [ABSTRACT FROM AUTHOR]

• Low temperatures improve compressive resistances of stub steel tubes. • Low temperature reduces the ductility of stub steel tubes due to early local buckling. • Classifications for cross section of steel tube at low temp requires modifications. • Modified code equations predict well compressive resistances of steel tubes at low temperature. • At low temp circular steel tubes exhibited more ductile behavior than square tubes. This paper reports the compressive behaviour of stub steel tubes at the low temperatures relating to cold-region environment. Axial compression tests were performed on 12 circular and 16 square/rectangular stub steel tubular columns at different temperatures of 20, −30, −60 and −80 °C. Including the low temperature levels, different D / t ratios were also studied for circular and square stub steel columns at low temperatures. The test results reported detailed information on failure modes, load-shortening behaviours, and strengths of stub steel tubular columns at low temperatures. Effects of low temperatures and wall thickness (i.e., slenderness ratio of the tube) on compressive behaviours of stub steel tubular columns were extensively analysed and discussed. The test results also revealed that current slenderness limit for Class 3 type of square cross section is less conservative at low temperatures due to that the increased yield strength delays the occurrence of local buckling. Modified prediction equations in Eurocode 3, AISC 360-10, and GB50018 were adopted to predict the ultimate compressive resistance of stub steel tubular columns at 20 ∼ −80 °C environmental low temperature. Their accuracies were also validated by 28 test results in this paper. Some conclusions were then drawn through these experimental and analytical studies. [ABSTRACT FROM AUTHOR]