Study on Flexural Behavior of High-Strength Reinforced Concrete Columns Confined by Steel Tube

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An experimental study of monotonic flexural behavior of reinforced concrete columns confined by square or circular steel tube was conducted to investigate effects of lateral confinement by steel tubes on ultimate flexural properties of the confined high-strength reinforced concrete columns. Twenty test columns, ten of which were confined by square steel tubes and the others by circular steel tubes, were made of high-strength concrete with compressive strength of 60MPa or 100MPa, and all of these columns were tested under combined monotonic bending moment and constant axial compression. Test results indicated that flexural response as well as ultimate properties of a confined high-strength concrete column not only depended upon wall thickness of the steel tube, but the axial load level as well. It has also been shown that confinement effect of circular steel tube was much higher than that of square steel tube when the steel tubes had the same wall thickness. The most interesting experim … More ental result obtained in this study was that the compressive strain at the extreme concrete fiber of the confined concrete section developed at the maximum moment was not a constant as recommended in many current design codes, but a variant increasing as the confinement degree provided by the steel tubes almost regardless of the axial load level.Parallel to the experimental study described above, an analytical study of the flexural response of confined high-strength reinforced concrete columns was carried on to develop a simple and practical method of theoretically evaluating the ultimate moment and deformation capacity of confined concrete column. Based on the results of parametric study, a mathematical expression for predicting the ultimate strain of confined concrete was developed. Furthermore, two rectangular equivalent stress blocks were proposed to simplify the calculation of ultimate moment capacity. One stress block was proposed for columns confined by square steel tube and the other was for columns confined by circular steel tubes. Good agreement between the measured results and the theoretical predictions verified validity of the proposed stress blocks. Less