Structural Behaviors of Steel Tube-encased Concrete Columns Confined by Bolted Circular Thin Steel Tube

Six square steel tube-encased concrete (SC) columns confined by bolted circular thin steel tube were fabricated and tested under cyclical reversed lateral load to investigate their structural behavior. The primary experimental variables included the axial load ratio, the grade of the encased square steel tubes (FB rank and FC rank), the infilling of concrete into the encased steel tube, and the thickness of outer circular bolted thin steel tubes. Experimental results revealed that confinement by the bolted circular thin steel tube with outer-diameter-to-thickness ratio of 189 could ensure sufficient ductility to the SC columns, and the bolted thin steel tube did not rupture until the drift angle of about 0.09 rad. Furthermore, a simple evaluation method for the ultimate flexural strength of SC column section was proposed along with a numerical analytical method to predict the overall behavior of SC columns. The proposed methods can take the confinement effect by the bolted circular steel tube into consideration. Fairly good agreement between the experimental results and the calculated ones verified the reliability and accuracy of the proposed methods.