Experimental Performance Evaluation of Concrete-Filled Steel Tube Columns Confined by High-Strength Steel Bolts

Concrete-steel composite columns have been utilized prevalently as load-bearing members in structures. The present study investigates the uniaxial behavior of concrete-filled steel tube (CFST) columns, which were proposed to provide high-strength steel bolts as external confinement to enhance the structural behavior of CFST columns. A total of 18 CFST columns were designed, out of which 12 columns were confined with steel bolts and 6 columns were unconfined; all columns were tested under axial compression. A mixture of self-compacting concrete (SCC) was utilized to fill all the test specimens. This study proposed an innovative approach to increase the restriction on lateral expansion of steel wall during elastic loading stage, which is expected to enhance structural behavior of CFST column. Test parameters included the effect of steel bolt spacing, three different (L/D) ratios, and two diffrent (D/t) ratios. The test findings demonstrated that the compression strength, axial stiffness, toughness, and ductility behavior of CFST columns increase as the spacing of steel bolts decreases and the improvement becomes more pronounced as the (L/D) and (D/t) ratios decrease. In addition, the compression capacity of the improved CFT short, medium, and long columns were enhanced by 25.3, 9, and 3.5%, respectively. A design model was developed to estimate the ultimate compression behavior of the improved CFST columns using steel bolts, and a close correlation was obtained between experimental findings and the proposed model.