Structural performance of circular CFST columns with circumferential gap imperfections

The present study investigated the influence of circumferential uniform gap imperfections ( <= 1.0%) on the structural behavior of circular concrete -filled steel tube (CFST) columns under axial compression. The study involved testing 20 CFST columns with circumferential uniform gaps with varying gap ratios (0.2% - 1.0%), diameter -to -thickness ratios (18 - 43), and concrete strength (68.5 MPa and 114 MPa). The presence of uniform gaps (0.2% - 1.0%) reduced the confinement effect of steel tube on core concrete, and peak load capacity decreased by 9.0% - 24.0%. The normalized concrete strength in high -strength concrete specimens with gap ratios ranged between 0.89 and 0.99, showing no interaction between steel tube and core concrete due to the less deformable nature of high -strength concrete. Core concrete exhibited a combination of vertical splitting and diagonal cracks distributed across the height of specimens with an increase in gap ratio, and local steel tube bulging was dispersed around the edge regions. A theoretical procedure was developed to assess the impact of uniform gaps on the stress state of steel tube and core concrete by determining their respective load contributions. The suggested approach reasonably estimated the gap ratio ' s nominal influence on the behavior of steel tubes and its considerable influence on the behavior of core concrete. Considering the experimental limitations, gap ratios were broadened through finite element modelling to investigate the gap ratio effect further. After examining the confinement mechanism of CFST columns and finite element results, an analytical formulation to predict the peak load capacity of CFST columns with circumferential uniform gaps was proposed and verified.