Experimental investigation of rectangular concrete-filled fiber reinforced polymer (FRP)-steel composite tube columns for various corner radii

To study the compressive behavior of rectangular concrete-filled fiber reinforced polymer (FRP)-steel composite tube (CFCT) columns, twenty-nine specimens were tested under compression loading in five series based on the corner radius. The variables are the corner radius of cross-section, the number of layers of FRP and the type of FRP wrapping. The experimental results revealed that the FRP wrapping may effectively suppress the local buckling for the steel tubes prior to FRP rupturing. The FRP fractures occurred at the corners in the middle of the specimens or at the longitudinal weld seams due to local stress concentrations. The stress-strain curves are influenced significantly by the corner radii for the rectangular CFCT columns. As the corner radius is increased, the linear ascending or descending slopes after the peak points of the stress-strain curve is generally enhanced, and a descending curve can be transformed into an ascending curve. For a given FRP confinement, the ultimate strength increased with increasing corner radius. Based on the experimental results and collected test data, a unified model with a simple and unified expression for predicting ultimate axial strength considering the shape and corner radius of the cross-section was proposed for the CFCT columns.