Experimental Investigation on Concrete Filled Double Skinned Steel Tubular (CFDST) Column with Concrete Imperfections

This chapter delves into the experimental investigation of Concrete Filled Double Skinned Steel Tubular (CFDST) columns, focusing on the impact of concrete imperfections. Four key areas are explored: the types and effects of concrete imperfections, the ultimate load-carrying capacity, the load-deformation behavior, and the failure modes of CFDST columns. The study reveals that concrete imperfections, such as circumferential and rectangular gaps, significantly reduce the load-carrying capacity and alter the failure modes of CFDST columns. Circumferential concrete imperfections (CCI) show a reduction of approximately 19% for a 1 mm gap and 23% for a 2 mm gap, while rectangular concrete imperfections (RCI) yield a reduction of about 5% for a 4 mm gap and 16% for an 8 mm gap. The research also highlights the importance of proper construction practices to avoid such imperfections. The load-deformation behavior indicates that CFDST columns exhibit substantial axial deformation before failure, but concrete imperfections make them vulnerable to sudden failure due to premature local buckling. The failure modes observed include elephant foot-type local buckling, premature local plate bending, and sudden failure with local buckling of the entire cross-section. Confinement studies using strain gauges reveal that the confinement effect varies along the length of the column and is significantly affected by the presence and type of concrete imperfections. Overall, this chapter provides a detailed analysis of how concrete imperfections influence the structural performance of CFDST columns, offering valuable insights for improving design and construction practices.