Development of Circular Multicell Double-Skin Tubular Columns: Testing and Improved Axial Performance

Double-skin tubular columns (DSTCs) represent one of the most novel column systems for future structures, such as bridges, flyovers, high-rise buildings, and towers, due to their higher axial compressive strength and improved ductility characteristics. The enhanced confinement of the concrete is the primary reason for the improved axial compressive strength and superior ductility in DSTCs. However, the steel in DSTCs is localized to the outer and inner layers only, without any connecting element in-between inner and outer steel tubes. As a result, a significant sudden drop in the load-carrying capacity has been observed in DSTCs as soon as the outer or inner steel tube fails. However, the failure in inner and outer steel tubes can be significantly delayed or prevented if they are connected through stiffeners. The present study, through an extensive experimental investigation, focused on effectively strengthening and connecting both the outer and inner tubes in DSTCs by adopting various stiffener schemes. Additionally, the stiffeners split the concrete core into discrete concrete cells. Accordingly, in the current study, these columns have been referred to as multicell double-skin tubular columns (MC-DSTCs). From the experimental observations, axial compressive strengths in MC-DSTCs were higher than that in the unstiffened DSTCs by up to 62%. Further, MC-DSTCs exhibited larger axial deformations compared with the unstiffened DSTCs, improving their ductility index by up to 253%. The increased axial strengths and ductility in MC-DSTCs have been attributed to delayed and controlled failure of steel and concrete components because of the improved participation of inner and outer tubes through stiffener connections. Finally, a design model has been proposed based on the concept of concrete confinement in MC-DSTCs to accurately predict their axial compressive strength, which has been verified through a regression analysis. A good correlation (with R2=0.986) between the predicted and experimental axial compressive strengths of MC-DSTC was observed.