A study on connections of concrete filled steel tubes by using built-in steel bars

From 2006, authors started a series of experimental and analytical studies to establish a design method for a new connection system of concrete filled steel tube (CFT) by using built-in reinforcing steel bars (CFTR). Among the series, a pullout test of the built-in steel bars from the CFTRs has been early performed, which is orientated as a fundamental study for the connections of CFTR [1]. In the pullout test study, it is clarified that the influence of tube shapes (square and circular), the stress transfer capacities of bond by steel bars, ring bands in steel tubes and anchor plates of steel bars. The new connection system of CFTR may apply to an exposed-type CFT column base, where the built-in high strength steel bars contribute to transfer the axial force, bending moment and shear force from a CFT column to the foundation [2, 3]. The column base strength is much increased by the built-in steel bars. In other words, the built-in steel bars make the base plate and anchor bolts compact without any strength reduction in the column base. A stable elastic-plastic behavior is observed in the CFTR column base, and an evaluation method of the ultimate strengths is proposed for that. A further improved CFTR column base is the base without any base plate (non-base-plate CFTR column base), so that all of the stresses can be transferred through the built-in high strength steel bars [4]. A stable hysteretic behavior is observed in the column base, and the evaluation method of the ultimate strength is also proposed. It is noteworthy that the non-base-plate CFTR column base is applicable to the super high strength steel, because the base system does not require any full penetration welding, which sometimes causes brittle fracture in super high strength steel.