The Plastic Limit Stability Analogy between Centrifugal Concreted-Filled Steel Tubes and Cylindrical Shells with Spiral Stiffeners under Torsion

The plastic buckling calculation of the centrifugal concreted-filled steel tubes under torsion was derived from the cylindrical shells with spiral stiffeners analogy. In applying the analogy, it is assumed that under torsion, concrete compression parts between the diagonal cracks work as spiral stiffeners of steel shells, based on the plastic performance and mechanism of the tubes under experiment. An analytical method was proposed for the torsional plastic stability of the cylindrical shells with spiral stiffeners. The method was applied to the tubes of varied slenderness ratios (3, 5, 10, 12, 15) and varied steel ratios (0.12, 0.17, 0.236), and the results indicate that the torsion resistant plastic stability of the tube increases by 6% when the steel ratio is 0.12 or 0.17, and the slenderness ratio varies between 5 and 15; and that the torsion resistant plastic stability of the tube can increases by 20% reaching its top, when the steel ratio reaches 0.236.