Sugano, Nagashima, and Kei 1992

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This paper presented a brief overview of several tests of square and circular CFT beam-columns subjected to cyclic lateral loads. The strength and ductility of the beam-columns were examined with respect to the predicted capacities presented in the Japanese standard [Architectural Institute of Japan (AIJ)] which does not account for the confinement of the concrete.

Experimental Study, Discussion, and Results

Thirty-eight tests were performed; the test parameters were the type of tube (circular or rectangular), the amount of axial load, the width-to-thickness or diameter-to-thickness ratio (D/t ratio), and the strength of the materials. The cyclically loaded circular columns exhibited large hysteresis curves. The effect of the D/t ratio on the ultimate load was minor. For D/t ratios greater than 39, local buckling governed the ultimate capacity of the section, but the load did not decrease substatially after buckling had occurred. For smaller D/t ratios, the load continued to increase after local buckling. In the square tubes, local buckling determined the ultimate capacity for D/t ratios greater than 33. The square columns exhibited larger hysteresis curves for smaller D/t ratios, concrete strengths, and axial load ratios. The authors also observed rupturing of the steel tube at unspecified loads beyond ultimate.

Both the square and the circular sections provided a much larger amount of strength than that predicted by AIJ which using a simple superposistion of the strengths of the concrete and the steel. Ultimate strengths of 1.2-4.5 and 1.2-3.5 times the predicted value were realized in the tests of the circular and square CFTs, respectively. The strength enhancement due to the confinement of the concrete was larger in the circular sections.

The ductility of the CFT sections was gauged by the amount of displacement the beam-column could undergo while maintaining at least 95% of the ultimate load. The circular beam-columns showed a larger ductility than the corresponding square beam-columns (a rotation of 3.0-6.0% versus 2.2-3.3% for an axial load ratio of 0.3). Additionally, the ductility of the square sections decreased at a more rapid rate as the axial load ratio was increased. The higher strength materials produced marked decreases in ductility for both section types, as did an increase in the D/t ratio.

In conclusion, the authors reiterated their finding that both circular and square beam-columns show a significantly enhanced strength due to confinement, and that this fact must be accounted for in the design procedure. They also concluded that circular beam-columns are quite ductile even when subjected to high axial loads and square beam-columns are ductile for low values of axial load.

References

Sugano, S., Nagashima, T., and Kei, T. (1992). “Seismic Behavior of Concrete-Filled Tubular Steel Columns,” Proceedings of the ASCE Tenth Structures Congress '92, San Antonio, Texas, 13-16 April 1992, pp. 914-917.