Han and Yan 2001

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A series of monotonic tests were conducted on square CFTs including stub-columns, columns, and beam-columns. In addition, the authors presented analytical models to estimate the capacity and load-deformation response of the specimens.

Experimental Study, Results, and Discussions

The objective of the experiments was to investigate the strength and failure patterns of CFTs. Two sets of experiments were conducted. In the first set, twenty stub-columns were tested. Eight columns and twenty-one beam-columns were tested in the second set. The authors defined a confinement factor to account for the composite action between steel and concrete. This factor was used as a parameter in each set of experiments, with a range of values varying from 1.08 to 5.64. Other parameters included concrete strength, D/t ratio, eccentricity, and slenderness. The average measured yield strength of steel was 47.14 ksi and the measured cubic concrete strength ranged between 2.35 ksi and 7.15 ksi. The D/t ratio varied from 20.5 to 36.5.

The stub-columns were tested under load control. Two stiff base-plates were welded to the ends of the specimens. Local buckling was observed near or at failure. The columns with a high confinement factor maintained their strength after peak load. However, a rapid decrease in strength was observed for the specimens with a low confinement factor.

In the second set of experiments, the specimens were tested under axial load with eccentricities ranging from 0 to 3.15 inch. The column slenderness, which was defined as varied from 45 to 75 and the values of L/D ratio were between 11 and 18, approximately. The tests were performed under load control and pin-pin support conditions were simulated by the test setup. The eccentricity was provided by thick end plates with an offset triangular wedge. The failure of the specimens was due to overall buckling.

Analytical Study

The authors proposed stress-strain relations for steel and confined concrete. The stress-strain model for confined concrete was dependent on the confinement factor. Using these two models, a combined stress-strain curve was derived for the composite section. It was assumed that there was no slip between the steel and concrete. The predicted response was close to the stub column test results. The authors performed a regression analysis and obtained equations for cross-section strength and cross-section modulus (E). The equation for axial cross-section capacity was compared with AISC-LRFD (1993), AIJ (1997), and EC4 (1996) design code provisions using the experiments of different researchers. The proposed method showed the best correlation with the experimental results.

For columns and beam-columns, load versus mid-span deflection relations were derived using the proposed stress-strain models of confined concrete and steel. For this purpose, the deflection curve of the member was assumed as a sine wave. A basic layered approach was used, in which the cross section was divided into small rectangles parallel to each other along the width. The rectangle elements consisted of separate steel and concrete portions. The moment and axial load capacities were expressed as the summation of the moment and axial load from each rectangle element. Using these equations, the load versus mid-span deflection relations for the specimens were obtained. The predicted curves matched with the experimental results accurately. Simplified equations for axial load capacity and moment capacity were derived based on the linear regression studies. Interaction diagrams were generated for the specimens tested by several researchers using the proposed equations and the methods in the AISC LRFD (1993), AIJ (1997), and EC4 (1996) specifications. When the experimental and analytical results were compared, the proposed equations showed the best correlation.

References

Han, L.-H. and Yan, S.-Z. (2000). “Experimental Studies on the Strength with High Slenderness Ratio Concrete Filled Steel Tubular Columns,” Composite and Hybrid Structures, Proceedings of the Sixth ASCCS International Conference on Steel-Concrete Composite Structures, Xiao, Y. and Mahin, S. A. (eds.), Los Angeles, California, March 22-24, 2000, Association for International Cooperation and Research in Steel-Concrete Composite Structures, Los Angeles, California, pp. 419-426.