Cederwall, Engstrom, and Grauers 1990

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This paper presented results of experimental tests of rectangular CFT columns using high-strength concrete. Eighteen slender columns with warying degrees of eccentricity were tested to evaluate the advantages of high strength concrete in CFTs, the confining effects of composite sections, and the shear transfer at the steel/concrete interface. The main variable parameters in the tests were the concrete and steel strengths, the thickness of the steel tube, and the load eccentricity. Additionally, CFTs with interior reinforcement and CFTs with a debonded interface were examined.

Experimental Study, Discussion, and Results

The rectangular CFT columns in the test were simply supported and subjected to an increasing axial load until failure of the member occurred. Each long column test was accompanied by a corresponding stub column test having the same material properties and cross-sectional geometry to analyze the squash load. It was shown that the axial load capacity of the CFT section was about 6% higher than the capacity calculated by summing the individual strengths of the steel and concrete. The 6% increase in total capacity corresponds to a 15% increase in the capacity of the concrete core. The authors stated that this increase in strength has not, as a rule, been shown for rectangular CFTs. The experiments showed two distinct points of stiffness degradation as the member was loaded. The first occurred at the onset of compression steel yielding, and the second at the onset of tensile steel yielding. Therefore, the authors suggested that the maximum load bearing capacity of the section was determined by the strength of the steel tube. Furthermore, as the ratio of the nominal concrete load bearing capacity to the nominal capacity of the steel, Pco/Pso, increased, the ratio of the load bearing capacity of the long column to the capacity of the short column, Pu/Po, decreased.

Several parameters were varied in the course of the experimental study. The strength of the concrete in the CFT column had a greater effect on the ductility of the secdtion than on the strength of the section. As the concrete strength was increased, the column exhibited an increase in the amount of ductility, but a somewhat less significant increase in strength. The ductility at deflections beyond the section's ultimate strength was attributed to the reserve capacity of the concrete, which was underutilized at the ultimate load (the tests showed an average concrete load of 0.3*Pco at Pu). This underutilization of the concrete in slender columns decreases its contribution and relative importance to the axial load capacity of the section. An increase in the thickness of the steel tube resulted in an increase in the load bearing capacity of the section, as did an increase in the yield strength of the steel. Increasing the yield strength, however, resulted in a loss of ductility in the section. The effect of increasing load eccentricity was a decrease in the load bearing capacity and stiffness of the column, and an increase in the ductility. The high-strength concrete increased the load bearing capacity of CFT columns with little or no eccentricity. However, the contribution to the ductility of the section was realized only in the long CFT columns subjected to large eccentricities.

The ductile behavior of the CFT columns depended integrally on the existence of bond between the steel and the concrete. In tests where the materials were debonded, the concrete core did not contribute to the column behavior when the steel alone was loaded and the column behaved as a hollow steel tube. When the concrete alone was loaded, the strength of the section increased because the steel confined the concrete to some extent.

References

Cederwall, K., Engstrom, B., and Grauers, M. (1990). “High-Strength Concrete Used in Composite Columns,” Second International Sympsium on Utilization of High-Strength Concrete, Hester, W. T. (ed.), Berkeley, California, May, pp. 195-214.