Huang, Huang, and Zhong 1991

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The test results of 46 CFT members under combined compressive, flexural, and shear loading were presented. The objectives of the experiment were to plot hysteresis loops for cyclic loading, determine the ductility ratios and absorbed energy ratios of the tested members, observe the failure mode and the effect of different parameters (i.e., slenderness ratio, steel ratio, and the ratio of applied axial load to the section's squash load) on member behavior, and to analyze the difference between the behavior of CFT and reinforced concrete members.

Experimental Study, Discussion, and Results

The test members were oriented vertically, fixed at the base, and attached rigidly to a frame through which the lateral load was applied. An axial load P was applied first and then a lateral load H was applied until the member reached its maximum capacity. The plots of the hysteresis loops for the CFT member were very full and there was no deterioration of stiffness as the loads were cycled, except in specimens with small axial load ratio (P/Po). The failure of the specimens was due to a combination of compression and bending, manifested by a cracking of the concrete and local buckling around the entire perimeter of the tube at the top of the column where the loads were applied. Failure was preceded by four distinct stages of loading. Up to 0.4*Hmax, the member remained elastic. At this point, confinement began as Poisson's ratio for concrete exceeded that of the tube. The ratio of circumferential to longitudinal strain in the tube at the point of maximum compression at the top of the member was approximately 0.4. The portion of the load-deformation curve at this stage was non-linear. At 0.75*Hmax, the tube began to deform plastically. The concrete continued to expand laterally at a greater rate than the steel and the confinement increased. The ratio of circumferential to longitudinal strain at this stage was more than 0.8. Strain hardening occurred here for some specimens while others began to show the formation of local buckling. The behavior in the final stage (Hmax and the subsequent unloading beyond this point) was governed by the P/Po ratio. For very small P/Po, no unloading occurred. The authors' discussion of the relation between H and P for higher values of P at failure was very nebulous.

Most of the specimens were able to absorb over 90% of the energy during cyclic loading. The P/Po ratio had the greatest effect of the variable parameters. Increasing P produced an increase in both the ductility ratio () and the amount of energy absorbed (area within the hysteretic loop). Increasing the steel ratio (As/Ac) increased ductility, energy absorption, and Pu; increasing the slenderness ratio had the opposite effect. Comparing the CFT member to the reinforced concrete member with the same slenderness ratio, steel ratio, and axial load ratio showed a value of Pu for the CFT of 2.174 times that of the reinforced concrete member. This gave a corresponding hysteretic loop with much greater area signifying better seismic behavior. Despite this, the ductility ratio and the energy absorption ratio of the CFT and the reinforced concrete members were very similar, indicating that these parameters were not good measures of seismic performance. Therefore, the authors recommended calculating the absorbed energy per unit volume of the member. Doing this, they found that the energy absorption capacity of the CFT was 2.9 times the capacity of the reinforced concrete member.

References

Huang, S., Huang, X, and Zhong, S. (1991). “Experimental Research on Behavior of CFST Member Resisting Lateral Load,” Proceedings of the Third International Conference on Steel-Concrete Composite Structures, Wakabayashi, M. (ed.), Fukuoka, Japan, September 26-29, 1991, Association for International Cooperation and Research in Steel-Concrete Composite Structures, pp. 107-112.