El-Remaily et al. 1997

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In this paper, the seismic behavior of high strength concrete-filled tube columns was studied. Four simply supported circular specimens were tested under constant axial and cyclic lateral loads. At the end of the paper, a finite element model for CFT members was presented as part of ongoing research.

Experimental Study Results and Discussions

The main parameters of the tests were axial load level and D/t ratio. High strength concrete mixes with nominal compressive strengths of 10 ksi or 15 ksi were utilized for the specimens. The nominal yield strength of the steel tubes was 54 ksi and the D/t ratio was either 32 or 48.

The columns were tested in a horizontal position and displacement-controlled loading was applied in the lateral direction at the mid-height of the CFT. In addition, a rigid stub was located at mid-height to simulate the confining effect of floor system. The applied axial load ranged from 0.2Po to 0.4Po, where Po was the nominal axial strength of the specimen. At each ductility level from 1 to 10, the specimen was cycled two times. The ductility level for the specimens was defined as the ratio of the lateral displacement (Δ) to the ratio of the first yield displacement (Δy).

The specimens did not show a decrease in their moment capacities up to a ductility level of 10. They exhibited large hysteresis loops and nearly perfectly plastic behavior after the elastic range. Local buckling was observed near the midpoint of the specimens at a ductility level of 4 and this caused the axial deformations to increase. The range of axial shortening was 1.69 to 3.07 in.. The failure of the specimens was due to tensile cracking in the steel tube.

Analytical Study

The plastic moment capacity and AISC LRFD (1993) moment capacity were calculated for each specimen. The plastic moments closely estimated the test results with an error of 10% to 20% on the conservative side. The error was mainly due to lack of accuracy in predicting the effects of confinement. The AISC LRFD (1993) moments underestimated the test results by factors ranging from 2.57 to 4.54. For all of the specimens, the ratio of the measured stiffness to the transformed stiffness, which was calculated as the sum of the individual stiffnesses of the steel and concrete, was observed to decrease as the ductility level increased. The resulting stiffness degradations at the ductility level of 10 were between approximately 25% and 80%.

The finite element model, presented as ongoing research, consisted of two parallel beam-column elements and two spring elements lumped at the nodes. The two beam-column elements represented the steel and concrete portions of the member, respectively. The spring elements simulated the bond between the steel and concrete. This model is intended to be used for seismic analysis of composite structures.

References

El-Remaily, A., Azizinamini, A., Zaki, M., and Filippou, F. (1997). “Seismic Behavior of High Strength Concrete-Filled Tube Beam-Columns,” Proceedings of the PCI/FHWA International Symposium on High Performance Concrete, New Orleans, Loiusiana, October 20-22, 1997, PCI, Detroit, Michigan, pp. 383-393.