Hsu and Yu 2003

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This report documents a study of the seismic performance of square CFT beam-columns with restrained plastic hinge zones. The restraint is provided by pairs of tie rods at expected plastic hinge zones. A simplified analytical model was created in ABAQUS to identify the most effective location for the tie rods. Cyclic tests were performed on 18 beam-columns.

Analytical study

A three-dimensional finite element model of only the steel tube was created in ABAQUS. The modeling of only the steel tube was justified by noting that the behavior is governed by the steel tube and the insufficient information on the interaction of the steel and concrete. As a further simplification the top half of the beam-column was modeled as a one-dimensional element, whereas the lower half was modeled with shell elements. Deformation of the unrestrained members was presented and based on those results locations for the tie rods were chosen.

Experimental Study, Results, and Discussion

The steel tubes (11 in. × 11 in.) were manufactured by welding cold-bent thin plates. Three different thicknesses were used, providing D/t ratios of 45, 60, and 85. Three different tie rod diameters were chosen corresponding to the three thicknesses. The yield stress of the steel tube was 46 ksi and the compressive strength of the concrete was 5 ksi. The bottoms of the members were fixed by means of rigid clamps to a pair of stiffened bases. The lateral load was applied at the top of the member. For each combination of tube thickness and axial load (0.1 Po & 0.3 Po) three members were tested. One with no restraining rods, one with a single layer of restraining rods, and one with two layers of restraining rods.

The tie rods were found to be effective in delaying local buckling by noting the interstory drift at which local buckling first occurred. The specimens with the thinnest steel tube exhibited tearing at the tube corners. It was found that the tie rods had little effect on the ultimate strength of the members but did improve the post-buckling strength. It was noted that the additional restraint was most effective at improving the post buckling response for the members with large D/t ratios. The tie rods were shown to have an insignificant effect on the stiffness. The largest benefit of the tie rods was seen in the ductility performance which was measured in two ways: the displacement ductility and the energy dissipation capacity. In the best case, the displacement ductility was improved 38% and the energy dissipation was improved 61% over the unrestrained members.

References

Hsu, H.-L. and Yu, H.-L. (2003). “Seismic Performance of Concrete-Filled Tubes with Restrained Plastic Hinge Zones,” Journal of Constructional Steel Research, Vol. 59, pp. 587-608.