Wu et al. 2005

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A new design of bolted beam-to-column connections for CFT is proposed. In this new connection, the end of the H-beam is welded with an end-plate; the end-plate is then connected to the square concrete filled tube with tie-rods. After the compressive strength of the concrete is fully developed, the tie-rods are pres-stressed. Flange wing plates and upstanding ribs are welded to the end of the steel beam. Welding is completed in factory to ensure quality control. A mechanical model is established in order to derive theoretical equations for calculating the stiffness, the yielding shear strength and the ultimate shear strength of the panel zone. A series of cyclic loading experiments were conducted to validate the mechanical model.

Analytical Study

The steel and concrete in a rectangular concrete filled tube both contribute to shear stiffness, yielding shear strength and ultimate shear strength and can be assumed to have independent behaviors. The shear stiffness and shear strength of the panel zone are contributed by the shear behavior of the webs and the flexural behavior of the flanges. The rectangular steel tube in the panel zone is divided into two regions with and without holes from bolts. Both regions contribute in series to the shear stiffness of the column webs. The column flanges and the beam end-plates are combined tightly together under the compressive pres-stress of the bolts and can be considered as a single body known as the generalized column flanges. The elastic shear stiffness contributed by the rectangular steel tube to the panel zone is the superposition of the shear stiffness of the column webs and the shear stiffness of the generalized column flanges. Because of strain hardening of the material and restraint of the surrounding components after reaching yielding point, the shear stress-strain curve extends continuously at smaller stiffness until the strain is four times the yielding strain. The regions with and without holes also contribute in series to the shear stiffness of the column webs. After the stress of the region with holes in the column webs reaches the yielding point, the shear stiffness contributed by the rectangular steel tube to the panel zone is the superposition of the shear stiffness in the column webs and the shear stiffness of the generalized column flanges.

Due to the existence of holes, the concrete in the panel zone is also divided into two regions, both of which contribute in series to the shear stiffness of the concrete. The elastic shear stiffness of the panel zone is the superposition of the individual stiffness of the rectangular steel tube and concrete. When the shear stress in the panel zone reaches the yielding point its yielding shear strength is the superposition of the yielding shear strength of the steel tube and the ultimate shear strength of the concrete. When the shear stress in the panel zone reaches the ultimate point its ultimate shear strength is the superposition of the ultimate shear strength of the rectangular steel tube and the concrete.

Experimental Study, Results, and Discussions

The experiment simulates a high rise structure with a model that spans over 6 m with a height of 3.2 m. To simulate the upper and the lowed inflection points of the columns, and H-beam is used. The H-beam is bolted to the mid-point of the column and the behavior of the minor axis of each H-beam is used to simulate the behavior of the inflection point. To apply the static loading, a horizontal cross beam was attached to the top of the column. 39 mm diameter tie rods are used to connect the ends of the cross beam to the strong floor; pre-stressed forces are applied at the ends of the tie-rods with a jack such that the beam is subjected to a downward tension. Three sets of bolted beam to column connections were tested. Columns were made of 400x400 mm square steel tube having thicknesses of 6,8, and 10 mm. The 10 mm thick specimen had a lower width-to thickness ratio with thicker steel tube; thus it had a stronger panel zone. Energy was dissipated through the flexural yielding of the beam end followed by buckling failure. This specimen had a higher ultimate strength but the strength diminished more rapidly after buckling. The 6 mm thick specimen had a higher width-to-thickness ratio with thinner steel tube, thus it had a weaker panel zone. The energy dissipated through the shear yielding of the panel zone. The strength of the beam was not fully developed but it retained its strength after failure. The 8 mm thick specimen had an intermediate with to thickness ratio. Energy was dissipated the combination of flexural yielding of the beam and shear yielding of the panel zone. Its strength was developed and sustained.

Stiffness, ductility and energy dissipation of the bolted connection were determined to be excellent. Results from the experiment validated the mechanical model.

References

  • Wu, L. Y., Chung, L. L., Tsai, S. F., Shen, T. J., and Huang, G. L. (2005). “Seismic Behavior of Bolted Beam-to-Column Connections for Concrete Filled Steel Tube.” Journal of Constructional Steel Research, 61(10), 1387–1410. doi:10.1016/j.jcsr.2005.03.007