Cheng, Hwang, and Chung 2000

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Beam-to-circular CFT column connections were tested under cyclic loading in this work. The authors investigated the effect of concrete infill and D/t ratio on the seismic performance of the connections. The research program included four external diaphragm and two through diaphragm details. In this paper, test results of external diaphragm specimens were presented.

Experimental Study, Results, and Discussion

Each specimen consisted of two girders connected to the mid-height of a circular column. Three specimens had CFT columns and the other three had tubular columns. Only one specimen, which had a CFT column with external diaphragm detail, was designed for strong column-weak beam condition and the others were designed for weak column- strong beam condition. The L/D ratio for all of the columns was 6.5. The D/t ratio of the steel tubes was either 40 or 66.7 for 0.394 and 0.236 in. wall thicknesses, respectively. The yield strength of steel tube was 56.86 ksi for 0.236 in. thick tubes and 45.5 ksi for 0.394 in. thick tubes. The compressive strength of concrete was either 3.771 or 3.916 ksi. The yield strengths of the flange and web of the steel beam were 44.7 and 47.6 ksi, respectively. The columns were subjected to constant axial load throughout the test. Cyclic shear forces were applied at the girder-ends and displacement controlled loading was utilized.

Among the external diaphragm connections, the ones designed for weak column-strong beam condition experienced local buckling of the column wall. In addition, some inelastic deformation at the panel zone was observed. In the case of the strong column-weak beam connection, plastic deformation initiated at the girder-flange. The plastic deformation proceeded with local buckling of the column wall and inelastic deformation at the panel zone. The failure occurred at the weld between the girder-flange and external diaphragm. This connection exhibited a stable hysteresis loop and it had increasing stiffness after yield.

For all of the specimens, panel zone deformations did not contribute to the total girder-end deformations while the contributions of column and beam deformations were 60% and 40%, respectively. The concrete infill improved the cyclic behavior and CFT specimens had better strength, post-yield stiffness and ductility properties. As the tube wall thickness reduced, the gain in strength due to concrete infill increased. However, the specimens became more susceptible to local buckling and panel zone deformation.

References

Cheng, C., Hwang, P., and Chung, L. (2000). “Connection Behaviors of Steel Beam to Concrete-Filled Circular Steel Tubes,” Composite and Hybrid Structures, Proceedings of the Sixth ASCCS International Conference on Steel-Concrete Composite Structures, Xiao, Y. and Mahin, S. A. (eds.), Los Angeles, California, March 22-24, 2000, Association for International Cooperation and Research in Steel-Concrete Composite Structures, Los Angeles, California, pp. 581-589.