Morino et al. 1993

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This paper presented a study of three-dimensional cruciform subassemblies composed of four steel girders framing into a CFT beam-column. Axial load was applied to the top of the CFT beam-column, constant loads were applied to the ends of the two girders framing into the minor axis of the beam-column, and anti-symmetric, cyclic loads were applied to the girders in the major axis direction of the beam-column. The CFT/steel connection was designed for two modes of failure: shear failure of the connection and flexural failure of the beam-column. In addition to presenting the test results, the paper discussed the hysteretic behavior, the maximum strength, the energy dissipation, and the failure configuration of each subassembly.

Experimental Study, Discussion, and Results

In all, ten specimens were tested, five failing by connection shear, and five by flexural failure of the CFT column. Each subset of tests consisted of a planar specimen (no out-of-plane girders), a specimen with no loads on the out-of-plane girders, one specimen with equal constant applied loads on the out-of-plane girders, and two final specimens with unsymmetric loads (biaxial bending of the CFT beam-column). The load applied to the in-plane girders was anti-symmetric (i.e., one beam was loaded in an upward direction while the opposite beam was loaded in a downward direction, then the loads were reversed). The specimens were cycled by increasing increments of rotation until failure or the limit of the testing apparatus was reached.

The specimens loaded uniaxially (either no out-of-plane loads or symmetric loads) exhibited a larger deformation capacity. Nonetheless, the biaxially-loaded specimens achieved rotation angles of R = ±4/100. The subassemblies failing at the connection showed slight pinching behavior, but no significant strength deterioration. Deformation of these specimens consisted of an S-shaped pattern in the plane of cyclic loading with yielding above and below the connection. The connection displaced laterally in one direction, but the column was able to sustain the axial load until the limits of the appartus were reached.

The column-failing specimens exhibited unstable deformations under three-dimensional loading when the rotation approached ±3/100 to ±4/100 These specimens also produced a larger strength degradation than the connection-failing specimens. Two types of failure mechanism were observed. The first, which occurred in the symmetric cases, consisted of local buckling of the CFT beam-column above and below the connection. The connection then displaced laterally leading to column instability. The second type of failure was similar to the first except that the beam-column underwent large plastic deformations below the connection in the out-of-plane direction. The result was again a failure due to lateral displacement of the connection area, leading to an instability failure.

The connection fainling specimens show less strength degradation, a larger deformation capacity, and more stable hysteresis loops than the comparable column-failing specimens. Biaxial bending of the CFT beam-column (when asymmetric loads are applied to the out-of-plane beams), results in a noticeably smaller deformation capacity in both types of structures because the bending axis rotates with repeated loading which causes earlier local buckling of the tube.

References

Morino, S., Kawaguchi, J., Yasuzaki, C., and Kanazawa, S. (1993). “Behavior of Concrete-Filled Steel Tubular Three-Dimensional Subassemblages,” Composite Construction in Steel and Concrete II, Proceedings of the Engineering Foundation Conference, Easterling, W. S. and Roddis, W. M. (eds.), Potosi, Missouri, June 14-19, 1992, ASCE, New York, New York, pp. 726-741.