# Liu, Zhou, and Zhang 2008

This paper investigates the behavior of square concrete-filled steel tubular (CFT) square beam-columns subjected to biaxial moment. Nine tests on beam-columns are reported under a combined loading of constant axial load and cyclic load applied at varying angles to the axis of the cross-section. Specimens were prepared in order to evaluate the influence of different parameters on the overall structural response, their ductility, and energy dissipation ability; the parameters included the effects of axial load ratio, width-to-thickness ratio, concrete compressive strength, slenderness ratio, and load angle on the moment strength.

## Experimental Study, Results, and Discussions

A total of nine square CFT beam-columns, including five specimens filled with normal (7.08 ksi cubic strength) and four with high strength concrete (14.6 ksi cubic strength), were tested. All columns had a width of 5.91 in. Tube thickness was varied from 0.104 in. to 0.190 in. as a test parameter. Two different column lengths, 33.3 in and 43.3 in., were tested.

In the test procedure, the axial compressive load was applied and maintained constant before the cyclic transverse load was applied. The test setup consisted of the rigid L beam and a moveable truss system which allowed the L beam to move freely in vertical and horizontal directions with no rotation. In order to reproduce the type of restraint provided to a column in a real building frame, the test specimens were fixed at their ends by rigid steel beams.

Tests demonstrated that ductility and energy dissipation ability of biaxially bent normal and high strength concrete filled tube columns decreases with an increase in axial load ratio. Neither type of column are sufficiently ductile when subjected to axial load ratios greater than 0.5. The moment capacity decreases as the width-to thickness ratio increases for both normal and high strength concrete columns. Energy dissipation ability decreases with an increase in width-to-thickness ratio in normal strength concrete columns; energy dissipation ability is not greatly affected in high strength concrete columns. Moment capacity increases while ductility and energy dissipation ability decrease with an increase in concrete compressive strength. For both column types, ductility and energy dissipation ability increase with an increase in slenderness ratio. Diagonal load angle barely affects ductility and energy dissipation ability; moment capacity slightly decreases with increase of the diagonal load angle.

Experimental results were compared with moment strength predictions based on current Eurocode 4, Architectural Institute of Japan, American Concrete Institute, and American Institute of Steel Construction-Load and Resistance Factor Design code provisions for CFT columns. Moment capacity of biaxially bent square CFT columns can be predicted with reasonable accuracy using EC4 and AIJ code provisions. The ACI predicted moment capacity was slightly conservative compared with the test results while the LRFD code provisions greatly underestimated the capacity of square CFT beam-columns under biaxial bending moment.

## References

- Liu, J., Zhou, X., and Zhang, S. (2008). “Seismic Behaviour of Square CFT Beam-Columns under Biaxial Bending Moment.” Journal of Constructional Steel Research, 64(12), 1473–1482. doi:10.1016/j.jcsr.2008.01.013