# Konno, Kai, and Nagashima 1990

Experimental tests were conducted on square concrete-filled steel beam-columns to determine the ultimate strength of the member, focusing on the effects of local buckling of the steel tube and confinement of the concrete. The authors recognized a marked increase in both the strength and the ductility of the concrete due to the effect of confinement in the square tubes. They proposed a design formulation which accounts for confinement as well as local buckling of the steel tube.

## Experimental Study, Results, and Discussion

The tested specimens were subjected to combined axial load and bending, the latter of which was induced by an alternating transverse load at the midpoint of the section. The axial load on the specimen was kept constant throughout each test. A total of 19 specimens were tested. The variable parameters included the steel and concrete strength, the D/t ratio, and the amount of applied axial load.

The authors observed that the ultimate strength of the specimen was closely related to the onset of local buckling of the tube, which usually occurred prior to strain hardening of the steel. They also showed that square CFT members have a large capacity for energy-absorption, even members with a large D/t ratio or members subjected to high axial loads.

## Design Formulation

The analytical strength of a CFT was computed by superposing the individual strengths of the steel and the concrete. The steel strength was determined by computing an effective width, which was computed using an empirical relationship based on the D/t ratio and the yield strength of the steel tube:

The concrete strength was increased based on an empirical experiment-based formula to account for the effect of confinement:

The strength calculation using the modified effective width and concrete strength showed better agreement with the experimental values.

## References

Konno, K., Kei, T., and Nagashima, T. (1990). “Behavior of Concrete-Filled Square Steel Columns,” Proceedings of IABSE Symposium, Brussels, 1990.