# Varma, Ricles, Sause & Lu 2002

This paper experimentally investigates the flexural force-deformation behavior of concrete-filled steel tube beam-columns, with parameters including width-to-thickness ratio, steel yield stress, and axial load level. Eight square CFT beam-columns were tested under constant axial load and increasing flexural loading. Existing codes have determined limits for d/t ratios, and equations for axial load and moment capacity can be determined, assuming that the moment capacity for a CFT beam-column can be predicted using that for steel beam-columns. The EC4 provisions for calculating the moment capacity use a plastic stress distribution on the composite cross section, while maintaining compatibility between the steel and concrete. The P-M interaction curve is formed by joining four points: the pure axial load capacity, pure flexural moment capacity, combined axial and bending moment capacity where the plastic neutral axis is at the centroid, and the combined axial load and bending moment capacity resulting in the same Mu as the flexural moment capacity.

## Experimental Study, Results, and Discussion

The specimens are ¾ scale models of a base column in a six-story moment resisting frame. Eight specimens were tested with d/t ratios of 32 and 48, and with varying steel thicknesses and axial load levels. Two types of steels were tested including A500 Grade-B steel and A500 Grade-80 steel, filled with high strength concrete. The tests included pure axial loading tests and combined constant axial load and monotonically increasing flexural loading. The ends of the specimens were attached to bearings with pin-pin end conditions. The axial load was applied and maintained using the test machine in the Fritz laboratory, and flexural loading was applied by imposing increasing rotations at the ends of the specimens using two hydraulic rams. Linear variable displacement transducers were used to measure in-plane displacement, and rotation meters were used to measure rotation of the bearings. Moment-curvature graphs were created, and it is seen that yielding of the steel tube flanges occur with increasing flexural deformations. The specimen achieved its moment capacity and failed close to midheight due to local buckling of the compression flange and concrete crushing. Local buckling of the webs was also experienced close to midheight, at .91Mu. Many of the specimens behaved in a similar manner, and many of the specimens failed at midheight. The specimen with the highest moment capacity had a d/t ratio of 32, used Grade 80 steel, and used 40% of the axial load capacity. The study also indicates that increasing the nominal yield stress of the steel while maintaining b/t ratio and axial load level increases the moment capacity.

## References

Varma, A., Ricles, J., Sause, R., and Lu, L.-W.(2002). “Experimental Behavior of High Strength Square Concrete-Filled Steel Tube Beam-Columns.” Journal of Structural Engineering 128 (3), March, pp. 209-318 [[doi:10.1061/(ASCE)0733-9445(2002)128:3(309)]