# Hardika and Gardner 2004

An experimental study on square CFT beam columns with normal and high strength concrete was presented. Calculated values of strength for the beam columns were determined using conventional section analysis (i.e. equivalent rectangular stress block) with a modified concrete strength.

## Experimental Study Results and Discussion

Twenty four CFT cantilever beam columns were tested under constant axial load and cyclic lateral displacement. The main test parameters were concrete strength (5.8 or 14.5 ksi), thickness of the steel tube (3/8 or 3/16 in.), orientation of the plane of bending, and magnitude of the axial load. The lateral displacements were applied either, square (parallel to one of the sides of the square cross section) or diagonal (parallel to a diagonal of the cross section). For each combination of concrete strength, tube thickness, and orientation, a set of three beam-columns with different axial load were tested. The steel tubes were square HSS sections with an 8 in. side length and nominal yield strength of 50 ksi.

Columns loaded in the square orientation were deliberately placed such that the plane of bending was offset 1/2 in. from the centroid of the section, introducing torsion and small biaxial effects. This was done to obtain results that were less than those of perfectly oriented specimens. The lateral displacement was controlled 71 in. above the point of restraint, though the beam-columns only extended 60 in. above the point of restraint. The test results indicated that the moment capacities of the beam-columns loaded in the diagonal orientation were only slightly less than those loaded in the square orientation. The authors suggested that this finding could justify reduced calculations of biaxial moment capacities for various orientations. No local buckling was observed prior to yielding. The moment strength, ductility, and flexural stiffness measured from the experiments are presented.

## Analytical Study

A review of several design codes was presented, with an emphasis on the methods of section analysis. The authors proposed a method in which the concrete stress is represented by a rectangular block extending to the neutral axis and with equivalent stress equal to the product of three corrective terms and the concrete cylinder strength. The three corrective terms relate to the concrete cylinder strength, the confinement of provided by the steel tube, and rate of loading. Interaction diagrams were created assuming the proposed method and compared to experimental results.

## References

Hardika, M. S. and Gardner, N. J. (2004). “Behavior of Concrete-Filled Hollow Structural Section Beam Columns to Seismic Shear Displacements,” ACI Structural Journal, Vol. 101, No. 1, pp. 39-46.