# Nakahara and Sakino 2000a

Monotonic and cyclic beam-column tests were conducted on square CFTs under constant axial load and uniform bending. The authors presented the experimental results and proposed an elasto-plastic analysis method for square CFT beam-columns.

## Experimental Study, Results, and Discussions

The objective of the experimental study was to examine the axial and flexural behavior of square CFT beam-columns. Uniform bending moment and constant axial load were applied at the ends, which were attached to cylindrical bearings simulating pinned end conditions. The main parameters were axial load ratio, D/t ratio, and deformation histories (monotonic and cyclic). The D/t ratio ranged between 34 and 98. The axial load ratio was either 0.2 or 0.4. The nominal strengths of concrete and steel were 7.25 ksi and 58.01 ksi, respectively. The steel tubes were annealed and as such they were free from residual stresses. The L/D ratio was 3 for all of the specimens. The loading was controlled by a dimensionless parameter obtained by multiplying the mid-height curvature by the depth of the specimen.

The flexural behavior of the cyclic specimens was mainly affected by the axial load ratio. The specimens with high axial load ratio showed a sharp drop in strength after achieving the peak strength, as well as a relatively high axial strain. On the other hand, the response of the specimens with low axial load ratio was stable and exhibited low axial strain, as may be expected. The effect of the D/t ratio was negligible for low axial load ratios and the response of these specimens was similar to the monotonic behavior. A rapid decrease in moment capacity was observed as the D/t ratio increased for the specimens subjected to high axial load.

## Analytical Study

The moment-curvature behavior of the specimens was obtained numerically using a fiber analysis. The concrete and steel stress-strain curves were based on a previous study by the authors. However, the descending branches were calibrated to account for the strain gradient existing in a column section under bending. The unloading and reloading parts of the steel and concrete stress-strain curves were adjusted from other curves found in the literature. The analytical results predicted the experimental behavior accurately. The moment capacities of the specimens were also calculated by full plastic moment and the results matched with experimental values unless the D/t ratio was large.

## References

Nakahara, H. and Sakino, K. (2000a). “Flexural Behavior of Concrete Filled Square Steel Tubular Beam-Columns,” Proceedings of the Twelfth World Conference on Earthquake Engineering, Auckland, New Zealand, January.