# Inai and Sakino 1996

In this paper, a computational simulation study was described to predict the behavior of square CFTs subjected to constant axial load, bending, and shear. The computational formulation was applied to test specimens from the literature, and the findings were compared with the experimental results.

## Analytical Study

Stress-strain relations for the steel and concrete were proposed to be used in the numerical simulation of the behavior of square CFTs. It was assumed that the behavior of the concrete was the same as plain concrete up to the peak load. Confinement was assumed to occur in post-peak response. It was accounted for in the stress-strain relation by introducing a factor that depended on the confinement pressure, steel strength, and concrete strength. For the stress-strain relation of steel, local buckling was the key issue to take into consideration. If the compressive strength of the hollow steel tube decreased due to local buckling, no reduction in yield strength was assumed for the steel tube with concrete infill. This was because the concrete infill improved resistance to local buckling. After the peak load, the stress-strain curve of steel descended linearly down to a strain level and then the stress remained constant. The corresponding strain level depended on the steel strength and D/t ratio. The proposed stress-strain relations for steel and concrete were first used to estimate the behavior of the axially loaded square CFT specimens. Good correlation was obtained between experimental and analytical behavior.

An analysis procedure for square CFT columns subjected to constant axial load, bending moment and shear force was also developed. The purpose of the analysis was to calculate the moment and axial strain at the critical section, which was located at the middle of the column for the specimens considered in this research. A linear curvature distribution was assumed and an incremental relation between drift angle and curvature was derived in terms of flexural stiffness, shear stiffness, length of the column and length of the plastic hinge. The moment at the critical section was calculated incrementally with step size increments in curvature and drift angle. The axial strain was also calculated using an assumed strain distribution along the length of the column. The concrete and steel stress-strain relationships were adjusted for hysteretic loading and the moment-drift angle and axial strain-drift angle relations were compared to specimens tested by other researchers. The axial shortening and the behavior after the ultimate load were predicted well. However, the ultimate moments were found to be a little smaller than the experimental values.

## References

Inai, E. and Sakino, K. (1996). “Simulation of Flexural Behavior of Square Concrete Filled Steel Tubular Columns,” Proceedings of the Third Joint Technical Coordinating Committee Meeting, U.S.-Japan Cooperative Research Program, Phase 5: Composite and Hybrid Structures, Hong Kong, December 12-14, 1996, National Science Foundation, Arlington, Virginia.