# Sato, Saito, and Suzuki 1991

A new method for determining the interaction of the materials comprising CFTs was presented in order to analyze the interactive effects of the concrete and the steel and to elucidate the issue of higher cumulative strength and ductility of the combined materials. A very detailed series of figures was included in the paper, giving clear and concise results.

## Experimental Study, Discussion, and Results

The tested circular CFTs were oriented vertically and pinned at both ends. The three specimens were subjected to a transverse reversed cyclic shear force at midheight with a constant axial load. Loads were applied to a rotation angle R equal to 6.0% over 13 cycles. After this, load was applied monotonically until R equaled 10%, the limit of the testing system. In spite of local buckling, the maximum strength during loading exceeded the cumulative strength calculated using the individual material strength in the fully plastic state.

## Analytical Study

Based on the results of the experiments (i.e., axial and shear forces, and deflections), the local buckling in the steel was analyzed first. By equilibrating forces at the site of local buckling and estimating the buckled wave form of the steel tube as a cosine function, the authors were able to calculate the critical strain in the steel at buckling. The material properties of the tube were obtained from tests. Axial and circumferential stresses were obtained such that the equivalent biaxial stress obeyed the von Mises yield criterion. With these values, the interactive calculation along the critical buckled section was carried out using a fiber element model, assuming linear strain distribution. To obtain the forces taken by the concrete, the results of the steel analysis were subtracted from the experimental results. Both shear forces and axial forces in the concrete could be obtained in this manner. The authors then discussed a procedure to determine the relationship between the two using a fiber element model. Also, they formulated the relationship between stress and strain in the concrete at the critical section. From the forces, the eccentric distance of the axial force (the centroid of the compression block) in the critical section may be determined.

The calculated hysteresis shape of the steel tube was similar to that of the experimental results of the CFT, indicating that the CFTs mechanical behavior is very much like that of the steel tube. After a deflection angle R = 2.5%, the axial and shear force taken by the steel gradually decreases and local buckling occurs.

A series of detailed graphs provided the results of the analytical study and compared them to the experimental values.

## References

Sato, T., Saito, Y., and Suzuki, K. (1991). “Force Contribution Analysis of Elements in Concrete-Filled Steel Tube Column Under Seismic Load,” Proceedings of the Third International Conference on Steel-Concrete Composite Structures, Wakabayashi, M. (ed.), Fukuoka, Japan, September 26-29, 1991, Association for International Cooperation and Research in Steel-Concrete Composite Structures, pp. 125-130.