Sakino and Tomii 1981

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The experiments contained in this paper were performed to examine the hysteretic behavior of square CFT beam-columns. Fifteen specimens were subjected to a constant axial load and a cyclic shearing force. The main test parameters were the D/t ratio, the axial load ratio, and the shear span ratio. Detailed hysteretic plots describe the effect of varying each parameter.

Experimental Study, Results, and Discussion

Essentially the same materials and the same apparatus were used in these tests as those in the experiments described in Tomii and Sakino (1979b). The difference in these tests was the nature of the loading. Each specimen underwent 3 cycles of lateral loading at increasing increments of displacement.

A number of conclusions were drawn based on the results. An increase in the D/t ratio or an increase in the axial load ratio brought on a decrease in the shear resistance (moment capacity) of the section at the unloading point. The a/D ratio had only a negligible effect. For specimens with a high axial load ratio (P/Po = 0.5), after a certain amount of decrease in the shear resistance, the hysteretic loops tended to stabilize and even showed a slight increase in shear resistance. The reason for this was the behavior of the tube at the ends of the section (critical regions). The square tube began to form a circular shape as it underwent successive local buckling at the critical regions. This transformation in shape effectively increased the amount of confinement of the concrete and resulted in the stabilization of the hysteretic loops. There was also a considerable amount of axial shortening observed for columns with a P/Po of 0.5 due to the combination of steel local buckling and concrete crushing. Values of axial shortening ranging from 27% to 34% of the section depth were measured. Accompanying the shortening and considerable bulging near the end of the sections subjected to a high P/Po ratio, a second wave of local buckling formed on the tube closer to the midpoint of the section. Finally, the ultimate moment was 1.0 to 1.2 times the value calculated by the method described in Tomii and Sakino (1979a, 1979b). The authors suggested that this was due to a combination of strain hardening in the steel tube and moment gradient effects in the confined concrete at the critical section.

References

Sakino, K. and Tomii, M. (1981). “Hysteretic Behavior of Concrete Filled Square Steel Tubular Beam-Columns Failed in Flexure,” Transactions of the Japan Concrete Institute, Vol. 3, pp. 439-446.