Sakino and Ishibashi 1985
This study expanded upon Sakino and Tomii (1981) by examining short beam-columns under shearing forces. Nine specimens with an a/D ratio of 1.0 were subjected to cyclic shearing combined with a constant axial load. Twelve specimens with an a/D ratio of 1.5 were tested monotonically with combined shear and axial loads. An effort was made to study the failure mechanism of such beam-columns. The paper closed with an analytical formulation to determine the ultimate strength of short beam-columns.
Experimental Study, Results, and Discussion
The procedure and test setup were nearly identical to those described in Sakino and Tomii (1981) and Tomii and Sakino (1979b). The monotonic tests with an a/D ratio of 1.5 displayed a crack pattern indicative of a flexure failure with plastic hinges forming at the ends. Previous monotonic tests with a/D ratios of 1.0 (Tomii and Sakino, 1979b) displayed the characteristic diagonal concrete cracking of a shear failure. The cyclically loaded specimens with an a/D ratio of 1.0 failed in shear, as opposed to the cyclically loaded specimens with a/D ratios of 2.0 and 3.0, which failed in flexure (Sakino and Tomii, 1981). (The authors define a “shear failure” as a failure in which the concrete fails in shear, contrary to Wakabayashi et al. (1976) and Kato et al. (1978) who consider this failure “flexural” since both flanges of the steel tube yielded at ultimate). These beam-columns showed considerable energy absorption and displayed less strength deterioration than the longer columns that failed in flexure. Like the longer specimens, the beam-columns tested here showed an initial decrease in capacity and then a slight increase as local buckling in the critical regions transformed the shape of the tube from rectangular to circular. Also, specimens with larger D/t ratios showed a larger decrease in shear resistance with successive cycles of loading.
Analytical Study, Discussion, and Results
The analytical method proposed in this paper to compute the ultimate strength of a CFT beam-column in shear was based on the formulations by Wakabayashi et al. (1981) and Kato et al. (1978). The axial load is computed by summing the axial load on the concrete and the steel individually. The ultimate shear force is likewise computed by a summation of the ultimate concrete strength in shear and the ultimate steel strength in shear. The details of the individual strength calculations are given in the paper. Resulting from these computations are graphs of axial load ratio (P/Po) versus the shear strength ratio (V/Vmax). The analytical method presented agreed well for beam-columns with a/D ratios of less than 1.5.
Reference
Sakino, K. and Ishibashi, H. (1985). “Experimental Studies on Concrete Filled Square Steel Tubular Short Columns Subjected to Cyclic Shearing Force and Constant Axial Force,” Transactions of the Architectural Institute of Japan, No. 353, pp. 81-89.