CFT Members Subjected to Shear Load
Tests of concrete-filled steel tubes subjected to shear or shear in combination with axial load and bending have been conducted primarily in Japan. The test results have demonstrated that CFTs have excellent shear resistance under both monotonic and cyclic loading.
The behavior of a CFT member under a shear-type loading is dependent upon essentially the same parameters as beam-columns, including the D/t ratio, the axial load ratio, and the shear span ratio (a/D ratio) which is analogous to the L/D ratio for beam-columns. Based on the shear span ratio, shear behavior can be divided into two types. For a small shear span ratio (0.83 to 1.0), diagonal shear cracking indicative of shear failure occurs in specimens that are also subjected to axial load (Tomii and Sakino, 1979a). For shear span ratios of 2.0 to 3.0, columns exhibit a flexure-type failure with plastic hinges forming at the specimen ends.
The D/t ratio and P/Po ratio have much the same effect as discussed for beam-columns in Section 4. The point of local buckling typically occurs at or near the peak shear stress, but well in advance of the maximum rotation and occurs earlier as the D/t ratio of the section increases. Overall shear resistance (moment capacity) decreases with an increase in the D/t ratio or an increase in the axial load ratio. Such columns will behave in a brittle manner.
Concrete-filled steel tube members subjected to shear forces display a large amount of energy dissipation and ductility. Circular members tend to have more stable hysteresis loops and a greater ductility than rectangular tubes. However, experiments have shown that rectangular tubes tend to behave as circular tubes after a few cycles, as the buckling of the steel tube at the point of maximum shear transforms the critical regions from rectangular to circular in shape (Kawaguchi et al., 1991, Sakino and Tomii, 1981; Sakino and Ishibashi, 1985).
Members with relatively thin tube walls show some strength deterioration with successive cycles of loading, but still display a large amount of energy dissipation (Tomii, 1991). The strength deterioration results from the buckling of the steel tube and subsequent crushing of the concrete. Members with thick tube walls exhibit deformation behavior similar to thin-walled tubes. However, these members resist local buckling and concrete crushing well into the plastic range of strains (Council on Tall Buildings and Urban Habitat, 1979) providing greater overall shear resistance.
Axial load has been found to have little appreciable effect on the shear-carrying capacity of CFTs (Tomii et al., 1972). CFT specimens subjected to a high axial load (P/Po = 0.5), tend to show a stabilization of the hysteretic loops and even a slight increase in shear resistance. As described earlier, rectangular sections buckle at the critical region and become circular in shape. The circular shape provides an increased confinement of the concrete, increasing the shear resistance. Sakino and Tomii (1981) also observed a considerable amount of axial shortening 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.
Cyclically loaded rectangular specimens with an a/D ratio of 1.0 fail in shear, as opposed to the cyclically loaded specimens with a/D ratios of 2.0 and 3.0, which fail in flexure, much like the monotonic specimens (Sakino and Tomii, 1981; Sakino and Ishibashi, 1985). Short beam-columns show considerable energy absorption and display less strength deterioration than the longer columns that fail in flexure. Both lengths exhibit an initial decrease in capacity and then a slight increase as local buckling in the critical regions transforms the shape of the tube from rectangular to circular.