Han, He and Liao 2011
17 CFTs, and one hollow tube, all with circular cross sections were tested under axial tension. The main parameters were steel ratio (the ratio of cross sectional area of steel tube to concrete), the type of concrete (self-consolidating or steel fiber-reinforced), and finally bonded vs unbonded conditions between steel and concrete.
Experimental Study and Results
17 CFTs, and one hollow tube, all with circular cross sections were tested under axial tension. The main parameters were steel ratio (the ratio of cross sectional area of steel tube to concrete), the type of concrete (self-consolidating or steel fiber-reinforced), and finally bonded vs unbonded conditions between steel and concrete. In order to test the specimens, a constant tensile load was applied until the strain reached 40,000με, when an unloading program was used to reduce the tensile load to zero. Many specimens did not reach this limit, and failed before reaching this longitudinal strain. Failure was mostly seen at the area that was welded, however some cracks were found distributed uniformly on the concrete surface. When the CFT was ‘unbonded’, tensile cracks were seen in the concrete core, and ‘pinching’ of the steel tube was seen. Overall, the CFT increased in tensile strength by 11% as compared to that of the hollow tube.
Analytical Study
An ABAQUS module was used to study the tensile behavior of CFTs using 4-node shell elements for the steel tube and 8-node brick elements for the end plates. Another study was also performed to determine the adequate mesh density for the most accurate results. Tensile strain tests can be divided into four stages on a tensile load vs. tensile strain. The first is a linear curve until the concrete core cracks, followed by a decrease in rigidity until the steel reaches the proportional limit. Next, the steel tube exhibits pinching, and finally, because of the hardening effect of steel, the curve goes upwards slightly.
Reference
Han, L., He, S., and Liao, F. (2011). “Performance and Calculations of Concrete Filled Steel Tubes (CFST) under Axial Tension.” Journal of Constructional Steel Research, 67 (11), November, pp. 1699–1709., doi:10.1016/j.jcsr.2011.04.005