Effect of External Pressure on Axial Ultimate capacity of Concrete Filled Steel Tube Stub Column

Concrete -filled steel tube are widely used today in many civil engineering structures. The advantage of steel members is their high tensile strength and ductility and, on the other hand, concrete members have high compressive strength. Composite members combine steel and concrete, which have positive properties of both materials. In members under compressive loading, circular tube columns, for a given cross section area, have the large uniform bending strength in all directions in comparison to other cross-sections. Filling the pipe with concrete will increase the ultimate strength of the member without significantly increasing costs. On the other hand, the concrete in the tube delays the local buckling of the pipe wall. In this type of section, the outward buckling will reduce the amount of confinement, ductility and ultimate strength.

Subsea and offshore marine structures are mainly made of hollow steel circular sections, where water pressure reduces their load carrying capacity. By converting these sections to concrete filled tube, external pressure can improve the behavior by increasing the confinement. This paper tries to investigate the effect of external pressure on the ultimate strength of CFT, so that the use of this kind of composite sections in construction and retrofitting of marine structures would be investigated. This paper tried to evaluate the effect of lateral pressure on improving the behavior of concrete-filled steel tubes (CFT) by conducting laboratory studies. For this purpose, tri-axial testing set up, with capability of 400 bars pressure, was designed and constructed by the authors. Parameters such as lateral pressure, concrete strength and diameter to thickness ratio (D/t) of steel tube were tested.

Concrete with strength of 15 to 45 MPa was cast in pipes of 0.5 to 2 mm thickness and subjected to axial loading under external pressure between 0 and 150 bars. All specimens have a constant diameter of 100 mm and a height of 250 mm and are filled with ordinary concrete. All specimens have a diameter of 100 mm and a height of 250 mm and are filled with normal concrete. To evaluate effect of lateral pressure on the final strength, the ratio of d Parameters such as ultimate strength and failure mode of specimens along with their displacement load diagrams were investigated. Diameter to thickness in some samples was considered higher than the values proposed in the standards. Experimental tests results were compared with the relationships presented in the Eurocode 4 and AISC standards.

According to the calculations, the AISC standard result in conservative numbers compared to the EC4 standard for the ultimate strength of the specimen. External pressure has increased the loading capacity, as well as the ductility of the specimens by preventing the buckling and sudden crushing of the core concrete. Increase in load carrying capacity due to external pressure was up to 91% in some specimen. The effect of increasing on ultimate strength on the lower thickness specimens was significant. In conclusion, results of the experiments showed a significant effect of lateral pressure on the final strength of the CFT with normal concrete.