Wheeler and Bridge 2004

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Full scale flexure tests were performed on four circular CFTs as well as two circular HTs. A peak moment was not obtained for the CFTs due to the testing equipment; however, the maximum achieved moment was shown to be larger than the capacity determined from design formulas. The study indicated that the stiffness of the CFT members is close to that of the theoretical stiffness of the bare steel tube.

Experimental Study, Results, and Discussion

Four circular CFTs and two circular HTs subjected to pure bending were tested. The tubes had diameters of 16 in. and 18 in. with a thickness of 0.25 in. The steel tubes were produced with a cold rolled process with continuous seamless welds and had a yield stress of 50 ksi. The concrete strength varied between 5.8 ksi and 8.1 ksi. A four-point loading scheme was used with a saddles at the supports and loading points to ensure the loads were applied evenly to the cross-section.

Due to insufficient stroke in the actuators, the peak load was not obtained for the CFT specimens. The peak load for the HTs was reached. The maximum achieved moment was compared to the design capacity from the Australian standard, a theoretical moment capacity calculated from a cross-section software package (SECTPROP), and two other simple design equations found in the literature. All of the methods were found to be conservative when compared to the experimental results.

At one end of the specimen, two horizontal transducers, placed at opposing sides of the concrete portion of the cross-section and fastened to the steel tube were used to measure the slip between the concrete core and steel tube. Three regions were identified in the slip response: negligible slip for low loads, gradually increasing slip for medium loads, and significantly increasing slip for high loads (however, the maximum slip did not exceed 0.5 mm). Stiffness was also measured during the experiment. It was noted that the initial stiffness was near the theoretical uncracked stiffness. At a relatively low load the stiffness decreased and became close to the theoretical stiffness of the bare steel tube. The authors suggested that the size of the cross-section has an influence on the evolution of stiffness of the section.

References

Wheeler, A. and Bridge, R. (2004). “The Behaviour of Circular Concrete-Filled Thin-Walled Steel Tubes in Flexure,” Composite Construction in Steel and Concrete V, Proceedings of the 5th International Conference, Leon, R. T. and Lange, J. (eds.), Kruger National Park, South Africa, 18-23 July 2004, United Engineering Foundation, American Society of Civil Engineers, Reston, Virginia.