Elchalakani, Zhao, and Grzebieta 2004
Constant amplitude cyclic pure bending tests were performed on 23 circular CFT beams. Using the results from these tests, conclusions were drawn about the seismic performance of these members.
Experimental Study, Results, and Discussion
Under the chosen loading protocol, twenty three circular CFT beams were cycled back and forth at a given rotation until failure. The amplitude of rotation that the beams were subjected to was equal to a multiple of the yield rotation. That multiple, termed member ductility index, was varied from 6 to 12 between tests. The specimens were tested in a machine with two rotating wheels which supported and loaded the member, ensuring pure bending and negligible axial load for large rotations seen by the specimens. The beams were 59 in. in length from end to end and the length of pure bending was 31.5 in. The steel tubes were manufactured by roll forming and electric resistance welding. The steel had an average yield strength of 60.6 ksi. The concrete had an average compressive strength of 3.35 ksi. The diameter of the steel tubes varied from 4.3 in. to 2.4 in. and the thickness varied from 0.123 in. to 0.027 in. resulting in D/t ratios ranging from 20 to 162.
The ratio of maximum moment to maximum moment in the first cycle was plotted versus the number of cycles in groups defined by the ductility index. From these graphs, it is clear that the members with higher D/t ratios degrade faster. Also, given a performance criteria (e.g., less than 20% degradation after four cycles) and required ductility, a maximum D/t ratio can be identified. Conversely, given a performance criteria and D/t limit, the member ductility index can be identified. Further, by following a recommendation from the New Zealand standards, an approximation of the structure ductility index can be found by dividing the member ductility index was divided by 1.25.
The results of these experiments were also compared to similar studies by the authors, including constant amplitude cyclic tests on HTs and monotonic tests on circular CFTs. In particular, it was noted that the ratio between seismic and standard slenderness limits ranges between 0.5 and 1.0, with the low ratio corresponding to high ductility demands.
Reference
Elchalakani, M., Zhao, X.-L., and Grzebieta, R. (2004). “Concrete-Filled Steel Circular Tubes Subjected to Constant Amplitude Cyclic Pure Bending,” Engineering Structures, Vol. 26, pp. 2125-2135.