Elchalakani and Zhao 2008
Increasing amplitude cyclic pure bending tests were performed on ten circular CFT beams. Using the results from these tests, conclusions were drawn about the seismic performance of these members. The authors have previously studied circular CFT beams under monotonic and constant amplitude cyclic loading.
Experimental Study, Results, and Discussion
Ten circular CFT beams were cycled back and forth at increasing multiples of the yield rotation. One specimen was subjected to three repeat cycles at each deformation level, but all others were subjected to only one cycle at each deformation level. 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 cold formed and in some cases machined to achieve higher D/t ratios. The steel had an average yield strength of 61.4 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 0.121 in. to 0.035 in. yielding D/t ratios ranging from 20 to 120.
Graphs of moment vs. rotation were presented with indication of initiation of local buckling and fracture. For specimens with various D/t ratios, the envelope of cyclic moment vs. rotation was superimposed with monotonic moment vs. rotation response of similar, previously tested specimens. From these graphs, several observations can be made. First, for higher D/t ratios, the strength is seen to be significantly less under the cyclic load, whereas, for lower D/t ratios, there is not significant strength degradation. Also, in all cases the ductility is seen to be reduced under the cyclic load. Additional comparisons were made between similar filled and hollow tubes.
Reference
Elchalakani, M., and Zhao, X.-L. (2008). “Concrete-Filled Cold-Formed Circular Steel Tubes Subjected to Variable Amplitude Cyclic Pure Bending.” Engineering Structures, Vol. 30, No. 2, pp. 287-299.