Cyclic Axial Compressive Behavior Of Elliptical Concrete-filled FRP Tubular Stub Columns (ECFFTCs) And Elliptical FRP-concrete-steel Double-skin Tubular Stub Columns (EDSTCs)

Fiber reinforced polymer(FRP)composites have found increasingly wide applications in civil engineering due to their high strength-to weight ratio and high corrosion resistance.Many hybrid columns applied with FRP composites have been proposed by researchers.In recent years,extensive research has been done on concrete filled FRP tubes and hybrid FRP-concrete-steel double-skin tubular columns(hybrid DSTCs).Ellipse has major axis and minor axis,the aspect ratio can be changed according to the load conditions and then the structural efficiency can be maximized.The oval cross section also has advantages of small fluid resistance.Besides,conversion of rectangular columns into elliptical columns(i.e.shape modification)followed by fiber-reinforced polymer jacketing provides an effective strengthening method.Elliptical hybrid columns have wide application prospect in both the retrofit of existing columns and in new construction.This thesis presents a systematic study on the structural behavior of elliptical concrete-filled GFRP tubular stub columns(ECFFTCs)and elliptical FRP-concrete-steel double-skin tubular columns(EDSTCs)under monotonic and cyclic axial compression,with particular attention to the aspect ratio,layers of FRP and loading scheme.(1)An experimental study of 25 ECFFTCs specimens has been conducted on the cyclic axial behavior of ECFFTCs.both ECFFTs and EDSCTs cover 4 aspect ratio and 2 FRP thickness.The test parameters included aspect ratio,FRP thickness and loading scheme.The research revealed that the failure mode of elliptical specimens is the rupture of GFRP tube at around the apex of long axis of elliptical section.The range of ultimate strain and ultimate stress improvement decrease with the increase of aspect ratio.Different loading schemes had little effect on ultimate loading capacity.(2)The test results have been compared with Teng et al.(2016)monotonic stress-strain model and Yu et al.(2015)cyclic stress-strain model.Based on Teng et al.(2016)model,a new stress-strain model was proposed.Aspect ratio has little effect on unloading/reloading cycles.Yu et al.(2015)model can provide accurate prediction for elliptical FRP confined concrete.Then a new cyclic stress-strain model for ECFFTCs was proposed consists of the new monotonic model mentioned above for envelope curve and Yu et al.(2016)model for unloading/reloading cycles.(3)A total of 25 ECFFTCs specimens were tested under monotonic and cyclic axial compression.The test parameters included aspect ratio,FRP thickness and loading scheme.The research indicated that the failure mode of elliptical specimens is the rupture of GFRP tube at around the apex of long axis of elliptical section.The crushing of concrete and the deformation of inner steel tube were also observed at the same location.The range of ultimate strain and ultimate stress improvement decrease with the increase of aspect ratio.Different loading schemes had little effect on ultimate loading capacity.(4)Based on the monotonic stress-strain model of ECFFTCs proposed in this paper,a monotonic stress-strain model of EDSTCs was proposed.The test results of cyclic specimens were compared with Yu et al.(2015)model.The applicability of Yu et al.(2015)model was analyzed.