Experimental And Analytical Study Of Double-Tube Concrete Columns With An FRP External Tube And A High-Strength Steel Internal Tube

This thesis systematically presents an experimental and analytical study on double-tube concrete column with an FRP external tube and a high-strength steel internal tube(DTCCs)under monotonic/cyclic concentric compression and eccentric compression.DTCC is a novel form of hybrid columns incorporating an internal HSS tube which was recently proposed at The Hong Kong Polytechnic University.The novel column comprises two concentric circular tubes with all the space inside the tubes filled with concrete.The external FRP tube affords confinement to the in-filled concrete as well as the internal high-strength steel(HSS)tube,thus enhancing the loading capacity of the hybrid column.The internal HSS tube under confinement can make the most use of its high-strength property,and afford extra confinement to the concrete part inside the HSS tube.The interaction between different components can motivate an optimum result and the hybrid DTCC has advantages of higher loading capacity and better ductility comparing to simple superposition of each component.This thesis mainly contains the following parts:In Chapter 2,a novel test method,named curved coupon tensile test,for the determination of hoop elastic properties of FRP tubes is proposed based on a well-rounded discussion on the most commonly used test methods.Both advantages and disadvantages of these existing test methods are firstly analyzed,and taking account of the precision and simplicity of the experimental procedure,the suggested curve coupon tensile test method is designed and a systematic experimental study is conducted,where both hoop elastic modulus and Poisson's ratio can be determined.The test results show that the proposed test method is easy to implement and delivers reliable results.Chapter 3 and 4 present an experimental and analytical study on DTCC specimens under monotonic compression.All the DTCC specimens possess a bilinear axial load-strain curve with no descending stage,and all the specimens failed by the hoop rupture of FRP in the mid-height region.The experimental results confirm that the high-strength characteristic of the internal HSS tube in a DTCC specimen can be fully exploited in a twofold manner:the local buckling of internal HSS tube is successfully suppressed under effective confinement from the external FRP tube,leading to excellent column performance;the internal HSS tube can afford a considerable lateral confinement to the in-filled concrete,thus further promoted the axial loading capacity of DTCCs.Based on Prandtl-Reuss theory for the internal HSS tube and path-independent assumption for confined concrete,the behavior of two concrete-filled HSS tubes is first examined to verify the reliability of the analytical modeling.An analytical model for DTCCs under monotonic compression is then proposed based on the experimental results and FE modeling.The interaction between different components in DTCCs is revealed by the dilation equation.The comparison between the prediction and the test results demonstrates the accuracy of the proposed analytical model.Chapter 5 and 6 present a thorough research program on DTCC specimens under cyclic compression.Different loading schemes,(repeated)full cyclic loading and(repeated)partial unloading/reloading,are designed.The test results indicate that DTCCs possess excellent loading capacity,as well as seismic ductility.The failure of hybrid DTCCs under cyclic compression occurred when the external FRP tube ruptured.The analytical model of DTCCs under monotonic loading proposed in Chapter 3 is found to have good performance in modeling the envelope curve of the axial load-strain relation of cyclic compressed DTCCs.The influence of Baushinger's effect of HSS tube in DTCCs cannot be ignored and is simulated with two existing models.Five existing unloading/reloading models of confined concrete are compared and the influence of the basic parameters including plastic strain,secant modulus at unloading/reloading point,tangent modulus of unloading point,strain accumulation coefficient,and stress deterioration coefficient are studied based on the experimental results in this paper.Chapter 7 presents a thorough research program on DTCC specimens under eccentric compression.Ten DTCC specimens were tested,and the main parameters studied herein are the initial eccentricities of applied load and the slenderness ratio of the specimens.The test results demonstrate that hybrid DTCCs under eccentric compression have excellent axial shortening and lateral deflection performance,and are characterized by bending failure.With the increase of eccentricity and/or slenderness,the axial load-strain curve of DTCCs exhibits a more distinct strain-softening stage after the peak.Nevertheless,the descending branch of the axial load-strain curve after the peak is considerable flat,while the potential lateral deflection capability of DTCCs can still be utilized.This phenomenon demonstrates the favourable ductility and energy dissipation capability of DTCCs.The FRP confinement of concrete in DTCCs under eccentric compression is less effective comparing to that in DTCCs under concentric compression.Both the loading capacity and stiffness of DTCC dramatically decline along with the increase of eccentricity and/or slenderness of the specimen.The influences of eccentricity,axial load level,and slenderness to the behavior of DTCCs under eccentric compression are explored with Lam and Teng's stress-strain model of concrete as the backbone model.The experimental results are used to update the backbone model for concrete in hybrid DTCCs.The proposed stress-strain model of concrete is applied in a section analysis based on fiber element approach,through which both the axial load-strain relation and the axial load-lateral deflection relation of DTCCs under eccentric compression can be closely predicted.