Behavior of large-scale hybrid FRP-concrete-steel double-skin tubular columns subjected to concentric and eccentric compression

Abstract

Hybrid FRP-concrete-steel double-skin tubular columns (DSTCs) are a new form of hybrid structural members. A hybrid DSTC consists of an outer FRP tube, an inner steel tube, and a concrete infill between them. Hybrid DSTCs have attracted increasing research attention worldwide since their invention due to the many important advantages they possess over conventional structural members, including their excellent corrosion resistance as well as excellent ductility. However, the existing studies on hybrid DSTCs have generally been limited to small-scale specimens. Besides, the use of self-compacting concrete (SCC) has not been paid sufficient attention despite the fact that SCC is obviously more suitable than normal concrete (NC) as the infill material for the relatively narrow annular space of hybrid DSTCs. Against this background, the present thesis presents an in-depth investigation into the structural behavior of large-scale SCC-filled hybrid DSTCs subjected to concentric and eccentric compression. To this end, several issues related to the use of filament-wound FRP tubes and SCC in concrete-filled FRP tubes (CFFTs) are also examined in this thesis as a prerequisite. The thesis first proposes a compression test method and a hydraulic pressure test method to characterize the longitudinal and the circumferential properties of filament-wound FRP tubes for confining concrete, respectively. A theoretical model for CFFTs subjected to axial compression is next developed, in which the biaxial stress state and the material nonlinearity of the FRP tube are properly accounted for. In parallel, results of concentric compression tests conducted on 23 CFFTs filled with NC or SCC of four different sizes are presented. The test results reveal that the behavior of SCC-filled FRP tubes is appreciably different from that of NC-filled FRP tubes due to the relatively large shrinkage of SCC, especially under weak confinement. The experimental program on large-scale hybrid DSTCs comprised concentric compression testing of 11 short hybrid DSTCs and eccentric compression testing of six short and nine slender hybrid DSTCs, under various combinations of test parameters, which include mainly the load eccentricity, column slenderness, thickness of FRP tube and void ratio. The majority of the specimens were filled with SCC. The test results show that large-scale hybrid DSTCs possess excellent ductility under both concentric and eccentric compression although the relatively large shrinkage of SCC may lead to a delayed activation of the confinement action of the FRP tube. To capture the effects of slenderness and eccentricity on the behavior of hybrid DSTCs, a theoretical column model, which traces the lateral deflection of columns using the numerical integration method and incorporates an eccentricity-dependent stress-strain model for concrete in hybrid DSTCs, was formulated. It is shown that the column model is accurate in predicting the axial load capacity of hybrid DSTCs and reasonably accurate in predicting the lateral deflection. Finally, a slenderness limit expression, which differentiates short hybrid DSTCs from the slender ones, is proposed, based on the results of a comprehensive parametric study performed using the theoretical column model, to complete an existing design procedure for short hybrid DSTCs.