Structural behavior of hybrid FRP-concrete-steel double-skin tubular columns

Abstract

Hybrid FRP-concrete-steel double-skin tubular columns (DSTCs) are a new form of hybrid columns recently proposed by Prof. J.G. Teng of The Hong Kong Polytechnic University. The column consists of an outer tube made of fiber-reinforced polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. In this new hybrid column, the three constituent materials are optimally combined to achieve several advantages not available with existing columns. This thesis presents a combined experimental and theoretical study aimed at developing a good understanding of the structural behavior of and reliable design methods for this new hybrid column to facilitate its acceptance in practical applications. The first phase of the research was experimental, involving laboratory tests of DSTC specimens under axial compression, bending and eccentric compression to study the compressive, flexural and beam-column behavior of the new hybrid column. In addition to axial compression tests on short DSTCs, tests were also conducted on stub columns of circular solid and annular concrete sections confined with an outer FRP tube to gain a better understanding of how the three components in a DSTC interact under axial compression. The test results have confirmed that the concrete in the new column is very effectively confined by the two tubes and local buckling of the inner steel tube is either delayed or suppressed by the surrounding concrete, leading to a very ductile response. The test results have also shown that the new DSTC is very ductile under both flexure and combined flexure and axial compression. The bending tests showed that when the new section form is employed as a beam, the outer FRP tube not only enhances the structural behavior by providing confinement to the concrete but also provides a significant contribution to the shear resistance. Apart from the experimental study, finite element (FE) analysis of hybrid DTSCs under axial compression was also conducted. Existing Drucker-Prager (D-P) type concrete plasticity models for confined concrete were first critically assessed. It was found that D-P type plasticity models lead to reasonable predictions for both actively- and passively-confined concrete (e.g. FRP-confined concrete) only if the flow rule is suitably related not only to the confining pressure but also to the rate of increment of the confining pressure. A constitutive concrete model which takes into account the conclusions drawn from the assessment of existing D-P type models and other distinct characteristics of non-uniformly confined concrete was then proposed and verified with test results. A parametric study was next conducted using a finite element analysis incorporating the proposed constitutive model, from which a simple one-dimensional stress-strain model for the concrete in DSTCs for design use was formulated. Making use of the proposed one-dimensional stress-strain model for the concrete in DSTCs, a simple theoretical method based on section analysis was also developed for DSTCs under flexure or combined axial compression and flexure. The section analysis method was then verified with test results and used in a parametric study to examine the beam-column behavior of hybrid DSTCs.