Axial compressive behavior of FRP-concrete-steel double-skin tubular columns

Abstract

A new type of composite structural system has been proposed in terms of FRP-concrete-steel double-skin tubular columns (DSTCs). This composite system consists of a steel tube inside, an FRP tube outside with concrete in between, and it combines the advantages of all three materials to achieve a high-performance structural member. This thesis is aimed at developing an improved understanding of the axial compressive behavior of DSTCs. To this end, six experimental studies were undertaken at the University of Adelaide. In each of these studies, the key parameters that influence the axial compressive behavior of DSTCs were identified and investigated. The results of these experimental studies indicate that concrete in a DSTC system is confined effectively by FRP and steel tubes. Both the normal-and high-strength concrete DSTCs exhibited a highly ductile compressive behavior under monotonic and cyclic axial compression. However, it is found that, for a given nominal confinement ratio, an increase in the concrete strength results in a decrease in the ultimate axial strain of DSTCs. The results also indicate that increasing the inner steel tube diameter leads to an increase in the ultimate axial stress and strain of concrete in DSTCs. It is observed that the concrete-filling of the inner steel tubes of DSTCs results in an increase in the compressive strength and a slight decrease in the ultimate axial strain of concrete in DSTCs, compared to the values observed in companion specimens with hollow inner steel tubes. It is also observed that cyclically loaded normal-strength concrete (NSC) DSTCs developed similar strength and strain enhancement ratios to those of monotonically loaded NSC DSTCs. The results also show that concrete in hollow DSTCs manufactured with square inner steel tubes develops significantly lower ultimate axial stresses and strains than those of concrete in companion hollow DSTCs with circular inner steel tubes. It is found, however, that the performance of these specimens improves dramatically when the square inner steel tube is filled with concrete. Apart from these experimental studies, this thesis also presents analytical models that were developed to predict the compressive strength and ultimate axial strain of concrete in DSTCs. The first of these models was developed to predict the compressive strength and ultimate axial strain of concrete in hollow circular DSTCs. After undertaking additional studies to expand the test database of square and concrete-filled DSTCs a second model that is applicale both circular and square and hollow and concrete-filled DSTCs was proposed. Comparison with experimental test results show that of the proposed models are in close agreement with the test results, and the models provide improved accuracy compared to the existing models.