Abstract
Hybrid FRP-concrete-steel double-skin tubular columns (hybrid DSTCs) are a novel form of hollow columns consisting of an outer FRP tube, an inner steel tube, and an annular layer of concrete between the two tubes. Due to the effective confinement of the two tubes, the concrete in hybrid DSTCs is well confined, leading to excellent ductility and strength enhancement. Hybrid DSTCs also have excellent corrosion resistence due to the effective protection of the outer FRP tube. However, existing studies mainly focused on hybrid DSTCs with a circular cross-section. When subjecting to different loads in the two horizontal directions, elliptical columns are preferred as they can provide different bending stiffness and moment capacity around two axes of symmetry without significantly reducing the confining effect of the FRP tube. This paper extends the existing work on circular DSTCs to elliptical DSTCs with a particular focus on four issues: the effect of elliptical aspect ratio (i.e., the ratio of the major axis to the minor axis of the outer elliptical cross-section), the effect of the FRP tube thickness, the effect of void area ratio (i.e., the ratio of the area of concrete void to the area of the outer elliptical section), and the effect of the cross-section of the inner steel tube (i.e., both rectangular and elliptical steel tubes were used). Experimental results show that, the averaged peak stress of the confined concrete in elliptical DSTCs increases with the increase in the elliptical aspect ratio, whereas the elliptical aspect ratio has no obvious effect on the ultimate axial strain; the cross-section shape of the inner steel tube has significant effect on the axial stress-strain behavior of the confined concrete in elliptical DSTCs; elliptical DSTCs with an elliptical steel tube exhibit much better ductility and strength enhancement than those specimens with a rectangular steel tube. A simple stress-strain model of confined concrete was proposed for elliptical DSTCs to account for the effects of the elliptical aspect ratio, the inner void, and the shape of the inner steel tube, which can provide reasonably accurate but conservative predictions.
Highlights
Fiber-reinforced polymer (FRP) composites have many advantages compared with traditional building materials, such as high strength-weight ratio and excellent corrosion resistance
Hybrid DSTCs consist of an inner steel tube, an annular layer of concrete, and an outer FRP tube (Figure 1(a)). e inner steel tube provides the main longitudinal reinforcement for hybrid DSTCs and prevents the concrete from inward spalling during the earthquake. e outer FRP tube usually consists of fibers oriented in the hoop direction or close to the hoop direction to confine the concrete and to enhance the seismic performance of the member
Hybrid DSTCs are quite suitable for structural members in harsh environments due to their excellent corrosion resistance, which is enabled by the use of the outer FRP tube
Summary
Fiber-reinforced polymer (FRP) composites have many advantages compared with traditional building materials (i.e., concrete and steel), such as high strength-weight ratio and excellent corrosion resistance. Hybrid DSTCs constructed with high strength concrete (HSC) was first reported in Zhang et al [12], which confirmed that hybrid DSTCs still possess excellent ductility if the FRP tube has sufficient confinement stiffness and sufficient rupture strain capacity. Zhang et al [16] conducted a study on large-scale hybrid DSTCs subjected to combined axial compression and cyclic lateral loading, which indicates that hybrid DSTCs possess excellent ductility and seismic resistance even when high strength concrete with a cylinder compressive strength of around 120 MPa is used. To extend the existing study on hybrid DSTCs, the present study presents an experimental study of elliptical DSTCs under monotonic axial compression with a particular focus on four issues: the effect of elliptical aspect ratio (i.e., the ratio of the major axis to the minor axis of the outer elliptical cross-section), the effect of the FRP tube thickness frp, the effect of the void area ratio (i.e., the ratio of the area of the concrete void to the area of the outer elliptical section), and the effect of the cross-section of the inner steel tube (i.e., both rectangular and elliptical steel tubes were used, Figures 1(c) and 1(d))
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