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The concrete-filled square steel tubular (S-CFST) structure has been widely used as a resistance system against earthquakes due to its good seismic performance. However, the application of the S-CFST structure is limited by its complex joint formulation. To overcome this shortcoming, a sleeve semirigid joint has been proposed, while its seismic performance has not been well examined. This study aims to discuss the behavior of sleeve joints with different parameters under low cyclic loading. The analysis results show that the simulation results of the finite element model established by ABAQUS were in good agreement with the experimental results, which further verified the good energy dissipation performance of the sleeve joint, and gave the design suggestions of the joint under different parameters to promote the engineering application of this type of joint. [ABSTRACT FROM AUTHOR]

FRP-confined concrete-encased cross-shaped steel columns (FCCSCs) are a new form of hybrid columns recently developed at the University of Wollongong. An FCCSC consists of a square FRP outer tube, a cross-shaped steel section and concrete filled in between. This sectional configuration ensures that the concrete is very effectively confined despite the square shape of the column. In addition, the cross-shaped steel section serves as the ductile longitudinal reinforcement for loads in the two lateral directions and its possible buckling is constrained by the FRP outer tube and the concrete, leading to a column that is highly ductile. In this paper, results from a series of stub column tests are presented to demonstrate the concept of the new column form. The experimental program involved the testing of FCCSC specimens as well as four types of similar column forms, namely, square FRP-confined plain concrete columns (SFCPCs), circular FRP-confined plain concrete columns (CFCPCs), concrete-encased cross-shaped steel columns and square plain concrete columns. The test results confirmed the excellent performance of FCCSCs. The test results also showed that compared with the concrete in SFCPCs and that in CFCPCs, the concrete in FCCSCs has a much larger ultimate axial strain and a larger compressive strength, when the same FRP tube is used. [ABSTRACT FROM AUTHOR]

• Reliable 3D finite element models are developed for FRP-confined concrete-encased cross-shaped steel columns (FCCSCs). • An in-depth understanding on the complex confinement mechanism of FCCSCs is achieved. • The effects of key parameters on the mechanism and behaviour of FCCSCs are thoroughly examined. Fibre-reinforced polymer (FRP)-confined concrete-encased cross-shaped steel columns (FCCSCs) are an emerging type of hybrid columns. In an FCCSC, the concrete is subjected to combined confinement from the steel section and the FRP tube, leading to an enhanced load capacity and excellent ductility of the column. While several recent experimental studies have demonstrated the excellent structural performance of FCCSCs, the complex confinement mechanism behind their structural behaviour has not been clarified or thoroughly examined. This paper presents a comprehensive study aiming to investigate the confinement mechanism of FCCSCs. In this paper, the experimental observations of FCCSCs are first summarized, followed by three-dimensional FE modelling of the columns. The FE models, after being verified against the test results in various aspects, are used for a systematic parametric study. In this study, the complex interaction between the three components of FCCSCs (i.e., FRP tube, steel section and concrete) is illustrated explicitly and the effects of key parameters (i.e., flange width, flange thickness, web thickness and FRP tube thickness) are examined separately and thoroughly, leading to an in-depth understanding of the confinement mechanism of FCCSCs. In particular, through the examination on the distribution of steel flange-to-concrete normal pressure, it is shown quantitatively that the variations of the dimensions of the steel section in a practically wide range have only marginal effects on its confinement to the concrete, while the increase of FRP tube thickness tends to reduce the confinement effect from the steel section. [ABSTRACT FROM AUTHOR]

FRP-confined concrete-encased cross-shaped steel columns (FCCSCs) are an emerging form of hybrid columns, consisting of a square FRP outer tube, a cross-shaped steel section and a concrete infill. The steel section in FCCSCs serves as ductile longitudinal reinforcement, and its buckling is well constrained by the concrete and the FRP tube. In addition, the steel section provides confinement to the concrete together with the FRP tube, so that the concrete in FCCSCs is effectively confined despite their square cross section. This paper presents a systematic experimental study on FCCSCs. The test variables included the flange width, flange thickness and web thickness of the steel section, and the thickness of FRP tube. It is shown that the various steel sections in the tested FCCSCs all provided substantial confinement to the concrete, while the effect of the steel confinement is less pronounced for FCCSCs with a thicker FRP tube. The test results show that the variation of the cross-sectional dimensions of the steel section, within a reasonably wide practical range, has marginal effects on the behaviour of the concrete in FCCSCs. In addition, a simple method has been proposed to predict the axial load capacity and ultimate axial strain of FCCSCs. [ABSTRACT FROM AUTHOR]

• A study on slender FRP-confined concrete-encased steel columns (FCCSCs) is presented. • The test variables include column slenderness, load eccentricity and FRP thickness. • The excellent structural performance of slender FCCSCs was demonstrated. • The confinement effects are remarkable for slender FCCSCs under compression. Fibre-reinforced polymer (FRP)-confined concrete-encased cross-shaped steel columns (FCCSCs) consist of a square FRP outer tube with round corners, a cross-shaped inner steel section and concrete filled in the between. The unique configuration of FCCSCs ensures that the concrete in the columns is well confined despite the square column shape, as demonstrated by the relevant existing research. The existing work on FCCSCs, however, has been limited to the behavior of short FCCSCs under axial compression. With their optimal configuration, the use of FCCSCs means that considerably reduced section dimensions may be adopted for the same load demand, leading to relatively slender columns in practice. In addition, the load eccentricity is an important parameter to consider in the column design. This paper presents the first-ever experimental study on the behaviour of slender FCCSCs. The test variables in this study included the load eccentricity, the slenderness ratio of the column and the thickness of FRP tube. The test results confirm the excellent structural performance of slender FCCSCs, and show that the load capacity of FCCSCs decreases with the slenderness ratio and the load eccentricity. Nevertheless, the confinement effects on the behavior of FCCSCs were found to be remarkable even when the column slenderness ratio and load eccentricity are large. [ABSTRACT FROM AUTHOR]

A hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular column (DSTC) consists of an outer FRP tube and an inner steel tube, with the space in between filled with concrete. This paper presents results from the first ever experimental study on hybrid DSTCs with a large rupture strain (LRS) FRP tube, namely, polyethylene terephthalate (PET) FRP tube. The experimental program involved the testing of 12 hybrid DSTC specimens with or without additional concrete inside the inner steel tube. The test results confirmed the ample ductility of hybrid DSTCs with a PET-FRP tube despite the severe local buckling of the inner steel tube at large axial deformations. The test results also suggested that the diameter-to-thickness ratio of the inner steel tube is a more critical parameter in such DSTCs than in DSTCs with a glass, carbon or aramid FRP outer tube. [ABSTRACT FROM AUTHOR]

Using ultra-high-performance concrete with coarse aggregate (UHPC-CA) instead of plain concrete, the mechanical performance of FRP tube-confined concrete-filled steel-encased columns (FUSCs) can be significantly improved with controllable cost. In this study, the axial compressive behavior of FUSCs was investigated, in which the thickness and fiber winding angle of FRP tube, section size of profile steel, and substitution rate of coarse aggregate were the variables considered. The failure mode and the load-deformation relation were analyzed, upon which the interaction mechanism between the profile steel, UHPC, and FRP tube was uncovered. The results showed that FUSCs mainly failed in two distinct failure modes, i.e., shear failure and crush failure respectively, depending on the magnitude of FRP confinement and the size of profile steel. Meanwhile, the incorporation of coarse aggregate could enhance the constraint effect of FRP tube, resulting in an increase in the strength index and peak strain up to 16.2% and 38.2%, respectively. With extra restraint after the peak load, the profile steel provided a significant contribution to the FUSCs' ductility, leading to a transition of FRP confinement effect from non-uniform to uniform. The research outcome serves as a reference for design of FUSCs and promotes the application of FUSC structure. [ABSTRACT FROM AUTHOR]

• A detailed investigation on bond performance of non-uniform corrosion in concrete. • A new impressed current method was used to induced non-uniform corrosion of rebar. • The effects of longitudinal rib of rebar on corrosion and bon degradation are discussed. In this work, the bond degradation of non-uniformly corroded steel rebars embedded in concrete was studied. A total of 105 concrete specimens with accelerated corrosion of steel rebar were evaluated using the pull-out test. The influences of corrosion mode, corrosion level, orientation of the longitudinal rib, and the position of steel rebar on the bond degradation were analyzed. The cracking patterns of corrosion-induced cracks, bond failure modes, and bond stress-slip relations were discussed. The results showed that the orientation of longitudinal rib and the thickness of concrete cover play a vital role in determining the cracking patterns of corrosion-induced cracks. In comparison to the uniformly corroded specimens, the bond performance of specimens with non-uniform corrosion of steel rebar degraded more significantly, and this phenomenon became even more pronounced as the corrosion level increased. Moreover, the beneficial improvements caused by the expansive rust on the bond performance were greatly dependent on the degrees of concrete constraint and corrosion level. [ABSTRACT FROM AUTHOR]