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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]

This paper presents a study on the compressive behavior of a novel type of highly deformable hybrid FRP tubular columns. The columns consist of an FRP tube filled with large-size recycled concrete lumps (RCLs) as aggregates and flowable grout as the binding material, and are thus termed FRP-RCL-Grout (F-R-G) columns. The use of large-size RCLs (e.g., up to 63 mm RCLs for 200 mm circular columns) leads to a relatively high recycling ratio and facilitates the manufacturing process. The flowable grout can be easily poured/pumped into the FRP tube, leading to ease of construction, while the FRP tube acts as an effective lateral confining device. The experimental part of the study involved compression tests on F-R-G columns and similar columns for comparison, with the main test variables being the FRP tube thickness, the size of the RCLs, the compressive strength of the concrete for producing RCLs and the WP ratio of the grout. The test results showed that F-R-G columns possess excellent structural behavior including ample ductility, and clarified the effects of the key parameters. In addition, based on the test results, a simple yet accurate model is proposed to predict the strength of F-R-G columns. • A novel FRP tubular column form incorporating recycled concrete lumps and flowable grout is investigated. • The novel column is highly deformable with ample ductility. • The novel column form provides an effective method of using recycled concrete lumps for structural applications. • The use of flowable grout highly eases the construction of the column. [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]

Rubberized geopolymer concrete (RuG) has become increasingly popular due to its capability of sustainable utilization of resources, but the mechanical properties of RuG could deteriorate significantly with the increase of rubber content. To mitigate the adverse effects of rubber and further promote the engineering applications of eco-friendly building materials, a new type of composite column, i.e., steel tubed rubberized geopolymer concrete (ST-RuG) columns, was proposed and tested in this paper. The influence of concrete types, rubber replacement ratios, the configurations of steel tubes, and diameter-to-thickness ratios on the compressive behavior of specimens was analyzed. The test results show that the incorporation of rubber has a deterioration effect on the strength of ST-RuG columns with a maximum degradation of 37.6%, but it can improve the ductility of specimens to a certain extent, which is slightly superior to steel tubed rubberized ordinary concrete (ST-RuO) columns. Due to the inherent brittleness of geopolymer concrete (GPC), more obvious residual behavior is detected for ST-RuG compared with ST-RuO under the same rubber replacement ratio R f. Moreover, with the increase of R f , the confining pressure of ST-RuG specimens gradually decreases, and a significant decreasing trend is observed for both the peak stress ratio and residual strength ratio of confined RuG. Finally, a new stress-strain model considering the effect of rubber replacement ratio is developed, and comparisons with test results reveal that the proposed model possesses satisfactory accuracy in predicting the axial stress-strain curves of steel-confined GPC and RuG. • The axial compressive behavior of steel tubed rubberized geopolymer concrete columns was investigated. • The effects of key test parameters on the interaction mechanism of specimens were explored. • A new stress-strain model considering the effect of rubber particles was developed for steel tubed geopolymer concrete. [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]

• The fire resistance tests on twelve full-scale circular columns were performed. • The fire resistance behavior of UHPCFST columns subject to the thermal-mechanical coupling action were analysed. • The effect of UHPC mixture proportion on fire resistance of UHPCFST columns were analysed. • The tailored modifications to the average temperature-based model proposed by the author previously were made. To contribute a better understanding of the behaviors of ultra-high performance concrete filled steel tube (UHPCFST) under fire, fire resistance tests on twelve full-scale circular columns were performed in this study, with the emphasis on the evaluation of the effects of steel fiber content, coarse aggregate content, and load level. The failure mode, temperature distribution, axial deformation, and most importantly, the fire resistance of UHPCFST columns subject to the thermal-mechanical coupling action were analyzed. The test results showed that irrespective of the mix proportion of UHPC and the level of applied load, all the columns fail in the overall instability. Although the inclusion of steel fiber and coarse aggregate facilitates the heat transferring considerably, the temperature within the UHPC core is mostly below 400 °C during the whole heating process because of the low thermal conductivity. Moreover, as the content of steel fiber and coarse aggregate increases, better fire resistance of the UHPCFST column is generally observed, benefiting from the enhancement in global stability and the further hydration below the temperature of 400 °C. Finally, given the insufficient predictions of the fire resistance of UHPCFST columns upon the thermal parameters and the high-temperature reduction factors obtained from existing codes and research works, tailored modifications to the average temperature-based model proposed by the author previously were made. The satisfactory validation with the test results using material high-temperature reduction factors from independent research demonstrated the wide applicability of the proposed model. [ABSTRACT FROM AUTHOR]