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• SWSSC-filled CFRP-stainless steel composite tube columns were (SCFSSCTs) presented. • Influences of FRP layers, thickness of stainless steel tubes and specimens size were evaluated. • Stress–strain curves calculation models for SCFSSCTs were proposed. • Bearing capacity models under different loading modes were verified. The structure of concrete confined by carbon fiber-reinforced polymer (CFRP) and stainless steel tubes was investigated. The columns, which take advantage of the ductility and corrosion resistance of stainless steel, are composed of stainless steel tube, core seawater and sea sand concrete (SWSSC) and outer FRP. Forty circular SWSSC-filled FRP-stainless steel composite tube columns (SCFSSCTs) were manufactured and subjected to axial compression tests. After fracturing of the FRP, the failure modes of the specimens mainly showed extrusion expansion, with a small amount of shear failure. The increase in the thickness of the stainless steel and the number of FRP layers resulted in a significant improvement in the load capacity and deformation capacity of the structure, which was more evident in the specimens with higher steel contents. With the increase in the number of FRP layers, the increases in ultimate stress and ultimate strain were 40.4% − 130.0% and 118.6% − 282.4%, respectively, compared with the column with single stainless steel confinement. The stress–strain curves for the SCFSSCTs were similar to those of structures reinforced by FRP and carbon steel and were divided into four stages. Models were proposed to predict the full axial stress–strain curves of SCFSSCTs, including the ultimate point and the peak point. [ABSTRACT FROM AUTHOR]

A new structure termed "concrete-filled FRP-grooved steel composite tube (CFGCT) column" is proposed, which is composed of a stress-released steel tube (i.e., grooved steel tube), fiber-reinforced polymer (FRP) and concrete. Axial load tests were carried out on twenty-four specimens to investigate the constraint effect of this structure. Three main experimental variables were considered: the steel tube thickness, the FRP type, and the FRP layer. The failure modes, stress-strain relationships and the effect of the main experimental variables were discussed. The stress-strain curves of this new structure are composed of an initial linear part, a nonlinear transition part, a strengthening part and a residual part. The test results demonstrate that the bearing capacity of the structure was improved, and that the mechanical mechanism of the structure was simplified due to the stress-released grooves. Based on the test results and previous studies, formulas for calculating the ultimate stress (fcu), ultimate strain (εcu), peak stress (fcc) and peak strain (εcc) were proposed. In addition, models for predicting the stress-strain curves of CFGCT columns were put forward, and the models could precisely simulate the stress-strain curve of this new composite structure. Hence, this study indicates that a structure composed of FRP and stress-released steel tube can effectively constrain concrete. [ABSTRACT FROM AUTHOR]

Different cross-sectional shapes of ultrahigh-performance concrete (UHPC)-filled stainless-steel tubular (UHPCFSST) columns result in their stress-strain curves exhibiting softening or hardening stages, and currently, there is no unified calculation model available that can effectively simulate these variations. Based on experimental data, models for predicting peak stress, peak strain, ultimate stress, and ultimate strain of UHPCFSSTs with high accuracy has been provided. Comparative analysis reveals that the stress transfer capability and stability of UHPCFSSTs with the different cross-sectional shapes from highest to lowest are as follows: circular, square, and rectangular. Building upon Wei et al.'s general stress-strain calculation model, a high correlation pattern between parameter b and hoop confinement coefficient (ξ s) has been obtained through correlation analysis, elucidating the post-peak behavior of stress-strain curves of UHPCFSSTs. By modeling parameter b , Wei et al.'s model has been extended to accommodate the stress-strain relationship under axial compression for UHPCFSSTs with different cross-sectional shapes. This establishes a unified stress-strain calculation model for UHPCFSSTs with different cross-sectional shapes under axial compression. Comparative analysis indicates that the proposed unified calculation model for axial compression stress-strain behavior of UHPCFSSTs can effectively quantify the influence of cross-sectional shape on stress-strain curves, demonstrating high predictive accuracy and generality. • The peak stress, peak strain, ultimate stress and ultimate strain models of UHPCFSSTs have been derived. • Identified the influence patterns of cross-sectional shape on the general stress-strain curve of UHPCFSSTs. • The parameter b influencing the post-peak behavior of stress-strain curve has been innovatively modeled. • Filled the gap in the unified stress-strain calculation model for UHPCFSSTs with different cross-sectional shapes. [ABSTRACT FROM AUTHOR]

• Axial compression tests on the ultrahigh-performance concrete (UHPC)-filled FRP-steel composite tube (UHPCFSCT) columns were conducted. • The influences of the FRP layers, FRP types, FRP hybrid and loading mode were examined. • The stress–strain curves of UHPCFSCTs can be divided into weak, medium and strong restraint curves. • Existing bearing capacity models under different loading modes are evaluated. • A new strength model suitable for steel tube-confined UHPC, FRP-confined UHPC and UHPCFSCTs was proposed. A new ultrahigh-performance concrete (UHPC)-filled fibre-reinforced polymer (FRP)-steel composite tube column (UHPCFSCT) was designed. This structure uses not only UHPC with high strength and durability to reduce the section size and improve the stiffness of the column but also FRP–steel composite tubes as confinement tubes to reduce the brittleness of the UHPC. Thirty-two UHPCFSCTs were subjected to monotonic axial compression tests. The key parameters include the number of FRP layers, FRP type, FRP hybrid condition and loading mode. The full-section-loaded specimens mainly exhibit shear failure, while the core-loaded specimens mainly exhibit middle expansion deformation. The stress–strain curves of UHPCFSCTs represent three types of columns with different numbers of FRP layers and FRP types: columns with weak, medium and strong confinements. The weak and medium confinement curves exhibit a softening section after the elastic–plastic stage, and the strong confinement curve shows a bilinear response. With the increase in the number of FRP layers, compared with those of the unconfined UHPC column, the ratios of the increases in peak stress, ultimate stress, and ultimate strain are 1.436∼2.794, 2.046∼4.822 and 0.778∼2.315, respectively. The bearing capacity of the CFRP-confined specimens is stronger, while the deformation capacity of the BFRP-confined specimens is stronger. The ultimate stresses of the core-loaded specimens are 19.6%∼30.6% higher than those of the full-section-loaded specimens. Existing strength models were used to evaluate the test data of UHPCFSCTs, and the Hu et al. model was used to predict the strength of UHPCFSCTs under core loading. Finally, through database regression analysis, a strength model that can be used to predict steel tube-confined UHPC, FRP-confined UHPC and UHPCFSCT strength under full-section loading was proposed. [ABSTRACT FROM AUTHOR]

• A database including 449 published square and rectangular concrete-filled steel tube (SCFST and RCFST) columns was established. • The constraint mechanism of rectangular steel tubes on the concrete was analyzed. • The parameters affecting the load-carrying capacity of SCFST and RCFST columns were analyzed. • A unified calculation model of axial compressive load-carrying capacity for SCFST and RCFST columns was proposed. Due to the nonuniform constraints of the steel tubes on concrete, the calculations of the axial compressive load-carrying capacity of square concrete-filled steel tubular (SCFST) columns and rectangular concrete-filled steel tubular (RCFST) columns are complex problems. In this work, the calculation of axial compression load-carrying capacity of RCFST columns was finely divided into five parts creatively by applying superposition principle. It analyzed the constraint mechanism of rectangular steel tubes on the concrete, the stress state of the concrete in the strong and weak constraint zones, and steel tubes on the height and breadth sides. A unified model of axial compression load-carrying capacity of SCFSTs and RCFSTs was established. Based on the established database including 449 published RCFST and SCFST columns, the rationality, accuracy and wide applicability of the proposed model was verified. The parameter sensitivity analysis of the factors affecting the load-carrying capacity of RCFSTs was carried out, the influences of parameters on the improvement of concrete load-carrying capacity and the reduction of the axial load-carrying capacity of the steel tube were studied in detail. [ABSTRACT FROM AUTHOR]

• A novel concrete-filled steel tube columns with inner distributed seawater and sea sand concrete-filled fiber-reinforced polymer tubes was proposed. • The stress–strain relationship, failure mode and parameters influence of novel structure were studied. • Models based on different theories for ultimate strength were established. To effectively solve the problem of the corrosion of undisturbed seawater and sea sand concrete (SSC) in practical applications, a new structure of concrete-filled steel tube (CFT) columns with inner distributed SSC-filled fiber-reinforced polymer (FRP) tubes was proposed. A total of 40 specimens were tested to investigate the axial compression behavior according to three parameters. The results indicated that the ultimate strength of CFT columns with inner distributed SSC-filled FRP tubes increased with the increase of FRP tube thickness, FRP tube diameter and steel tube thickness. The arrangement of inner FRP tubes effectively improved the axial compressive behavior and ductility of CFT columns. The ultimate strength of the CFT columns improved by 6.23 %–91.14 % due to the inner FRP tube. The failure of the specimens was always accompanied by fracture of one of the inner FRP tubes, while the remaining inner FRP tubes continued providing confinement. Lastly, two models based on different theories were proposed and evaluated to predict the ultimate load of this structure. The evaluation results indicated that both models provided accurate and reasonable predictions for the design of this structure. [ABSTRACT FROM AUTHOR]

• Two different strengthening technology for CFST columns were developed. • Strengthened CFST columns were manufactured and tested. • Eccentric compression behavior of FRP/steel strip strengthened CFST columns were studied. • Ultimate bearing capacity models of FRP/steel strip strengthened CFST columns were proposed. Concrete-filled steel tube (CFST) column structures have been widely applied in structural engineering because of their superior bearing capacity and ductility. During the service lives of CFST columns, due to the change in the service function, the increase in the load, and the corrosion of the steel tubes, some existing CFST columns face the strengthening demand of increasing bearing capacity and durability. In this study, two efficient strengthening methods for CFST columns were proposed, namely, winding carbon fiber-reinforced polymer (CFRP) or welding steel strips around the columns. A total of 11 specimens were tested to investigate the behavior of the strengthened CFST columns under eccentric compression according to parameters such as the height-diameter ratio, fiber-reinforced polymer (FRP) layer, eccentric distance, and steel strip thickness. The results indicated that the ultimate strength and initial stiffness decreased with increasing eccentric distance. The arrangement of the FRP and steel strips effectively improved the eccentric carrying capacity and ductility. The improvement in the ultimate load due to FRP confinement exceeded 34.31%, and that due to the steel strips exceeded 13.47%. Finally, considering the effect of the eccentric distance and height-diameter ratio on the behavior of the structure under compression, a model for predicting the bearing capacity under eccentric load was proposed and verified by a database that included various design parameters. [ABSTRACT FROM AUTHOR]

• Concrete-filled FRP and steel composite tube (CFCT) columns with stress-release grooves was developed. • Compressive behavior of the columns with stress-release grooves was investigated. • Discontinuous steel tubes can provide an effective confinement force on concrete. • A new model was suggested for calculating ultimate stress of the CFCT columns. To investigate the behavior of rectangular concrete-filled fiber-reinforced polymer (FRP) and steel composite tube (CFCT) columns with stress-release grooves under axial compression, sixty specimens were fabricated and divided into five groups for axial compression testing. The main variables were the corner radius, the number of FRP layers, the steel tube thickness and the presence of stress-release grooves. The test results indicate that steel tubes and FRPs can complement each other and discontinuous steel tubes can provide an effective confinement force on in-filled concrete. The stress–strain responses of CFCT columns with stress-release grooves consist of a linear elastic stage, a nonlinear transition stage, a linear hardening stage and a residual stage. The ultimate strength of a CFCT column is in direct proportion to the number of FRP layers, the thickness of the steel tube and the value of the corner radius. Stress-release grooves only play a positive role in specimens with relatively large values of the corner radii (r ≥ 30 mm). A new model was suggested for calculating the ultimate stress of CFCT columns with stress-release grooves. The model is applicable for CFCT and concrete-filled steel tube (CFT) specimens with stress-release grooves; the predicted results agree well with the experimental statistics. [ABSTRACT FROM AUTHOR]

• A novel concrete-filled steel tube (CFST) columns with in-built SWSSC-filled FRP tubes was proposed. • Composite column is comprised of outer steel tubes, sandwich ordinary concrete, inner FRP tubes and core SWSSC. • Influence of various parameters on compressive performance was investigated. • A model for the stress–strain curve of the composite column was developed. A structure termed as concrete-filled steel tube (CFST) columns with in-built seawater and sea sand concrete (SWSSC)-filled fiber reinforced polymer (FRP) tubes was investigated. The new composite columns are comprised of outer steel tubes, sandwich ordinary concrete, inner FRP tubes and core SWSSC. Thirty-six CFST columns with in-built SWSSC filled FRP tube and four CFST columns were manufactured, and the influences of the diameter of the FRP tube, FRP layers and the thickness of steel tube on compressive performance of the specimens were studied. The results indicated that the ultimate strength of the specimens was positively related to the FRP layers, diameter of the FRP tube and thickness of the steel tube. The stress–strain curve of new structure consists of four stages, and columns with larger FRP tube diameter and more FRP layers obviously underwent more stress-drop after the ultimate point. The installation of the FRP tube has a more significant strengthening effect on the columns with a thinner steel tube. Models to predict the ultimate stress, ultimate strain, peak stress and peak strain of new structure were proposed. Moreover, a simple model to predict the axial stress–strain response curve of CFST columns with in-built SWSSC-filled FRP tubes was suggested. [ABSTRACT FROM AUTHOR]