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Rubberized concrete (RuC) has emerged as a promising eco-friendly solution for structural elements, offering improved energy dissipation, damping characteristics, and ductility. Incorporating rubber particles into concrete-filled steel tubular (CFST) columns presents an environmentally conscious alternative for various structural applications. However, understanding its performance post-fire is crucial for safe design application, an area currently lacking comprehensive research. This study investigates the structural behavior of sixteen axially loaded circular RuCFST columns after exposure to fire. The columns, with diameters of 219 mm and 377 mm, are filled with normal concrete and RuC, containing 5 %, 15 %, and 30 % replacement of natural fine and coarse aggregate with rubber particles. Following ISO-834 fire standards, the specimens undergo elevated temperatures and subsequent application of compressive axial loads to determine their post-fire behavior. Analysis includes examination of failure modes, temperature-time relationships, axial load-deformation curves, and strain and stress development in the columns. Microstructural analysis using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) is conducted to study internal morphology before and after exposure to high temperatures. Results indicate non-uniform and nonlinear distributions of maximum temperatures across cross-sections, with historical temperature affecting residual capacity more significantly with increased rubber content and decreased column size. Load-bearing capacities decrease by up to 30 % and 22 % for small and large RuCFST columns, respectively, after exposure to fire. Residual axial stiffness also declines, with reductions of nearly 55 % and 42 % for small and large columns, respectively. In contrast, ductility improves in fire-exposed specimens, with columns having a 30 % rubber replacement ratio showing the highest enhancement. A novel method for calculating post-fire residual load capacity based on the average historical maximum temperature is introduced, demonstrating high accuracy and offering a valuable tool for assessing and reinforcing RuCFST columns after fire exposure. • Residual performance of RuCFST columns exposed to fire were examined. • The influence of rubber particle content and cross-sectional size on the residual performance of RuCFST columns was studied. • Microstructural analysis was performed on the concrete cores. • A design model for predicting the residual strength of RuCFST columns was developed. [ABSTRACT FROM AUTHOR]

Concrete-filled double skin steel tubular columns with large hollow ratios (LHR-CFDST) have been commonly researched in the past few years. The excellent performance of this structure renders it promising as a novel form of wind turbine tower. Increasing the diameter-to-thickness ratio is one efficient way to reduce steel consumption as the size of this engineering structure continues to expand. There is a paucity of research on thin-walled LHR-CFDST. Meanwhile, these structures may be susceptible to local buckling without stiffening measures. Furthermore, the size effect of this type of structure is more complicated. This paper reports the first experimental investigation on super-sized thin-walled LHR-CFDST columns under combined compression-torsion-shear-bending loads. Two LHR-CFDST columns with varying stud spacings are discussed. Investigations are conducted into failure modes, local buckling behaviors, load-strain responses, and a few popular mechanical indices. According to the test results, the specimen with smaller stud spacing has an 8.1% greater ultimate bearing capacity. It is assumed that a more intensive stud arrangement can delay the onset of local buckling, thereby increasing the peak load. Besides, the arrangement of the studs leads to an obvious effect of section confinement, thus improving the overall performance of the specimens. The failure modes and load-displacement relationships simulated by the FE models agree well with the experimental results. Then, a modified method for predicting the ultimate strength of LHR-CFDST based on the previously summarized formulae for CFST under compression-bending-shear-torsion is proposed and utilized for the evaluation of two specimens in this research. Since the tested value of the specimen with narrower stud spacing is higher by 5.6% than the predicted value, it can be concluded that the large-scale thin-walled LHR-CFDST may be evaluated using this method, complemented by appropriate stiffened measures. • Two thin-walled CFDST columns were tested subjected to combined loads. • The stiffened studs greatly postpone the appearance of local buckling. • The studs produces a clear impact of section confinement, which enhances the specimen's overall performance. • This experiment utilizes a novel mutual verification model. • A modified method for CFDST under combined loads is proposed and utilized for the evaluation of two specimens. [ABSTRACT FROM AUTHOR]

Reinforced ultra-high-performance concrete (RUHPC)-encased concrete-filled steel tubular (CFST) columns in composite construction have exceptional fire and corrosion resistance, superior load-bearing capacity, and enhanced ductility compared to standalone RUHPC and CFST columns. However, research on their behavior is limited. This paper presents a 3D finite element (FE) simulation model for accurately predicting the behavior of axially loaded RUHPC-encased CFST short columns. The model accounts for the effects of the concrete confinement provided by the steel reinforcement and circular steel tube. The existing experimental data is used to validate the FE model. The validated model is then utilized to investigate the performance of RUHPC-encased CFST columns with various geometric and material properties. The range analysis technique is applied to an orthogonal design to assess the relative significance of the factors affecting the structural behavior. The findings reveal that RUHPC-encased CFST columns have high ultimate strength and ductility. The range analysis identifies the importance order of parameters as: the compressive strength of UHPC, the steel fiber concentration, tube diameter, stirrup spacing, tube thickness, and confined concrete's compressive strength. Lastly, a simplified design model is developed to estimate the compressive capacity of RUHPC-encased CFST stub columns, providing accurate results that are verified by tests and numerical analysis. • Finite-element simulation model for UHPC-encased CFST stub columns is developed. • The behavior of UHPC-encased CFST stub columns is parametrically studied. • A range analysis is carried out to specify the importance of design key factors. • The proposed finite-element simulation and design models provide accurate predictions. [ABSTRACT FROM AUTHOR]

This paper introduces a novel cross-sectional design for concrete-filled steel tubular (CFST) columns termed "concrete-filled double square steel tube" (CFDSST). The CFDSST columns incorporate an inner square steel tube positioned diagonally at 45° relative to the outer square steel tube, which serves as a stiffener for the outer tube. A three-dimensional finite element simulation model is presented to accurately predict the behavior of axially loaded CFDSST short columns. This model considers the influence of concrete confinement by the steel tubes. Experimental data are employed to validate the accuracy of the finite element model. The validated model is then utilized to assess the performance of the proposed columns, considering various geometric and material properties. The results highlight the substantial enhancement in structural performance achieved by the innovative CFDSST column configuration compared to conventional square CFST columns. The integration of the inner CFST components enhances both buckling resistance and the confinement mechanism of the column. This contributes to the increased load-bearing capacity, ductility, and residual strength of the columns. Notably, the steel tube thickness, the yield strength, and the concrete compressive strength greatly affect the performance of the column. Lastly, a simplified design model is formulated to predict the compressive capacity of the CFDSST stub columns, providing accurate outcomes validated by numerical analysis. • A new configuration of CFST columns is introduced. • Finite-element simulation model for CFDSST stub columns is developed. • The behavior of CFDSST stub columns is parametrically studied. • The proposed finite-element simulation and design models provide accurate predictions. [ABSTRACT FROM AUTHOR]

Under the action of complex earthquake loads, the elastic-plastic local buckling of a concrete-filled steel tubular column will most likely occur in the steel tube, which is difficult to be simulated using the traditional fiber beam-column finite element model. In this paper, 30 steel specimens with two strength grades, Q235 and LYP160, were prepared and tested considering various cyclic loading paths to obtain the hysteretic stress-strain relations and the elastic-plastic buckling behavior of steel material, aiming to develop an effective uniaxial constitutive model of steel material considering elastic-plastic buckling. Based on the available Légeron Model, Gomes and Appleton Model (GA Model), and Dhakal and Maekawa Model (DM Model) for determining the stress-strain relation of steel material considering the elastic-plastic buckling behavior, a comparison analysis was made between the test results and the predicted results. It is found that the buckling behavior of the steel in compression after yielding cannot be simulated well by Légeron Model; the compressive buckling stress and compressive strength of the steel with buckling behavior predicted by GA Model agrees well with the test results; and the unloading stiffness of the steel in tension and compression can be well predicted by DM Model. [ABSTRACT FROM AUTHOR]

Based on a quasi-static test on six concrete-filled double skin steel tube columns subject to cyclic pure torsion, the torsion behavior of concrete-filled double skin steel tube columns with various section types, hollow ratios and steel ratios was studied. The failure modes, torsion-rotation angle hysteretic curves and strain distribution law of concrete-filled double skin steel tube columns under reversed cyclic loading are obtained. The test results show that the hysteretic curves of concrete-filled double skin steel tube columns under pure torsion are plump. The unloading stiffness was close to the initial elastic stiffness. The good energy dissipation capacity of concrete-filled double skin steel tube columns can be observed. Under a cyclic torsion moment, the torsion-resistance capacity of circular concrete-filled double skin steel tube columns was better than that of square and rectangular concrete-filled double skin steel tube columns. With the same thickness of the inner steel tube and the same section size and shape, the hollow ratio of concrete-filled double skin steel tube columns has little effect on the cyclic torsion behavior. [ABSTRACT FROM AUTHOR]

• A new type of SPSW structure is proposed. • The mechanical properties and failure modes of the specimens were analysed. • The design method of the shear capacity of buckling-restrained beam-only-connected SPSWs was proposed. In this paper, a new type of steel plate shear wall (SPSW) structure comprising square concrete-filled steel tube (CFST) boundary frame and infilled cold-formed steel (CFS) buckling-restrained beam-only-connected SPSWs is proposed. To study the lateral behaviour and failure modes of the buckling-restrained beam-only-connected SPSWs with square CFST frame, tests on a buckling-restrained beam-only-connected SPSW and a comparison specimen with unstiffened beam-only-connected SPSW with square CFST frame were conducted. The failure mode, hysteretic curve, shear capacity and energy dissipation capacity of the specimens under horizontal cyclic load were reported. The results showed that the two specimens had good lateral resistance and stiffness, which indicated that this structural system was a reliable SPSW structure. Besides, the arrangement of the CFS buckling-restraining strips could improve the seismic performance of beam-only-connected SPSWs. A total of 84 finite element (FE) models were developed and validated. Upon validation, the parametric analysis was carried out, and design method of the shear capacity of buckling-restrained beam-only-connected SPSWs was proposed. The results show that the newly developed design method is accurate and can better predict the yield capacity of CFS buckling-restrained beam-only-connected SPSWs. [ABSTRACT FROM AUTHOR]

Abstract: When subjected to the horizontal earthquake load, compression–bending–torsion combined actions often exist in the piers of curved bridges because the center of stiffness and the center of mass of curved bridges do not locate at the same position. A new fiber beam-column element considering the nonlinear torsion behavior for concrete filled steel tubes with circular section was developed based on the traditional fiber beam-column model, which had widely been used. The calculation and solution procedures of the proposed fiber beam-column model were also illustrated. The reasonable geometrical relations, material constitutive law and force equilibrium equations were all applied. Based on the principles of fiber beam-column element considering torsion effect, a nonlinear solution program was developed using FORTRAN language. The entire loading history of concrete filled steel tube columns subjected to combined compression–bending–torsion load could be predicted with good agreement with test results. Using the proposed fiber beam-column model for the time-history analysis of curved steel–concrete composite girder bridges, the high solution precision and efficiency could be obtained, compared with the traditional finite element model. [Copyright &y& Elsevier]

Abstract: Concrete filled steel tube (CFST) columns are currently being widely used in the construction of modern buildings and bridges. In practice, the torsion effect will be often created in CFST columns when the horizontal earthquake happened. Based on geometrical, material constitutive and equilibrium equations, a theoretical model called laminated tubes model for analyzing the mechanical behavior of CFST columns under axial force–torsion combined action was proposed. The confined effect provided by the steel tube and the compression softening effect of the concrete could be considered in the proposed concrete material model. Based on the theoretical model, the non-linear analysis program was developed for obtaining the entire loading history of CFST columns under axial force–torsion combined action. The predicted results have good agreement with test results. The torsion behavior of CFST columns was detailed discussed based on the theoretical model, and the simplified formulae for practical design are also proposed based on regression methods. [Copyright &y& Elsevier]

Abstract: Based on the force–displacement mixed control quasi-static test on eight CFST columns subjected to combined compression, bending and torsion cyclic load, the mechanical behavior of CFST columns with various section types, bending moment to torsion moment ratios and axial load levels was studied. The test results showed that the hysteretic curves of CFST columns under combined compression, flexure and torsion are plump due to the good seismic behavior and the ductility was also good. But for rectangular CFST columns with high bending moment to torsion moment ratio, the strength degradation was observed due to the local buckling of the steel plate at the bottom. The torsion capacity of CFST columns would be reduced by the bending moment. The plane section assumption of axial strain of CFST columns could be satisfied. The shear strain of the steel tube has good linear relationship with the rotation angle of the section when CFST columns subjected to combined compression, flexure and torsion. Based on the test results and literatures available, the mechanism of CFST columns was qualitatively analyzed. [Copyright &y& Elsevier]

Abstract: Based on the quasi-static test on eight CFST columns subjected to pure torsion and compression–torsion cyclic load, the torsion behavior of CFST columns with various section types, steel ratios and axial load levels was studied. The test results showed that the hysteretic curves of CFST columns under pure torsion and low compression–torsion cyclic load are very plump, which indicate that CFST columns have good seismic capacity. The unloading stiffness of CFST columns was equal to the initial elastic stiffness. The torsion capacity of CFST columns could be improved by the low compressive load, and the ductility was also good. But the torsion capacity of CFST columns would be reduced by the high compressive load. When CFST columns subjected to pure torsion, spiral diagonal compressive struts will be created in the in-filled concrete, and the axial components of the diagonal compressive force of the in-filled concrete was equal to the axial tensile force of the steel tube in order to satisfy the axial load equilibrium condition on the section, so the axial strain will be produced in the steel tube. The shear strain has good linear relationship with the rotation angle of the section when CFST columns subjected to pure torsion and compression–torsion combined action. Based on the test results and literatures available, the torsion mechanism of CFST columns was preliminarily analyzed. [Copyright &y& Elsevier]

Concrete-filled steel tubular (CFST) columns are widely utilized in structures owing to their cost-effectiveness, high strength, ductility, and proven fire resistance. Despite these advantages, concerns persist regarding the performance of CFST columns with locally corroded steel tubes, impeding their widespread adoption in offshore structures. This research employs three-dimensional finite element (FE) simulations for circular CFST stub columns subjected to axial compression, validated against experimental data. Verification includes full load-displacement histories, ultimate axial strengths, and failure modes. The validated FE models are then employed to assess and compare the structural behavior of CFST columns with various shapes of localized corrosion, analyzing overall load-deformation curves, interaction stress-deformation responses, and composite actions. To acquire critical data for understanding stub column behavior, 81 specimens underwent non-linear finite element analysis, with local corrosion represented by artificial notches in steel tubes. Test parameters, such as notch orientations, lengths, depths, widths, and concrete compressive strength, are varied. Results elucidate the impacts of these parameters on different aspects of stub column responses with corroded steel tubes. Furthermore, the accuracy of existing design models in predicting the ultimate axial strengths of CFST columns with corroded steel tubes is assessed. Finally, through parametric analysis of test results, a practical model is proposed and verified for computing the peak resistance of a CFST stub column with a locally corroded steel tube. • Finite-element simulation model for circular CFST stub columns with locally corroded steel tubes is developed. • The influence of different shapes of localized corrosion on the performance of circular CFST stub columns is investigated. • The behavior of circular CFST stub columns with locally corroded steel tubes is parametrically studied. • The proposed finite-element simulation and design models provide accurate predictions. [ABSTRACT FROM AUTHOR]

The concrete-filled steel tube (CFST) column circumferentially confined by carbon fibre reinforced polymer (CFRP), i.e. CFRP-CFST column, is a promising structural member in engineering practice. In this paper, the torsional capacity of CFRP-CFST columns was investigated by means of finite element (FE) simulation and theoretical derivation. Comprehensive FE models were established considering suitable material constitutive laws and interface relationships, which were then verified by experiment results. Besides, parametric analysis was conducted to reveal the geometrical and material parameters' effects on the torsional capacity of CFRP-CFST columns. The results show that the yield strength, the thickness, and the outer diameter of the steel tube are the most influential factors, and the number of CFRP layers also affects the torsional capacity considerably. Meanwhile, the comparison between the CFRP-CFST and CFST column indicates that the confinement effect provided by the CFRP can enhance the torsional capacity. Finally, equations for calculating the torsional capacity of the CFRP-CFST column were proposed and proved to be accurate through comparison with the test data. Unlabelled Image • Comprehensive finite element models were established and verified by test data. • Parametric study on torsional behaviour of CFRP-CFST columns was conducted. • Torsional capacity was compared between CFRP-CFST columns and CFST columns. • Formula for predicting the torsional capacity was proposed and verified. [ABSTRACT FROM AUTHOR]

It has been shown that fire disaster normally occurs following an earthquake disaster, which might cause even more serious losses than the earthquake itself. This study investigates the post-earthquake fire performance of concrete-filled steel tube (CFST) columns. The experimental programme contains two parts - quasi-static tests and fire tests. During the quasi-static tests, the square CFST columns were loaded under constant compressive force and cyclic horizontal force, until reaching different degrees of seismic damage. Afterwards, the pre-damaged columns were loaded under a constant axial compression ratio (i.e. ratio of the applied compressive load to its bearing capacity) of 0.3 at elevated temperatures. The fire resistance time and failure modes of these pre-damaged CFST columns were reported. Finite element (FE) models were developed and validated against the test results; a total of 48 numerical results were generated in the parametric study using the validated model. In the parametric study, the influence of key parameters including the thickness of steel tube, the yield strength of steel tube, the axial compression ratio and pre-damage degrees on the fire resistance of square CFST columns were analysed. The experimental and numerical results are used to propose design rules for the fire performance of square CFST columns with different degrees of seismic damage. • Quasi-static tests and fire tests were carried out in the experimental programme. • Post-earthquake fire performance of square concrete-filled steel tube columns was investigated. • The design rules for the post-earthquake fire performance of square CFST columns were proposed. [ABSTRACT FROM AUTHOR]

An innovative composite structural member formed by concrete filled steel tube (CFST) circumferentially confined by carbon fiber-reinforced plastics (CFRP), i.e., referred to as CFRP-CFST, has great potential to be used in marine environment due to its excellent structural performance and corrosion resistance. This paper aims to investigate the behaviour of CFRP-CFST subjected to combined compression-bending-torsion load. Quasi-static tests are conducted on 5 CFRP-CFST members and 3 reference bare CFSTs. The test parameters include the loading scenario (pure torsion, bending-torsion, and compression-bending-torsion), the axial load ratio and the layers of CFRP wrapping. Experimental results show that the CFRP-CFST members exhibit good energy dissipation capacity under pure torsion and compression-bending-torsion loads, while the applied axial force benefits the bearing capacity and the stiffness. However, 'pinch' effect is observed in the load-deformation hysteretic curves of CFRP-CFST under bending-torsion load. It indicates that the confinement provided by CFRP can significantly improve the column bearing capacity and ductility under torsion. When subjected to combined compression-bending-torsion, the CFRP wrapping can effectively improve the column ductility. The energy dissipation capacity increases with multiple CFRP layers. Eventually, the structural mechanism at the ultimate limit state is analyzed, with the calculation methods for the bearing capacities evaluated. [ABSTRACT FROM AUTHOR]

Based on the completed experimental study, the structural response of steel tube confined RC (STRC) columns under pure torsion, bending-torsion and compression-bending-torsion load were investigated through the finite element method (FEM). Experimental results were used to verify the proposed finite element model and the verification results revealed that the assessment capability of the developed models was efficacious. The model considered the 3D equilibrium, compatibility, and constitutive laws of materials at the section and structural level. In addition, a parametric study was performed to visualize the influence of steel tube, section size, concrete, longitudinal reinforcement, stirrup, and axial force on the ultimate bearing capacity and the ultimate bearing capacity correlation of STRC columns. To this aim, a total of 72 STRC columns were modeled. It was noticed that the thickness of steel tube, the yield strength of longitudinal reinforcement, the diameter of longitudinal reinforcement (longitudinal reinforcement ratio) and axial force were the most crucial parameter, influencing the ultimate bearing capacity of STRC columns significantly. As well, it was overtly beheld from the study that the correlation curves of flexural and torsional capacity were similar under the influence of different parameters, and relevant equations of flexural and torsional capacity were proposed through curve fitting. • The finite element model was developed to investigate the behavior of STRC columns under combined load. • The parametric study was carried out and the most crucial parameters were figured out. • The correlation curves of ultimate bearing capacity were obtained through curve fitting. • The relevant equations of ultimate bearing capacity were proposed. [ABSTRACT FROM AUTHOR]

• Experiment and numerical analysis of built-up hot-rolled steel sections were carried out. • The design rules for hot-rolled built-up section stub columns under compression were proposed. • The design rules of the stability bearing capacity of columns have been developed. In this paper, a new type of steel section is proposed comprising adjacent H-sections linked together by intermediate steel plates welded along their flanges. Experiments on three hot-rolled steel built-up H-sections and five columns infilled with concrete are conducted, with results showing that both variants having much better ductility compared to conventional columns (i.e. hollow section columns and concrete filled steel tube (CFST) columns). Experimental results showed that the bearing capacity of the built-up hot-rolled steel hollow section column improved by 45.1% to 55% after it was filled with concrete. A finite element software ABAQUS is used for parametric study. The newly developed finite element models were validated against test results in terms of failure modes and load-displacement curves. A total of 482 numerical results, 132 hot-rolled steel built-up section columns and 350 concrete-filled built-up section columns, have been generated. Based on the experimental and numerical results, the compressive behaviours of hot-rolled steel built-up section columns and concrete-filled built-up section columns are studied. A new design method for the compression capacity of these two types of columns is proposed. It is found that the newly proposed design method could provide accurate and consistent predictions – the mean value of experimental to predicted results are 0.963 for hollow section columns (with coefficient of variation being 0.061) and 1.04 for composite section columns (with coefficient of variation being 0.043). [ABSTRACT FROM AUTHOR]

Carbon fibre reinforced polymer material has many advantages, including high strength, light weight and good durability, resulting in good performance when it is used in concrete structures; however, the construction procedures of concrete columns confined by a CFRP tube are very complicated, leading to a relatively high cost. Therefore, CFRP-steel composite tubed concrete columns are suitable for application in concrete structures because of their excellent mechanical properties and convenient construction. In this paper, the axial static mechanical behaviour of circular CFRP-steel composite tubed concrete stub columns was investigated, including an experimental study and a theoretical analysis. Based on investigation of the strain efficiency of CFRP material, the calculation methods for the axial ultimate loading capacity of circular CFRP-steel composite tubed concrete stub columns were compared, and recommendations for practical design were provided. [ABSTRACT FROM AUTHOR]

In this study, a two-side-connected steel plate shear wall (SPSW) reinforced by cold-formed steel (CFS) strips on free edges is proposed. This structure has several advantages, including the ability to avoid additional bending moments on the frame column, convenient layout, and lightweight structure with good assembly effects. To understand the mechanical performance of the novel structure, horizontal reciprocating tests were conducted on two specimens; one was a two-side-connected SPSW reinforced on free edges without a frame and the other with a concrete-filled steel tube frame. The results showed that the two-side-connected SPSW reinforced on free edges still possessed good bearing and energy dissipation capacities. Finite element (FE) models were established, and the numerical analysis results were compared with the test results. Based on the verified FE model, a parametric analysis was performed, leading to the derivation of formulas for calculating the minimum stiffness ratio and buckling bearing capacity. Additionally, formulas for the yield bearing capacity and ultimate bearing capacity were derived based on the calculation formula for the buckling bearing capacity. To improve the design efficiency, a simplified truss model (STM) was proposed. The stress–strain relationship of the equivalent truss was established based on the aforementioned bearing capacity calculation formulas. The STM calculation results were compared with the test results, indicating a high level of accuracy and significantly improved calculation efficiency. • A two-side-connected steel plate shear wall reinforced on free edges is proposed. • The mechanical properties of the specimens were derived and analysed. • The calculation formulas for the elastic buckling, yield and ultimate bearing capacity were introduced. • To improve the design efficiency, a simplified truss model (STM) was proposed. [ABSTRACT FROM AUTHOR]

The concrete-filled steel plate (CFSP) composite coupling beam is a newly developed form of coupling beam that possesses high deformation and energy dissipation capacities. In this paper, the shear capacity of concrete-filled steel plate composite coupling beams was investigated. An analytical model was proposed based on the force mechanism of the composite coupling beam and was proved to have adequate accuracy compared with the available test results. The shear capacity of the composite coupling beam was found to be governed by flexural strength, shear strength, or flexural-shear strength, depending on the material and geometrical properties of the coupling beam. By incorporating the effect of flexural-shear interaction, a unified equation was finally developed. [ABSTRACT FROM AUTHOR]

Tapered concrete-filled double-skin steel tubular columns with large hollow ratio (LHR-CFDST) have a good application potential in wind turbine towers, where they are commonly subjected to the combined action of compression, bending, shearing and torsion. Up to now, research work on the behaviour of tapered LHR-CFDST columns under combined loads is seldom reported and relevant experimental data are limited. This paper presents an experimental and numerical study on the tapered LHR-CFDST column under combined compression-bending-shear-torsion loads. Seven specimens with different parameters including the tapered angle, the hollow ratio, the torsion-to-bending ratio, the axial compression ratio and the height-to-diameter ratio were tested. The experimental results in terms of failure modes, load versus deformation relations and strain responses were discussed. Reliable finite element models were established to conduct numerical analysis in an extended parameter range. The mechanism of columns and the effects of torsion-to-bending ratios, axial compression ratios, height-to-diameter ratios, tapered angles, hollow ratios, diameter-to-thickness ratios of steel tubes and the material strength were revealed. The suitability of the existing ultimate capacity prediction method was evaluated for tapered LHR-CFDST columns. It is found that the failure mode of columns under compression-bending-shear-torsion loads is related to the torsion-to-bending ratio. The columns behave in a ductile manner under moderate axial compression. Tapered angle has a slight effect on the bending capacity of the column, whereas the capacity declines with the increase of hollow ratio and torsion-to-bending ratio. The existing method gives a conservative prediction for the ultimate capacity of tapered LHR-CFDST columns. • Tests on tapered LHR-CFDST columns under combined compression-bending-shear-torsion loads are reported. • Behaviour and mechanism of tapered LHR-CFDST columns under combined loads are discussed. • Numerical analyses are conducted and effects of important parameters are revealed. • Suitability of the existing ultimate capacity prediction method is evaluated. [ABSTRACT FROM AUTHOR]

• Three hexagonal CFDST stub columns with varying stiffened measures were tested. • Stiffened measures such as ribs and studs can effectively increase the ultimate strength. • Mechanistic analysis further reveals the positive effect of the stiffening measures during axial compressive stress. • Extensive parametric analyses are carried out on the material strength, and so on. • Modified formulas based on previous studies for circular CFDST structures obtained more consistent predicted values. With the escalating height of wind turbine structures, wind turbine steel-concrete hybrid towers have gradually become a new type of structure to replace pure steel towers. As a component connecting the upper steel tower with the lower concrete tower, the transition piece assumes an indispensable role in the overall structure of the hybrid tower. The traditional reinforced concrete-type transition piece has necessitated enhancements owing to the complicated construction and concrete cracking problems. A hexagonal concrete-filled double skin steel tubular (CFDST) transition piece is proposed to fit the hexagonal steel-concrete hybrid tower. Studies that investigate the hexagonal CFDST structures are currently few. Based on this, three hexagonal CFDST stub columns were tested, with the variables in stiffened measures. The experimental portion included a thorough investigation of local buckling behaviors and failure modes in addition to the analysis of typical mechanical indexes. Further, the validated finite element (FE) model was then utilized for mechanism analysis about the contact stress between different interfaces, the stress distributions, and so on. Besides, a parametric study was done to identify the sensitive parameters affecting this kind of structure's peak loads. Ultimately, the peak load prediction formulas found in published studies and specifications were modified by incorporating the findings from this investigation. [ABSTRACT FROM AUTHOR]

Concrete-filled double skin steel tubular columns with large hollow ratios (LHR-CFDST), as quintessential elements within offshore wind turbine structures, have received significant attention in contemporary times. As the scale of this type of structure continues to expand, an effective strategy to reduce steel consumption involves elevating the diameter-to-thickness (D/t) ratios of the steel tubes. Presently, there is a dearth of studies that explore the performance of LHR-CFDST columns in the case of thin-walled inner steel tubes. Four LHR-CFDST columns with varying D/t ratios of the inner tubes and one double-skin hollow steel tubular column were produced and tested under axial compression. The study encompassed an investigation into failure modes, local buckling behaviors, and ductility characteristics. The local buckling performance index of specimens with varying D/t ratios differed by up to 16.9%. Furthermore, the verified finite element model was subsequently employed for mechanistic analyses pertaining to the sandwiched concrete's stress distributions, and so on. When the peak loads were attained, the maximum variation in the stress values of the concrete of the specimens with different D/t ratios reached 50.4%. Subsequently, a multi-parameter impact analysis was applied to give a reasonable limit on the D/t ratio (225) of the inner steel tube for engineering design. Finally, the prevailing ultimate strength formulas outlined in established specifications were adapted by integrating the characteristics uncovered through this investigation. • Thin-walled inner tubes have a significant effect on CFDST with large hollow ratios • Inner tube's D/t ratio is positively correlated with the local buckling • Stress distribution and contact stresses in concrete are inextricably linked to the inner tube's D/t ratio • Multi-parameter impact analysis gives a reasonable limit of the D/t ratio for design • The equation considering the effect of buckling predict the CFDST columns with thin-walled inner tubes more accurately [ABSTRACT FROM AUTHOR]

• A concrete-filled steel tube-gusset plate joint with ring stiffener plates is proposed. • The mechanical properties of the specimens were derived and analysed. • The equivalent interaction method (EIM) was introduced to predict the ultimate strength of CFST-gusset plate joint with ring stiffener plates under tension. Tube-gusset plate joints are increasingly adopted in large lattice towers for wind power, due to the advantages of simple fabrication and clear force transmission path. To investigate the mechanical performance of CFST-gusset plate joints with ring stiffener plates, tensile bearing capacity tests of three tube-gusset plate joints were performed in this study. The test results showed that setting ring stiffener plates and filling concrete could effectively improve the stiffness and tensile bearing capacity of tube-gusset plate joints. In addition, the finite element (FE) models were established in this study, and the accuracy of the FE models was verified by comparing it with the test results. Parameter analysis showed that the increase of the width ratio (R / D) and thickness ratio (t r / t) of the ring stiffener plate and the width ratio (L GP / D) of the gusset plate was a significant method to improve the ultimate strength of tube-gusset plate joints under tension. But when the width ratio and thickness ratio of ring stiffener plates exceeded the limit relationship, the ultimate strength of joints would not be further improved, which was easy to cause waste of materials. Finally, to simplify the calculation of CFST-gusset plate joints with the ring stiffener plates, an equivalent interaction method (EIM) was proposed in this study, which could accurately predict the ultimate strength of CFST-gusset plate joints with ring stiffener plates under tension. [ABSTRACT FROM AUTHOR]

• Two large-scale thin-walled CFDST columns with different stud spacing were tested under axial compression. • The stiffened studs delayed the occurrence of local buckling in these specimens significantly. • The arrangement of the studs leads to a obvious effect of section confinement, thus improving the overall compressive performance of the specimen. • The variability of the Poisson's ratio of concrete and steel during loading resulted in different stresses on the studs. • The current design specifications for calculating the peak load of CFDST columns are modified and used for the evaluation of the two large-scale thin-walled CFDST specimens in this study. Wind turbine towers offer good application potential for concrete-filled double-skin steel tubular columns with large hollow ratios (LHR-CFDST). With the increasing size of this engineering structure, the requirement to reduce construction expenses can be met by enhancing the diameter-to-thickness ratio of steel tubes. Few studies have been conducted to report on thin-walled LHR-CFDST columns. Meanwhile, local buckling and size effect shall be considered for this type of structure. Based on this, the first experimental investigation on large-scale stiffened thin-walled LHR-CFDST columns under axial compression is reported in this paper. Two large-scale thin-walled LHR-CFDST columns with different stud spacings are discussed. Failure modes, local buckling behaviours, load-strain responses, and some common mechanical indexes are investigated. According to the test results, adding studs was an effective method to increase the ultimate capacity of LHR-CFDST columns, as evidenced by the 16.6% increase in peak load for the specimen with smaller stud spacing than the other. Besides, the arrangement of studs leads to an obvious effect of section confinement, thus improving the overall compressive performance of the specimens. Then, four existing specifications all give a conservative prediction for the ultimate capacity of the specimen with smaller stud spacing, indicating that the large-scale thin-walled LHR-CFDST can be evaluated utilizing the present specifications with suitable stiffening measures. [ABSTRACT FROM AUTHOR]

• Typical failure patterns of TLHR-CFDST columns for wind turbine towers are investigated. • Coupled mechanisms of columns under compression-bending-torsion loads and parameter effects are analysed. • Ultimate strength correlations of TLHR-CFDST columns for wind turbine towers are revealed. • A strength prediction method of TLHR-CFDST columns under combined loads is proposed. Concrete-filled double-skin steel tubular (CFDST) tower has emerged as a promising type of wind turbine support structure with excellent strength and stiffness. As the basic member in CFDST towers, tapered concrete-filled double-skin steel tubular columns with large hollow ratios (TLHR-CFDSTs) are frequently subjected to complex combined compression-bending-torsion loads. This paper presents a comprehensive analytical study on the coupled behaviour of the TLHR-CFDST column under combined loads and the relevant ultimate strength prediction method. A FE model was established to conduct numerical analyses and extensive parameter studies while the data from the previous experimental research was used to provide a verification basis. It is found that the column subjected to combined loads has three typical failure patterns, which are mainly governed by the bending-to-torsion ratio (m t). Columns with moderate m t undergo the bending-torsion failure and show obvious coupled performances. The load-deformation relation, the ultimate characteristics and the effects of important parameters, including axial compressive ratio (n), tapered angle (θ), hollow ratio (χ) and height-to-diameter ratio (λ) were discussed. The ultimate strength correlations of the column were revealed by analysing strength data within the parameter scopes commonly used in wind turbine towers. Lastly, a practical and accurate prediction method for the ultimate strength of the TLHR-CFDST column under combined compression-bending-torsion loads was proposed. [ABSTRACT FROM AUTHOR]

Concrete-filled double skin steel tubular columns with large hollow ratio (LHR-CFDST) have been commonly used in recent years. With the increasing size of the engineering structures, one efficient way to decrease the consumption of steel is to increase the diameter-to-thickness ratio. Few studies have been conducted to report on thin-walled LHR-CFDST columns. At the same time, local buckling may occur in these structures in the absence of stiffening measures. Based on this, an experimental and numerical analysis was carried out for the stiffened thin-walled LHR-CFDST columns under axial compression in this paper. Five thin-walled LHR-CFDST columns with different stiffening measures (ribs, studs) were tested. The failure modes, load versus shortening curves, local buckling behaviours and ductility were investigated. The test results indicates that the stiffened measures effectively delayed the appearance of local buckling and increased the peak load. Besides, the finite element models validated by the test results were applied to the subsequent mechanical mechanism analysis of the stiffened thin-walled LHR-CFDST. It can be seen that the stiffened measures enhance the confinement of the inner and outer steel tubes on the concrete and greatly improve the ultimate strength of the concrete. Finally, the predicted peak load formulae in the existing design specifications were modified by combining the characteristics of the specimens in this paper. The peak load values obtained by the improved formulae agreed well with the test results. • Five thin-walled CFDST columns with different stiffening forms (ribs, studs) were tested under axial compression. • The effects of stiffeners on the local buckling and peak load of specimens were revealed. • The validated finite element model was developed to further analyze the structural mechanisms. • The interaction between steel tubes and concrete were investigated. • The predicted peak load formulae in the existing design specifications were modified. [ABSTRACT FROM AUTHOR]

This paper presents a pseudo-static test investigation and finite element analysis on the behavior of circular concrete-filled double skin steel tube (CFDST) members under axial compression and torsion. The hollow ratio (χ) and axial compression ratio (n) are considered in the test study. The ductility, hysteretic behavior, failure modes, and compression–torsion correlation of circular CFDST members are discussed. The comparison results show that the validated finite element model is capable to capture the behavior of circular CFDST members under compression and torsion. The effect of loading paths and various parameters on the compression–torsion correlation of circular CFDST members are analyzed. Parametric analyses are carried out to investigate the effect of key parameters, such as material properties, nominal steel ratio, the wall thickness of inner steel tube, and hollow ratio on the compression–torsion correlation of circular CFDST members. It can be seen that the circular CFDST members under compression and torsion possess good load-bearing capacity, energy dissipation, and ductility properties. The loading paths, yield strength of inner and outer steel tubes, nominal steel ratio, and the wall thickness of inner steel tubes have little effect on the compression–torsion correlation of the composite members. When the axial compression ratio is less than 0.2, the axial compression load can slightly improve the torque capacity of circular CFDST members. Finally, the compression–torsion correlation equation of these members was proposed by regression analysis, and design formulas with good accuracy for predicting the torque capacity of circular CFDST members under compression and torsion were proposed. • Test investigation and numerical analysis were conducted to study behavior of CFDST members under compression and torsion. • Parametric analyses and regression analysis were conducted to propose the Axial compression and torsion correlation equation of CFDST members. • Parametric analyses have been conducted considering key parameters. • The axial compression and torsion correlation equation of CFDST members were proposed via regression analysis. • Formulas with good accuracy for predicting the torque capacity of CFDST members under compression and torsion were proposed. [ABSTRACT FROM AUTHOR]

This paper studies the load introduction and transfer mechanisms for steel-encased concrete-filled steel tube (CFST) K-type connections experimentally and numerically. Six K-type CFST connections with encased built-up latticed angles were tested. The finite element (FE) models were established and verified by the experimental results. Effectiveness of the reinforcing plates, i.e., direct bearing in an extended range was evaluated based on the parametric analysis. The upper chord length of 2 D mm is sufficient to transfer the required load from the chord wall to inner concrete via the direct shear interaction only. Though employing insufficient confinement provided by the chord wall, the chord with non-compact and slender sections cannot fully transfer the required load, even if the reinforcing plates are adopted. In general, due to the non-uniformed load introduction from single-side braces in K-type CFST connections, the reinforcing plates cannot effectively provide direct bearing to the inner concrete due to the localised stress effect and plying force. Furthermore, the ability of load transfer in the interface of inner concrete and encased angles should be ensured to achieve full utilisation of angle strengths. Finally, the equations for the lengths of load introduction between the tube-concrete and concrete-angle interfaces were proposed according to the numerical results. The comparison with the numerical results demonstrates that the proposed equations can predict load introduction lengths accurately and reliably. • Load introduction within steel-encased CFST K connections is assessed experimentally and numerically. • Non-uniform load transfer is found for single-side brace CFST connections. • Effectiveness of reinforcing plates is studied in contributing direct bearing to the load transfer. • Shear interaction and direct bearing mechanisms for steel-encased columns are evaluated. • Design equations for load introduction lengths are proposed. [ABSTRACT FROM AUTHOR]

Steel tubes adopted in wind turbine towers generally sustain the combination of compression, bending, and torsion loads. The study of compressive behavior is crucial for investigating the combined bearing capacity of the tower and proposing the corresponding design method. Local buckling tends to affect compressive behavior due to the large diameter-to-thickness ratios of the tubes. Thus, the authors proposed a new form of tower based on longitudinally stiffened steel tubes, which were experimentally and analytically studied in this paper. A total of six tube specimens (four stiffened ones and two unstiffened ones) were tested under axial compression. The experimental results indicate that the local buckling of the tube can be substantially prevented by stiffeners and the T-type stiffener exhibits a superior effect. Finite element models were built and validated by the test results, and the parametric analysis was conducted, indicating that the diameter-to-thickness ratio of the steel tube, the yield strength of steel, and the proportion of stiffener area are the prime sensitive parameters affecting the ultimate strength. Ultimate compressive strength prediction methods in design specifications were evaluated. A practical method for calculating the ultimate strength of stiffened steel tubes was subsequently developed in this paper. • Six steel tube specimens with different stiffener types and diameter-thickness ratios were tested under axial compression. • Finite element models were established and validated by the test results. • Parametric analysis was conducted and the prime sensitive parameters affecting the ultimate strength were presented. • The applicability of calculation methods suggested by several specifications was evaluated. • The calculation method for the ultimate axial compressive strength was proposed by parametric and regression analysis. [ABSTRACT FROM AUTHOR]

Tapered concrete-filled double-skin steel tubular columns with large hollow ratios (LHR-CFDSTs) possess light self-weight and good lateral rigidity. They exhibit tremendous potential in lateral load-critical structures, including large-span buildings, wind turbine towers and offshore infrastructures. In these structures, in addition to axial force and bending moment, columns are also commonly subjected to considerable torque. Although the compressive/flexural behaviour of tapered LHR-CFDST columns has attracted attention from researchers, studies on the torsional performance and mechanism are limited. This paper thus presents a comprehensive analytical study on the torsional behaviour of tapered LHR-CFDST columns on the basis of previous experimental research. Numerical analyses were conducted to investigate the influence of the key parameters. Full-range torsional characteristics of the column were revealed with regard to the typical torque–torsional angle relation, the stress development of the concrete and the contact between components. Load distributions as well as the effective twist length were discussed to understand the mechanism of the column under torsion. It is found that significant torsional deformation, oblique concrete cracks and struts, the compressive stress of concrete and the contact pressure all develop concentratedly within the effective twist length, which reflects the segments of the tapered column with a relatively weak torsion-resistant ability. Lastly, a torsional capacity prediction method was proposed by refining the previous theoretical model and it exhibits favourable accuracy for tapered LHR-CFDST columns with tapered angles higher than 1.50°. • Comprehensive numerical models of tapered LHR-CFDST columns are developed and verified. • Parametric effects and full-range torsional behaviour are analysed. • Load distribution and effective twist length are discussed to reveal torsional mechanism. • A refined torsional capacity prediction method of tapered LHR-CFDST columns is proposed. [ABSTRACT FROM AUTHOR]

• 1) Completing the monotonic axial loading test on the STCLAC stub columns. • 2) Proposing a formula that calculated the equivalent lateral confining stress. • 3) Proposing formulae that calculated the compressive stress and the corresponding strain for confined LAC. • 4) Proposing the constitutive model for confined LAC. To study the constitutive model of lightweight aggregate concrete (LAC) constrained by the circular steel tube, the monotonic axial loading test was carried out on steel tube confined LAC (STCLAC) stub columns, considering the effects of the diameter-to-thickness (D / t) ratio (30.4, 19 and 12.7) and steel grade (Q235 and Q355). The test results showed that the steel tubes of STCLAC columns were bulging outward as a whole without local buckling during the loading process. The D / t ratio for the STCLAC column is recommended that be greater than 19.0. Due to friction stress accumulating at the interface between the steel tube and the LAC core, the steel tube is subjected to longitudinal stress. When the confinement factor (ξ) is less than 2.313, the transverse stress of the steel tube reached its yield strength before the peak load, otherwise it reached the proportional stress without yielding. Using the average transverse stresses along the height of the steel tube, this study calculated the equivalent lateral confining stress of the STCLAC column. The calculation formulas of the compressive stress and corresponding strain of confined LAC were proposed based on the test results. And the constitutive model was also proposed, which is more capable of predicting the stress–strain relationship of the constrained LAC by a circular steel tube. [ABSTRACT FROM AUTHOR]

Both concrete-filled steel tube (CFST) column and buckling-restrained steel plate shear wall (BRSPSW) have the advantages of high load-carrying capacity and superior seismic performance. The structural system composed of CFST columns and BRSBSWs will have the superior mechanical performance and be more suitable for high-rise buildings. In this study, the pseudo-static tests were carried out on the specimens of unstiffened steel plate shear wall and cold-formed steel (CFS) BRSPSW which were infilled in the single-span two-story 1/3 scaled composite frame respectively. The test results show that the CFS stiffeners have the obvious effect of restraining the outward buckling deformation on the steel plate, improving the lateral stiffness and resistance of the steel plate, and reducing the deflection deformation of the steel plate to the surrounding composite frame. The finite element (FE) model was established and validated, and the interaction between the BRSPSW and the composite frame was analysed. In combination with the theory and the FE analysis results, the maximum deflection limits of CFST column and steel beam were proposed to ensure the strength of the infill BRSPSW to give full play. Considering the separation of the steel tube wall of square CFST from the core concrete in the process of loading, the deflection limit of square CFST is proposed considering the separation effect of the steel tube wall of square CFST from core concrete under the action of infilled steel plate. • A novel buckling-restrained SPSWs with composite structure was investigated. • The interaction behaviour of buckling-restrained SPSW and boundary composite frame were discussed. • The deflection limit of square CFST and steel beams is proposed under the action of infilled steel plate. [ABSTRACT FROM AUTHOR]

Tapered Concrete-Filled Double Skin Steel Tubular (CFDST) columns have been commonly used in structures. To report further information, it is necessary to perform more studies on the structural behaviour of tapered CFDST columns, especially for those with high hollow ratios. This paper thus conducted an investigation on the tapered CFDST stub columns under axial compression. Specimens with varied tapered angle, hollow ratio and tubes' thickness were tested, and then typical failure modes, load versus displacement relations as well as strain development were revealed. Validated finite element models were given to further analyse the columns' structural mechanisms including the stress distributions, the interaction between the steel tubes and the sandwiched concrete, and the influence of main parameters. The ultimate strength of specimens was calculated tentatively using the methods in the existing design guidelines, and their accuracy and applicability for the tapered CFDST stub columns with high hollow ratios were preliminarily evaluated. A simplified method was suggested to predict the ultimate strength based on one of the guidelines, and the predictions show reasonable agreement with the test results. [Display omitted] • Tests on tapered CFDST stub columns under axial compression are reported. • Experimental performance of tapered CFDST stub columns is revealed. • Mechanism analysis is conducted through verified finite element models. • Methods for predicting the ultimate strength of tapered CFDST columns are checked. [ABSTRACT FROM AUTHOR]

• Seismic performance of H-CFST columns under cyclic torsional and compression–cyclic torsional loads is investigated. • Two types of H-CFST columns under cyclic pure torsion and compression-torsion are tested. • Shear capacity of steel and concrete under combined normal and shear stress conditions is firstly studied. • Design methods of torsional capacities of H-CFST columns under cyclic pure torsion and compression-torsion are proposed. Concrete-filled steel tube (CFST) columns have been widely used in the construction industry because of their excellent compression capacity and torsion resistance. Regarding traditional CFST columns, the H-CFST column with an additional H-section steel member embedded in the core concrete provides improved fire and compression resistance. In this study, the seismic performance of nine H-CFST columns under cyclic torsional and compression–cyclic torsional loads was experimentally analysed. In the experiments, the failure mode, torque versus torsional angle hysteresis, torque versus torsional angle skeleton, torsional stiffness degradation, and energy dissipation capacity were determined. The hysteretic curves of the H-CFST columns under torsion were relatively plump in shape and exhibited no 'pinching' phenomenon, and the torsional stiffness of the H-CFST columns under unloading and reverse loading was approximately equal to the initial torsional stiffness. In addition, the applied compression had remarkable effects on the torsional capacity, torsional ductility, torsional stiffness, and hysteretic loop energy dissipation of the H-CFST columns. The shear capacities of the steel and concrete under normal stress or combined normal and shear stress were determined based on the latest theories and test results. Moreover, design method for the determination of the torsional capacity of H-CFST columns under compressive torsion is proposed based on the test results, and the theoretical contribution ratio of the H-section steel, steel tube, and concrete in the H-CFST columns to the torsional capacity was determined. [ABSTRACT FROM AUTHOR]