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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 gravity and the horizontal earthquake load, the eccentric compression and torsion combined action will be produced in the piers of curved girder bridges and the main arch of arch bridge. In order to study the torsion behavior of concrete filled steel tube columns subjected to eccentric compression, tests were carried out on eight specimens of concrete filled steel tube column subjected to static pure torsion, cyclic pure torsion and eccentric compression-torsion cyclic load. The results of tests have indicated that, the hysteresis curve of concrete filled steel tube column subjected to torsion cyclic load was plump; no pinch phenomenon can be found; the damage and degradation degree of the strength and stiffness of specimens is lower; and high energy dissipation capacity can be achieved. The stiffness degradation of the specimens with square section subjected to eccentric compression-torsion cyclic load is more obvious. The eccentric compression has led to weaken of the energy dissipation capacity of the square section concrete filled steel tube column. Under the ultimate state, the wave angle of steel tube was affected by the eccentric compression. [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]

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]

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]

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]

A newly developed reinforced concrete (RC) beam to concrete-filled steel tube (CFST) column joint without welding in the construction field is introduced in this paper. The seismic experiment is conducted on the joint, and the hysteretic curve of the specimen under cyclic loading on the top of the column is obtained. By analyzing the mechanical characteristics and the failure mode of the joint, a three-stage skeleton curve model is presented. In the elastic stage, the initial stiffness is evaluated by solving the stiffness matrix. In the perfect plastic stage and strength degradation stage, the ultimate strength and the degradation stiffness of the joint can be obtained with the limit equilibrium method. Moreover, the test is also simulated with FEM, and the results agree well with the skeleton curves obtained from the test. So this joint shows good ductility and the ability to dissipate energy, and it can be used to replace the joints welded with reinforcement on the outer annular plate. And the finite element analysis results are compared with the theoretical results, which verifies the theoretical model presented above. It is suggested that the three-stage skeleton curve model can be used to depict the mechanical behavior and bearing capacity of this new type of joint. In addition, Parametric analysis is conducted on this joint, with the consideration of different designs of beams, columns and steel brackets, and failure mode is discussed. [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]

• 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]

Highlights • Full-scale angle-encased CFST members under axial tension are investigated experimentally and numerically. • Load-transfer mechanism between components is assessed. • Component interactions that help to achieve high tensile performance are evaluated. • Design method based on load-transfer mechanism and component interactions is proposed. Abstract Load-transfer mechanism of concrete-filled steel tubes (CFSTs) with encased built-up angles under concentric tension is assessed experimentally and numerically in this paper. Seven full-scale angle-encased CFST test specimens with the length of 4000 mm were tested. The test parameters were the diameter-to-thickness ratio (D / T), connecting type, and configuration of encased-angles. Finite element analyses (FEA) were performed to further investigate the tensile behaviour of the composite members. The load distribution, load-transfer mechanism, and interactions between components, i.e. the tube, inner concrete, and encased-angles, were extensively evaluated. All these components could synergistically work under axial tension, though the encased angles were disconnected from the endplates. The strength of the angles could be effectively used through the load-transfer from the concrete, owing to the interactions between each component. In addition, the connecting types exerted minor effects on the ultimate tensile strength despite the existence of an insufficient confining region within the connecting area. The tensile strength design method was proposed for the angle-encased CFST member, based on experimental and numerical analyses on the load-transfer mechanism and component interactions. The comparisons between the calculated predictions and the results from the tests and simulations demonstrated the proposed design method was accurate and reliable. [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]

• 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]

Steel tubes with large diameter-thickness ratios are widely used in tubular steel towers of wind turbines. These steel tubes are susceptible to local buckling under compression-bending load, thus leading to a failure of the overall structures. In this paper, a new structural form of steel tubes with longitudinal stiffeners was proposed. Six specimens were tested to study the effects of stiffeners forms and diameter-thickness ratios of steel tubes. Finite element models were compared and validated by test results and used to further study the structural mechanisms. Lastly, the ultimate strength of specimens was evaluated by existing design guidelines preliminarily. According to the results, by setting stiffeners, especially the T-type stiffeners under the same consumption of the steel, the local buckling was prevented and the plastic deformability of the steel tubes was increased, resulting in the great enhancement of ultimate strength and ductility. The limits of quantity and width-thickness ratio of stiffeners to obtain a more stable and significant improvement effect were suggested through parametric analysis. The current methods agree well with the test results of unstiffened steel tubes but might be inaccurate for stiffened steel tubes. • Six specimens with different stiffener types and diameter-thickness ratios were tested under compression-bending load. • The finite element models were compared and validated by test results and used to further study the structural mechanisms. • The effects of stiffeners on the compression-bending behavior of steel tubes were revealed. • The limits of quantity and width-thickness ratio of stiffeners to obtain a significant effect were suggested. • The applicability and accuracy of the calculating methods in existing design guidelines were preliminarily evaluated. [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]