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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]

Cold-formed steel (CFS) buckling-restrained steel plate shear wall (SPSW) is a highly efficient lateral force resisting structure, which integrates architectural and structural functions. CFS stiffeners significantly reduces the out-of-plane deformation and improves the strength of SPSW. To investigate the effect of different arrangements of CFS on the buckling restraint of SPSW regardless of the effect of the surrounding frame, a finite element (FE) model of CFS buckling-restrained SPSW was developed and validated against the test results. Upon validation, a series of FE models were generated for parametric analysis. Based on the FE elastic buckling analysis method, the economic stiffness ratio of CFS to the steel plate was presented. Using the steel bar buckling theory, a simplified analysis model of the equivalent brace model considering the buckling-restrained effect of CFS on the steel plate was proposed. The simplified analysis model can consider the influence of buckling constraints and spacing on the buckling stress of the steel plate. Moreover, it can accurately simulate the stiffness and bearing capacity of the SPSW structure. The results show that the proposed simplified analysis model is accurate and consistent. • The FE model validated against test results was employed in the parametric study. • The economic stiffness ratio of CFS to steel plate was presented. • An equivalent brace model considering the buckling-restrained effect of CFS on steel plate was proposed. [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]

Prestressed concrete filled steel tubular (CFST) lattice tower is an innovative application to combine prestress and CFST composite structure to onshore tall wind turbines. In order to explore the seismic behavior of prestressed CFST lattice tower under a complex load state, there was pseudo-static testing carried out for prestressed four-chord CFST lattice columns under compression-bending-torsion combined load. The hysteretic performance of the specimens was analyzed by obtaining the failure mode, load-displacement hysteretic curve, energy dissipation, stiffness degradation, variation of strands stress and steel strain. The influence of prestress on seismic behavior of CFST lattice chords was investigated from prestress loss rate. Results showed that: the inclined battens of CFST lattice columns were seriously damaged with or without strands, while the inclined battens of non-prestressed specimen were damaged first and then the CFST chords. The existence of inclined battens had a great influence on the structural stiffness, while the application of strands significantly affected the initial stiffness and structural ductility of the prestressed specimen, but did not have any significant effect on ultimate bearing capacity. Post-tensioned strands could protect the main bearing chords and reduced the cracking of concrete in the tension area under the coupling action of compression, bending, and torsion. • Pseudo-static test of a massive structure under complex loading conditions was carried. • The hysteretic performance was investigated experimentally and analytically. • The influence mechanism of post-tensioned strands was investigated from the prestress loss ratio. • Some specific suggestions for engineering design were proposed. [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]

The lack of test data on the fatigue crack growth behaviour of Q420C steel affects its application in the field of wind power engineering. To fill this gap, this paper carried out an experimental study and theoretical analysis of the fatigue crack propagation behaviour of Q420C steel under constant amplitude. Six compact tension specimens were separately tested under six loading conditions comprising two maximum loads (F max = 8 kN or 10 kN) and three load ratios (R = 0.1, 0.3, or 0.5). The test results show that for the two crack growth rate calculation methods given by the ASTM E647–15, the secant method provides large discrete results, while the incremental polynomial method can obtain a relatively smooth rate curve. In addition, for the incremental polynomial method, the normalisation of the number of cycles in the ASTM E647–15 should not be used to achieve acceptable results. The rate curves of Q420C steel show a clear load ratio effect. Quantitative analysis of the effect of load ratios has shown that two-parameter models based on 'unified theory', such as Walker's model, can accurately describe the effect of load ratios. However, the theory of plasticity-induced crack closure cannot account for the effect of load ratios, as the crack opening load F op is less than the minimum load F min for load ratios greater than 0.1. Finally, a comparison is carried out against the predictions by BS7910; results verify the applicability of BS7910 to Q420C steel material and demonstrate the different degrees of conservatism of all recommended design curves in BS7910. • This paper presents a thorough fatigue crack growth study of Q420C steel. • No normalisation of cycle number is needed in the incremental polynomial method specified in ASTM E647–15. • The load ratio effect has been confirmed that a higher load ratio leads to higher fatigue crack growth rates in Q420. • The effect of load ratio can be well explained by the unified theory, but not by the crack closure theory. • The design curves in BS7910 also hold for Q420C, of which the simplified curve is preferable in engineering applications. [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]

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 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 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]