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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]

Abstract: Openings often exist in the concrete slab of composite floors due to the functional requirements of structures. The strength and rigidity of steel-concrete composite beams are reduced by openings. Based on three tests of steel-concrete composite beams with full openings in the concrete flange, the elastic rigidity of composite beams is analyzed. Finite element analysis (FEA) considering the slip effect between the steel and concrete is conducted to simulate the composite beams with full openings in the concrete slab, and the results show that the FEA method is reliable. The analytical calculation method for the deflection of composite beams with full openings in the concrete slab is also proposed, and the results are verified by tests. The predicted deflections using the analytical method and FEA method both agree well with the test results. It is further verified that openings near the supports have insignificant effects on the deflection at the mid-span and this effect can be thus ignored. The simplified method to calculate the rigidity reduction factor is developed by a regression analysis. The analytical method and FEA method can be used for the serviceability limit state design of steel-concrete composite beams with full openings in the concrete flange. [Copyright &y& Elsevier]

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]

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]

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]