Concrete-filled Double Steel Tubular Research Articles

Calculation of ultimate bearing capacity of concrete-filled double steel tubular under compression based on the unified strength theory

Concrete-Filled Steel Tube (CFST) components are widely used in the engineering field due to their superior performance. However, the actual stress state of CFST components under axial compression is complex. As a result, calculations for CFST components are often simplified using material equivalent assumption formulas and empirical equations to determine their load-bearing capacity. These simplifications require the introduction of many coefficients and mechanical performance indicators, and the actual material principal stresses are not adequately considered. This approach weakens the completeness of the theory and adds to the burden of experimental research.The aim of this work is to develop a load-bearing capacity calculation formula for Concrete-Filled Double Steel Tubular (CFDST) components that considers the different stress states of the inner and outer steel tubes, the core concrete, and their interface interactions. This formula is designed to be convenient for engineers to understand and use. By comparing the results of this formula with experimental data collected from domestic and international scholars, it is evident that the results of this formula closely align with the collected experimental data. This validation confirms the accuracy and effectiveness of the formula developed in this paper. Furthermore, when compared with existing axial compression load formulas for CFDST, the formula presented in this paper demonstrates good accuracy. The research results provide a convenient and accurate calculation formula for the axial compression load-bearing capacity of CFDST components, bridging the gap between theoretical derivation and practical application.

Further analysis on mechanical behaviors of blind-bolted T-stub connections to CFDST columns

The mechanical behavior of blind-bolted T-stub connections between concrete-filled double steel tubular (CFDST) columns and steel beams was investigated by cyclic loading tests and numerical simulations. Five bolted joints were tested to evaluate the bending moment (M)-rotation (θ) hysteretic behavior, and the numerical results agreed well with experimental data including the failure mode, initial stiffness, and bending resistance. Results proved that T-stubs with different stiffeners deformed in three modes; the internal force-transferring behavior of blind-bolted joints was different from that of the through-bolted joint, especially the stress response of inner tubes; blind-bolted joints hold a similar bending resistance as the through-bolted joint, but the ductility and energy dissipation capacity were improved significantly. The bending resistance of blind-bolted T-stub connections was predicted based on the tensile T-stub model, and the accuracy of the proposed formula was verified in comparison to both test and numerical results since all the calculation errors were within 9.3%. Parametric study showed that installing stiffeners on the T-stub was the most effective way to enhance the bending resistance, the effect of thickening the T-stub flange ranked second, and the growth rate of ultimate bearing capacity slowed down with the bending stiffness increase of the T-stub. Lastly, the optimum strength matching range of the T-stub flange was suggested.

Numerical Analysis of Mechanical Behavior of Self-Centering Joint between CFDST Column and RC Beam

The existing connection between the concrete-filled double steel tubular (CFDST) column and the reinforced concrete (RC) beam is difficult to repair and reuse after damage. In this paper, a self-centering joint between the CFDST column and the RC beam is proposed. The self-centering of the joint is realized by prestressed steel strands, and the energy dissipation is realized by friction. The overall purpose of the research is to analyze the influence of steel strand and friction on the mechanical behavior of the joint. By comparing the envelope curve and the restoring force model of a numerical joint model with theoretical values, accuracy of the numerical model was verified. Then, joints with different parameters, including the friction, prestress of steel strands, and ratio of the resisting moment provided by steel strands to the resisting moment provided by friction in the opening moment of joints, were numerically analyzed. The results showed that the joints with greater friction and prestress of steel strands had higher bearing capacity. Increasing the friction could increase the energy dissipation capacity of the joint, but it would increase the residual deformation of the joint. To reduce residual deformation, the prestress of steel strands should be increased. When the resultant force of the pretension of steel strands was greater than friction, the steel head could be kept pressed on the connecting block, making the stress changes of steel strands and the self-centering performance of the joint stable.

Behavior and design of axially loaded square concrete-filled double steel tubular slender columns

A square concrete-filled double steel tubular (CFDST) column composed of a circular core concrete-filled tube offers the advantages of both square and circular concrete-filled steel tubular (CFST) columns. However, limited tests were performed to investigate the axial performance of CFDST slender columns. This paper investigates the behavior and design of square CFDST slender columns subjected to concentric loading. A total of eight columns, including six CFDST slender columns and two CFDST short columns were tested under concentric loading. The test parameter includes the slenderness ratio of the columns and the thickness of the inner tube. The ultimate load, failure modes and axial load-deflection relationships of CFDST slender columns are presented. It was observed that square CFDST slender columns failed due to the overall buckling of the columns together with the localized buckling of the steel tube and concrete crushing. Increasing the slenderness ratio and decreasing the thickness of the inner steel tube reduced the ultimate load of CFDST slender columns. The applicability of the existing design code of CFST columns in designing CFDST slender columns was evaluated. It was found that the existing design codes significantly underestimated the ultimate loads of CFDST columns.

Concrete-filled double steel tubular beams under lateral impact

Concrete-filled steel tubular members have the merit of high strength and ductility, and are widely utilized in civil structures. However, corrosion of the steel tubes and live loads induced fatigue or coupling of these factors usually result in fractures generated on the steel tubes. The fractures may lead to a decrease in structural capacity and threaten structural safety, especially when the damaged members are subjected to impact loads such as vessel or vehicle impact. In this work, experiments are conducted firstly to examine the dynamic behavior of five damaged concrete-filled double steel tubular (CFDST) beams under lateral impact. The experimental results indicate that the deformation of the CFDST increases with the increase of initial damage severity under the same impact condition. The initial damage would change the failure modes of the CFDST beams. Based on the test results, the finite element models of the CFDST beams are validated. Then the parametric study is conducted to identify the influence of the thickness of steel tubes and initial fracture length on the displacement, impact force, and the increment in fracture length of CFDST beams.

Performance of (1) concrete-filled double-skin steel tube with and without core concrete, and (2) concrete-filled steel tubular axially loaded composite columns under close-in blast

Composite structural members such as concrete-filled double-skin steel tube (CFDSST) and concrete-filled double steel tubular (CFDST) columns are increasingly being utilized in modern structures owing to their capability to integrate the beneficial properties of constituent materials to carry heavy loads as compared to conventional reinforced concrete columns. Axial compression performance of such composite columns has been extensively investigated and available in the open literature. However, their response under impulsive loadings such as those induced by explosions is not very well studied because not many investigations have been conducted on these columns. Performance of composite compression members under short-duration/high-magnitude blast loading is of considerable interest under the prevailing environment of hi-tech wars, subversive activities, and accidental explosions. The recent devastating accidental Ammonium Nitrate explosion at Beirut port (Lebanon), and the ongoing invasion of Ukraine by Russia raise the concern of researchers and engineers for the safety of structural elements/components. In this study, a 3-D finite element model of axially loaded 2500 mm long CFDSST column of ultra-high-strength concrete (170 MPa) is developed in ABAQUS/Explicit-v.6.15 computer code equipped with Concrete Damage Plasticity (CDP) model, and investigation has been carried out for its blast performance under the 50kg-TNT explosive load at a standoff distance of 1.50 m in free-air. The effects of strain rate on the compressive strength of the concrete are considered as per fib Model Code 2010 (R2010) and UFC-3-340-02 (2008). The non-linear behavior of the steel is also taken into account. Damages in the form of (1) a - concrete crushing on the explosion side of the column and b - concrete cracking on the tension side and their spread over the column length, and (2) yielding of tubes are observed. Computational results are validated with the available experimental observations. To improve the column response, the analysis has been extended to investigate the blast performance of axially loaded CFDSST columns with and without core concrete having an inner steel tube of circular/square cross-section and their response have been compared with the equivalent single skin concrete-filled steel tubular circular/square columns of same axial load capacity.

Behaviour of Concrete-Filled Double Skin Tubular Short Column with Plate Stiffeners Welded Intermittently under Axial Compression

This paper presents the experimental and finite element analyses of the behaviours of stub circular and square concrete-filled double steel tubular (CFDST) columns intermittently welded with plate stiffeners under axial compression. The plate stiffeners are welded to the inner surface of the outer tube through predrilled holes. Twenty specimens consisting of two concrete-filled steel tubular (CFST), two unstiffened CFDST, and sixteen stiffened CFDST were tested under axial compression to determine the failure patterns and obtain the load versus axial shortening curve. The key variables are the distance of weld spacing and the dimension of the plate stiffeners. Based on the experiment results, the authors have identified the possible local buckling locations, including the peak load of the specimens that were significantly affected by the intermittently welded plate stiffeners. The developed finite element model is verified by comparing the current and previous experimental results, and it provides an acceptable mean ratio of 1.01 and a standard deviation of 0.04. The validated FE model is employed in a parametric study to investigate the effects of critical parameters, including the two shapes of the specimens, dimension of the plate stiffeners, and weld spacing, on the load-carrying capacity. The findings of the parametric study can be used to justify the optimum dimensions of plate stiffeners for the best stiffened CFDST performance.

Axial compressive behavior of circular concrete-filled double steel tubular short columns

This article investigates the axial compressive performance of concrete-filled double steel tubular (CFDST) short columns composed of circular section loaded concentrically. An experimental program comprised of compression tests on short columns is carried out to examine their structural performance. Axial compression tests on conventional concrete-filled steel tubular (CFST) columns and double-skin concrete-filled steel tubular (DCFST) columns are also performed for comparison purposes. The test parameters include the diameter-to-thickness of the outer and inner steel tubes, concrete strength, and diameter ratio. The test results exhibit that CFDST short columns composed of the circular section have improved structural performance compared to its CFST and DCFST counterparts. A theoretical model is also presented to simulate the test ultimate strengths and load-axial strain relationships of CFDST columns. The existing design models proposed including the codified design specifications are evaluated against the collected test data for predicting the axial compressive strengths of circular CFDST columns. It is seen that the existing codified design models cannot yield their ultimate axial compressive strengths accurately. A practical artificial neural network (ANN) model is proposed to estimate the ultimate load of such columns loaded concentrically.

Experimental and numerical investigations of square concrete-filled double steel tubular stub columns

In this paper, the structural behavior of concrete-filled double steel tubular (CFDST) stub columns composed of square hollow sections is investigated experimentally and numerically. The experimental program comprises compression tests on short columns loaded concentrically. The test parameters mainly focused on the influences of the width-to-thickness ratios of steel tubes and concrete strength on the axial behavior of CFDST stub columns. Finite element (FE) models are also developed to investigate the influences of a wide range of structural parameters on their axial performance. It is observed that square CFDST columns have improved strength and ductility compared to their CFST and DCFST counterparts. Finally, a calculation formula is proposed to predict their ultimate compressive strengths under the axial compression load.

Seismic experiment on blind-bolted joint in concrete-filled double steel tubular structure

In this study, blind-bolted joints were designed for use between steel beams and concrete-filled double steel tubular (CFDST) columns to utilize the structural advantages offered by double steel tubes. Six joint specimens were tested under a constant axial load and lateral cyclic load for a CFDST column. The seismic behaviors of these blind-bolted joints were estimated in terms of the hysteretic curves, ductility, and energy dissipation. The effects of a T-plate section type and steel beam-type on the semi-rigid joints were studied. The test results showed that the failure of the joints began with the yielding of the T-plates, and that the final failure mode was plastic deformation at the steel beam ends. The obtained hysteretic curves were complete, and the energy dissipation ability of the bolted joint was strong. The bearing capacity and energy dissipation of ribbed joints were higher than those of unribbed joints. Based on a steel volume comparison, the installation of stiffening ribs proved to be considerably more effective than the thickening of the T-plate flanges. The strain response of the inner tube in each CFDST column was also estimated. It was concluded that the blind-bolted joints exhibit excellent deformation capacities and structural internal force redistributions. The results of this experimental research could be helpful for the application of blind-bolted joints in fabricated CFDST structures.

Performance analysis and bearing capacity calculation on circular concrete-filled double steel tubular stub columns under axial compression

Concrete-filled double steel tubular (CFDST) columns have been proposed in recent years. This paper presents the performance analysis and a new bearing capacity calculation method of circular CFDST stub columns under axial compression. In order to establish accurate finite element (FE) model, various constitutive models for both the concrete infill and steel tubes are analyzed and compared with experimental results in FE analysis, and the suitable constitutive models for sandwich concrete, core concrete and steel tube are proposed in the FE model developed. Based on the validated numerical model, the variation of confining stress between steel tube and concrete is analyzed in detail. In addition, parametric studies are conducted to investigate the effects of concrete strength, external tube thickness, inner tube thickness, diameter ratio of inner tube to external tube and yield strength of steel tube on the axial compressive performance of circular CFDST stub columns. A new simplified formula based on the confining pressure is proposed to evaluate the ultimate bearing capacity of the circular CFDST stub columns. Based on the comparisons with experimental results and numerical simulation results, it is shown that the proposed formulae for the circular CFDST stub columns appear to be satisfactory.

Nonlinear analysis of square concrete-filled double steel tubular slender columns incorporating preload effects

Thin-walled square and circular hollow steel tubes are designed to support the permanent and construction loads of several upper composite floors before filling the concrete into the tubes to form concrete-filled double steel tubular (CFDST) columns. The influences of preloads acting on the steel tubes on the structural responses of slender square CFDST columns have not been investigated either experimentally or numerically. This paper presents a fiber-based computational model for the determination of the interaction behavior of local and global buckling in axially and eccentrically loaded CFDST thin-walled square slender columns including preload effects. The computational modeling method accounts for the influences of the deformations induced by preloads, local-global interaction buckling, geometric imperfections, second-order, and geometric and material nonlinearities. The accuracy of the computational algorithms developed is validated by comparing computations with test data on concrete-filled steel tubular (CFST) columns and finite element results of double-skin CFST (DCFST) columns with preload effects. The computer algorithms are employed to quantify the influences of preloads on the local-global interaction buckling responses of CFDST columns with various important parameters. Proposed is a design method for calculating the ultimate loads of concentrically loaded slender square CFDST columns considering preloads. The computational and design models developed are shown to be efficient modeling and design tools for square CFDST slender columns taking into account the construction method of high-rise composite buildings.

Experimental and numerical investigations of eccentrically loaded rectangular concrete-filled double steel tubular columns

ABSTRACT Rectangular concrete-filled double steel tubular (CFDST) columns have increasingly been utilized in high-rise buildings to support heavy loads. However, the responses of eccentrically loaded rectangular CFDST short columns have rarely been studied. This paper reports experimental and numerical investigations into the behavior of rectangular thin-walled short CFDST columns composed of a rectangular inner steel tube loaded eccentrically. Tests on rectangular and square CFDST short columns under eccentric loading and axial loading were carried out to examine their responses to various design parameters, including the cross-sectional dimensions, loading eccentricity, and width-to-thickness ratios of the external and internal tubes. The experimental program and results are described. A mathematical model underlying the theory of fiber elements is developed for simulating the moment-curvature responses as well as axial load-moment interaction envelopes of short CFDST columns. The computational model explicitly incorporates the experimentally observed failure mode of the progressive local buckling of the outer thin-walled rectangular steel tube. A computer simulation procedure is proposed for capturing the nonlinear behavior of short CFDST columns loaded eccentrically together with efficient solution algorithms that implement the inverse quadratic method for solving nonlinear equations. The experimental verification of the computer modeling technique is undertaken. The verified modeling technique is utilized to ascertain the significance of various design parameters on the structural behavior of CFDST columns made of rectangular thin-walled sections. It is confirmed that the computer model developed is capable of predicting well the experimentally observed responses of CFDST short columns.

Internal Force Transfer Mechanism and Bearing Capacity of Vertical Stiffener Joints in CFDST Structures

This paper firstly studied the internal force transfer mechanism of vertical stiffener joints in concrete-filled double steel tubular (CFDST) frame structures on the basis of finite element modeling (FEM). Analytical models of shear force and bending moment were established through the appropriate material constitutive equations and equilibrium theory. Then, the proposed models were used to predict and evaluate the shear and bending resistance of the vertical stiffener joint. Six joint specimens were tested to verify the rationality of the theoretical models, and the design suggestions for construction were subsequently discussed. The analysis indicated that the vertical stiffener together with the anchorage web played a dominated role in the internal force transfer mechanism. The computed bending resistance obtained by the tension model agreed well with the measured experimental data, and the shear resistance in the panel zone was sufficient to guarantee the ductile failure in the test. The vertical stiffener determined the plastic hinge so as to ensure the strong connection between the CFDST column and the steel beam. The ribbed anchorage web was an effective way of increasing the shear and bending resistance.

Experimental and numerical studies of square concrete-filled double steel tubular short columns under eccentric loading

Square concrete-filled double steel tubular (CFDST) beam-columns consisting of an internal circular steel tube have increasingly been utilized in composite building structures because of their high structural performance. This paper describes experimental and numerical studies on the structural responses of square thin-walled CFDST columns loaded eccentrically. Tests on twenty short square CFDST columns were undertaken that included sixteen columns under eccentric loading and four columns under concentric loading. The parameters examined in the experiments included the cross-sectional dimensions, the width-to-thickness ratios of outer and internal tubes and loading eccentricity. The measured ultimate strengths, load-shortening responses, load-lateral displacement curves, stress-strain curves and observed failure modes are presented. A numerical model incorporating the fiber analysis is developed that predicts the moment-curvature responses and axial load-moment strength envelops of CFDST columns. The model explicitly accounts for the influences of the confinement exerted by the internal circular steel tube on the core concrete and the progressive post-local buckling of the external steel tube. Efficient computer algorithms implementing the inverse quadratic method is developed to produce converged solutions to the nonlinear dynamic equilibrium equations generated in the analysis. Measurements from the tests are employed to validate the proposed numerical model. It is shown that there is a good agreement between theory and experiment. The computer model is utilized to demonstrate the significance of various parameters on the behavior of thin-walled short CFDST beam-columns.

Local-global interaction buckling of square high strength concrete-filled double steel tubular slender beam-columns

High-strength square concrete-filled double steel tubular (CFDST) slender beam-columns with a circular internal steel tube subjected to eccentric loads may undergo interaction local-global buckling. No computational studies on the interaction local-global buckling of slender square CFDST beam-columns have been reported and their behavior has not been fully understood. This paper describes a mathematical model for the simulation of the interaction local-global buckling behavior of square high-strength CFDST slender beam-columns under axial compression in combination with uniaxial bending. The mathematical model is formulated by the fiber approach, accounting for confinement provided by the internal circular steel tube, and geometric and material nonlinearities. An incremental-iterative numerical procedure is designed to quantify the local-global interaction buckling responses of slender CFDST columns. Efficient numerical solution algorithms implementing the inverse quadratic method are developed for solving the nonlinear equilibrium dynamic functions of CFDST columns. The formulation proposed is verified by existing experimental data on CFDST columns as well as double-skin concrete-filled steel tubular (DCFST) slender columns. The developed computational model is employed to study the local-global interaction buckling behavior of CFDST columns made of high-strength materials with various important parameters. Simplified design models are proposed for determining the ultimate axial strengths of slender square CFDST columns under axial compression and the interaction curves of CFDST slender beam-columns loaded eccentrically. It is demonstrated that the computational and design models predict well the interaction local-global buckling behavior and strength of slender square CFDST beam-columns.

Numerical analysis of axially loaded circular high strength concrete-filled double steel tubular short columns

Circular high-strength concrete-filled double steel tubular (CFDST) columns are high performance members where the internal and external steel tubes offer significant confinement to the concrete infill. The confinement remarkably improves the concrete compressive strength and ductility. However, no fiber element models have been formulated for computing the responses of CFDST columns with circular steel tubes filled with high-strength concrete incorporating accurate confinement to the core and sandwiched concrete. In this paper, a new fiber-based numerical model is developed that computes the axial load-strain responses of circular high-strength CFDST short columns under axial loading. Based on existing experimental results, a new confining pressure model is developed for the determination of the confining pressures on the core-concrete in CFDST columns with circular sections. A new strength degradation parameter is also proposed that allows the concrete post-peak characteristics to be quantified. The fiber-based numerical model validated by experimental data is used to assess the responses of high-strength CFDST columns considering important parameters, which include the inner steel tube, external tube diameter-to-thickness ratio and concrete and steel strengths. A simple expression is derived for the estimation of the axial load-carrying capacities of circular short CFDST columns and comparisons with several design codes are made. The proposed fiber-based analysis technique and design equation can accurately determine the responses of short circular high-strength CFDST columns.

Shear Storage Capacity of Vertical Stiffener Joints between Concrete-Filled Double Steel Tubular Columns and Steel Beams

Based on the low cyclic loading test results of vertical stiffener joints between concrete-filled double steel tubular (CFDST) columns and steel beams, the shear transfer mechanism and shear resistance were analyzed in this paper. A conceptual model formulated was presented in terms of equilibrium and stress-strain relationships. The results calculated by the theoretical model and the available experimental data were compared, and then one new concept of shear storage coefficient was proposed for the determination of the shear storage capacity of the joint, which quantitatively explained the ductility failure progression of the joint specimens in the seismic performance test. It was concluded that the vertical stiffener joint had sufficient shear resistance, which met the seismic design principle of strong shear and weak bending. Results show that the ribbed joints have greater shear resistance than unribbed ones; lengthening the overhang of the vertical stiffener can both increase shear resistance and shear storage capacity of the joint; axial compression ratio can reduce the shear storage capacity. The paper also suggests that the joint design should ensure enough safety storage of shear resistance to improve the seismic performance.

Beam‐column connections of concrete‐filled double steel tubular frame structures

SummaryFour types of concrete‐filled double steel tubular (CFDST) columns were introduced in this paper, which had the advantage of higher bearing capacity, stronger stiffness, and better fire‐resistant and explosion‐resistant performance. To research a reasonable form of beam‐column connection is the top priority in CFDST frame structures. After a brief historical background concerning the development of CFDST columns, this study conducted an investigation and comparison of several existing beam‐column connections. The constructional detail and force transfer mechanism of each beam‐column connection were analyzed according to the present research achievements. Several finite element models were established to discuss the various connection configurations and main factors affecting their behaviors. In general, connection design can take full advantage of the characteristic of double tubes in CFDST structure, so a series of construction improvements for joint design parameters in CFDST frame structures was proposed to improve the seismic performance and the relevant study for the future work was also discussed.

Nonlinear analysis of rectangular concrete-filled double steel tubular short columns incorporating local buckling

Rectangular concrete-filled double steel tubular (CFDST) columns with inner circular steel tube possess higher structural performance than conventional concrete-filled steel tubular (CFST) columns. However, the local buckling of the outer steel tube of thin-walled rectangular CFDST columns has not been accounted for in the existing fiber element models and design codes that may overestimate the column ultimate axial strengths. This paper describes a computationally efficient fiber-based modeling technique developed for determining the behavior of concentrically-loaded rectangular CFDST short columns including the local buckling effects of the external steel tube and the confinement offered by the internal circular steel tube. The effective width concept is used to simulate the post-local buckling of the outer steel tube. Comparative studies are undertaken to verify the fiber-based model with the relevant test results. The computational model is then employed to investigate the axial load–strain responses of rectangular CFDST short columns with various key design variables. A design equation is developed for computing the ultimate axial loads of short rectangular CFDST columns and compared with design methods given in several international design codes. It is shown that the fiber-based modeling technique and the proposed design model predict well the structural performance of short CFDST columns.