Concrete-filled Double Steel Tubular Columns Research Articles

Investigation on behavior of concrete-filled double steel tubular columns infilled with nanomaterial-based concrete subjected to axial compression

Nanotechnology has been applied in construction to enhance the mechanical properties of concrete by using nanomaterials. A new stiffening scheme called catty-cornered propped concrete-filled steel tube (CFDST) column was proposed and analyzed under axial compression. The steel tube of CFDST specimens was filled with normal and nano material-based concrete, including nano-silica (NS), carbon nanotubes (CNT), and nano-titanium dioxide (NT). The study collected data on ultimate capacity, load vs strain behavior, and load vs deformation response, as well as ductility index (DI), secant stiffness, composite interaction, and confining effect variation. The results showed that the proposed stiffening scheme increased the ultimate capacity of unstiffened CFDST by approximately 14%, and the use of nanomaterials in CFDST infill concrete resulted in an approximately 7% increase in load capacity. Increasing the number of stiffening bars improved the ductility and stiffness of the column section, while the inclusion of nanomaterials decreased the ductility index and improved the stiffness of the section. The proposed stiffening scheme resulted in better composite interaction and increased confinement. The study concluded that the utilization of nanomaterial-based concrete as an infill in the stiffened CFDST column could enhance its performance under axial compression loading.

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.

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.

Experimental and numerical analysis of circular concrete-filled double steel tubular stub columns with inner square hollow section

A circular concrete-filled double steel tubular column (CFDST) with a square hollow section (SHS) as an inner tube is introduced as a new form of a composite column in the paper. A series of axial compression tests that are carried out on this new type of composite column is reported in this study. Three-dimensional models based on the finite element (FE) program ABAQUS are also developed for such columns employing a modified confined concrete constitutive model for the core concrete. The accuracy of the model is verified by comparing the FE model with the experimental results. The effects of various parameters on the load-axial strain response of CFDST short columns are studied by using the validated FE model. The suggested formulas based on the confined concrete model for calculating the bearing capacity of CFDST short columns are proposed.

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.

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.

Analysis of axially loaded concrete filled circular hollow double steel tubular columns exposed to fire

A series of new unified design formulas for calculating the temperature field and fire resistance under axial compressive loading of a circular solid or hollow concrete filled double steel tubular (CFDST) short and slender columns exposed to fire are presented in this paper. The results of the experiments are firstly derived from exploring the fundamental behavior of the CFSDT slender columns under the ISO 834 standard fire and temperature distributions. Failure modes and axial deformation versus time curves are also obtained and discussed. It was realized that a solid or slender CFDST column has different temperature fields, stronger limiting temperature and better fire resistance, as compared to columns filled with plain concrete. Also, the load ratio and steel ratio have no evident difference between the results of the solid and hollow CFDST columns. A three-dimensional finite element analysis (FEA) model is developed to calculate the temperature field of CFDST columns. After calibration of the FE model, the influences of important parameters on the temperature field of the composite columns are investigated and a unified formula for calculating average temperatures of the columns’ cross-section is obtained. Combining the average temperature with the “Unified theory” under axial compression, a simplified design method is put forward, which provides a unified formulation for both the plain CFST and CFDST columns relating to the axial bearing capacity at room and elevated temperature. The proposed formulas are calibrated through comparisons with fire tests on the columns, and it was found that the method produces safe results on average.

Study on Section Temperature Field of Concrete-Filled Double Steel Tubular Columns under Fire

Finite element model was built in order to study the section temperature field of concrete-filled double steel tubular columns through the software of ANSYS. Moisture of concrete and thermal contact resistance were considered in the model. Based on the Eurocode 2, specific heat of concrete was modified in order to consider the effect of moisture. Contact elements were created in the interface between steel tube and concrete in order to consider the effect of thermal contact resistance. The effects of diameter of steel tube and type of aggregate to section temperature field were studied. Distribution of section temperature field is given on the concrete-filled double steel tubular columns, which provides the basis for the analysis of fire resistance capability.

Mechanical Behavior of Concrete-Filled Double Steel Tubular Columns under Axial Compression Load

This article analyzes five concrete-filled double steel tubular column models based on the large finite element analysis software ABAQUS. An axial compression test was carried out on specimens with different diameter-thickness ratio of internal steel tube in order to get the whole longitudinal load-displacement curves and the whole load- strain curves for internal and external steel tube and concrete. We get the ultimate bearing capacity for five specimens, the result of simulation agree well with the theoretical calculation data, that provide reliable data for concrete-filled double steel tubular column in respect of finite element (FE) simulation.