Concrete-filled Steel Tube Members Research Articles

Flexural behavior of circular concrete-filled steel tube members reinforced with annular stiffener

This paper presents a study on the flexural performance of a novel concrete-filled steel tube (CFST) member reinforced with an internal annular stiffener. The annular stiffener is composed of curved steel plates, and the circumferential plate serves as the interface connector. Four-point bending tests were conducted on the four new composite beams and two traditional CFST counterparts to investigate the flexural performance of the composite members. The failure modes, deflection distribution, flexural strength, strain distribution, and moment-curvature relationship were analyzed. The test results showed that the new composite members exhibited higher bending capacity and stiffness than their CFST counterparts. Then, a finite element (FE) model was established and validated using the test results to numerically investigate flexural behavior. The available design models for predicting the flexural capacities of the composite beams was compared. Design formulae considering the stiffener contributions were deduced to calculate the flexural strength of the composite beams based on the plastic-stress distribution method. It was demonstrated that the developed FE modeling appropriately simulated the structural behavior of the composite members. Parametric studies indicated that the use of high-strength concrete was inefficient. In addition, the comparison results indicated that the proposed design formulae were feasible for predicting the bending capacity of composite members.

Waste rubber recycling in concrete-filled steel tube members: Mechanical performance, reliability and life cycle assessment

This paper focuses on the axial compression mechanical performance, reliability analysis and environmental impact of rubber concrete-filled circular steel tube (RuCFST) columns. Initially, the research of the axial compression performance reveals that enhancing the wall-thickness or yield strength of steel tubes variably increases the yield, peak, and ultimate loads of RuCFST columns. The degree of enhancement of each characteristic point by growing the wall thickness of the steel tube is 5 %–44 %, 6 %–44 %, and 9 %–44 %, respectively; while the degree of enforcement is 5 %–55 %, 23 %–53 %, and 24 %–61 %, respectively, upon yield strength increase. Reliability analysis indicates that higher concrete grades decrease the probability of axial compression failure in RuCFST columns. Raising the yield strength and wall-thickness of steel tubes leads to a reduction in the reliability index. Both larger and smaller live-to-dead load effect ratios adversely impact the reliability index. In addition, the life cycle assessment method was used to evaluate the environmental impact of RuCFST columns, and the analysis was made after considering the superposition effect of waste rubber incineration and recycling into rubber aggregates.

Reliability Analysis of Axial Compressive Strength of Concrete-Filled Steel Tube (CFST) under Coupled Corrosion and Load Effects

This paper presents a finite element analysis (FEA) of and reliability study on concrete-filled steel tube (CFST) members under the combined effects of corrosion and compressive loading. First, a stochastic-based FE model is established through the proposed secondary development program based on ABAQUS 2021 software. The model could account for the uncertainties of material, geometric, and corrosion effect on CFST members. The reliability of the built model was validated through experimental data of corroded CFST members under compression loading. Subsequently, the compressive performance of CFST under a combination of corrosion and loading was further investigated by numerical parameter analysis. A total of 1800 models were created to clarify the coupling mechanism among the core concrete strength, the steel tube strength, the steel ratio, and the maximum strength of the CFST member. Three theoretical formulas presented in classical design standards were used to calculate the axial compressive strength of the corroded CFST, and the uncertainty parameters μkp and δkp were also obtained for the discussed design formulas. Finally, the First Order and Second Moment (FOSM) method was employed to estimate the reliability indices β across different standards. The calculations revealed that the reliability indices β according to European standard ranges from 2.93 to 5.52, with some results falling below the target reliability index βT of 3.65. In addition, the multi-parameter coupling effects on reliability index β were investigated, and the main influencing factors were obtained. By leveraging the reliability analysis, reasonable design requirements can be proposed for CFST members under the coupling effects of corrosion and external load, which provides a design basis for the CFST member.

A Simple Method to Evaluate the Bearing Capacity of Concrete-Filled Steel Tubes with Rectangular and Circular Sections: Beams, Columns, and Beam–Columns

This study investigates the behavior and capacity of concrete-filled steel tubes (CFTs) with rectangular and circular sections under various loading conditions: axial loads, pure-bending, and combined axial load-bending moments. A finite element-based numerical model is developed and validated against existing experimental data. Using an extensive databank generated from finite element analysis, this research proposes analytical empirical relations for determining the bearing capacity of rectangular and circular composite beams, columns, and beam–columns. These relations provide a direct, concise, and efficient method for calculating the ultimate strength of CFT members, making them valuable for engineering design. Comparisons between the proposed analytical results and previous experimental studies demonstrate the high accuracy of this method in analyzing rectangular and circular CFT member behavior. The findings contribute to a better understanding of CFT structural performance and offer practical tools for designers working with these composite elements.

Axial behavior variation of 72-story concrete-filled steel tubes with different joint connections due to interface debonding considering long-term deformation of concrete core

Long-term concrete deformation including shrinkage and creep occurring seriously threats the longevity, safety and serviceability of concrete-filled steel tube (CFST) structures. In this study, numerical simulation on the variation of axial behavior of 72-story CFST structures with different joint connection details due to interface debonding considering shrinkage and creep of concrete core is carried out. Three connection details including connections with inner horizonal diaphragms only, two-direction through-beams only, and inner horizontal diaphragms combined with two-direction through-beam, are considered, respectively. Vertical loading from floors is treated as centralized vertical force applied on steel tube directly. The shrinkage of concrete core is simulated by decreasing concrete temperature. A cohesive constitutive law and a Coulomb friction model are employed to describe the interface behavior in tangential direction before and after debonding, respectively. The interaction between concrete core and steel tube in normal direction is treated as a hard contact. The concrete plastic-damage (CPD) constitutive model is used for concrete core and a plastic constitutive law is employed for the steel tube. The initiation of the interface debonding between steel tube, H-shaped steel beam flanges, inner diaphragms and concrete, the variation of the force transfer path, stress redistribution, steel tube yielding and the final failure are described in detail. Results show that the axial load-carrying capacity of each CFST structure has a decrease of 26-28% when compared with that determined by the current design code where the confinement effect is considered, and a decrease of 9-10% when compared with that determined by the current design code where the confinement effect of steel tube is not considered. Moreover, the axial stiffness decreases to 76-78% of their theoretical values when debonding is not considered. Numerical results show that the interface debonding negatively affects the long-term axial load-carrying capacity and stiffness obviously, which has not been considered in current design codes. Some comments on the design of CFST members are proposed based on the numerical simulation results.

Effect of vibration on the interface properties of welded steel joints and filled concrete in steel pipes

Concrete-filled steel tubes (CFSTs) have been increasingly utilized in engineering due to their excellent mechanical properties. Ensuring a solid bond between a steel tube and concrete is essential for optimizing their synergistic effect. This study introduces an internally welded steel bar structure within the inner wall of a steel tube to enhance the bond properties at the connection interface. The influence of various configurations of steel bars welded to the inner surface of the tube on the bond strength is investigated considering the impact of vibration on the load-bearing capacity of the component. This study comprises two groups of specimens, one with vibration and one without vibration, for a total of ten specimens. Each group included CFST members with five distinct internal welded steel bar structures. The experimental results, including load–displacement curves and strain data of the steel tube, were used to assess the impact of the internal welded steel bar configurations on the steel–concrete interface. The sliding process is described by correlating test data with curves and observed phenomena. To comprehensively compare the effects of structural dimensions on the bonding and slipping properties of the welded bars, finite element simulations replicating the experimental conditions were carried out using ABAQUS software, and the simulation results agreed with the experimental observations. The study demonstrated that incorporating internal welded steel bars substantially enhances the bond strength of steel pipe–concrete interfaces. While vibration weakens the bond strength in CFST members, internal welded steel bars mitigate this effect. These findings improve the structural performance of CFST structures and their resilience to external vibrations.

Experimental Study and Neural Network Predictions of Early-Age Behavior of Microexpansion Concrete in Large-Diameter Steel Tube Columns

The improved mechanical performance of concrete-filled steel tube (CFST) components has led to their widespread application in megastructures. Therein, CFST hybrid arch bridge has become an optimal selection of large span bridges. Nonetheless, for massive concrete poured in CFST members with large diameter, the thermal cracking and sustained rising temperature caused by early hydration heat of core concrete are urgently required to be studied. The two large-diameter CFST columns in this work, each having a diameter of 2.1 m, had their temperature and strain fields recorded in-situ. An existing CFST arch bridge served as the model for the two CFST columns’ design. Additionally, early-age characteristics of several scaled CFST columns with varying diameters were documented. A multi-field finite element (FE) model that combines linked chemical (hydration), heat, and mechanical fields is suggested in order to properly characterize the evolutions of temperature and strain fields. The model is validated by comparing the in-situ measurements with the numerical results. Finally, to investigate the affect factors on the hydration temperature of core concrete in CFST columns, early-age hydration behaviors of CFST columns was simulated using the validated FE models input parameters as water to cement ratio (w/c), cement dosage, heat release of cement and diameter of CFST columns. Based on the numerical results under the input parameters mentioned, the LSTM neural network was constructed, and the hydration temperature variances computed by FE models were selected as the input dataset. Afterwards, the temperature variance of core concrete of CFST columns was predicted using the established LSTM network. It is discovered that the LSTM neural network that was previously constructed was able to predict the peak temperature of CFST columns as well as the hydration temperature of CFST specimens with respect to time.

Uniaxial material model with softening for simulating the cyclic behavior of steel tubes in concrete‐filled steel tube beam‐columns

AbstractThis paper presents a new uniaxial constitutive material formulation with softening for simulating the inelastic behavior of steel rectangular tubes in concrete‐filled steel tube (CFST) members. The primary behavioral characteristics of the steel tube in CFST members are isolated and pronounced through a carefully designed experimental campaign with CFST specimens subjected to uniaxial strain‐based loading protocols. The model is expressed in an effective stress–strain domain, where the effective uniaxial strain is defined as the uniaxial displacement within a dissipative zone over a predefined length. In the pre‐peak state, the proposed model can effectively capture the combined kinematic/isotropic hardening and Bauschinger effect—characteristic of mild structural steels—within the framework of rate‐independent plasticity. In the post‐peak state, the proposed model traces strength deterioration due to outward local buckling, which is a characteristic nonlinear geometric instability in CFST members due to the presence of the filled concrete in the steel tube. The proposed constitutive formulation incorporates a softening branch that exponentially decays to trace the stabilization of the outward buckling wave within the buckling region in successive inelastic loading cycles. Cyclic deterioration of the effective stress is explicitly considered via an energy‐based rule. The proposed model is calibrated to a CFST dataset. Regression equations are proposed for predicting the input model parameters. These equations cover a wide range of geometric parameters and structural steel materials in CFST members. Comparisons with prior tests on actual CFST beam‐columns under planar symmetric cyclic loading suggest that conventional 2‐dimensional displacement‐based beam‐column elements can predict the full‐range of the hysteretic behavior of the CFST members with the proposed constitutive formulation including cases where the post‐peak response of CFST members exhibits negative stiffness.

Mechanical properties of concrete filled steel tube members constrained by carbon fiber reinforced materials under bidirectional coupled loading

Carbon fiber reinforced composite structures have been a research hotspot in recent years, with 9 specimens designed for static tests under bending and torsion loads of CFRP concrete filled steel tubes. The torque angle (T-θ) curve was studied from an experimental perspective. Subsequently, a reasonable finite element model was established using ABAQUS software. In addition, the effects of changes in parameters such as the number of steel concrete strength, bending ratio, and steel ratio on C–CF–CFRP-ST bending and torsion specimens were studied through numerical parameter research. Finally, equations of bearing capacity of CFRP-concrete filled steel tube under coupled bending and torsion are proposed.

Experimental Study on the Detection of the Existence and Location of Mimicked and Unexpected Interface Debonding Defects in an Existing Rectangular CFST Column with PZT Materials.

Interface bonding conditions between concrete and steel materials play key roles in ensuring the composite effect and load-carrying capacity of concrete-steel composite structures such as concrete-filled steel tube (CFST) members in practice. A method using both surface wave and electromechanical impedance (EMI) measurement for detecting the existence and the location of inaccessible interface debonding defects between the concrete core and steel tube in CFST members using piezoelectric lead zirconate titanate (PZT) patches as actuators and sensors is proposed. A rectangular CFST specimen with two artificially mimicked interface debonding defects was experimentally verified using PZT patches as the actuator and sensor. By comparing the surface wave measurement of PZT sensors at different surface wave travelling paths under both a continuous sinusoidal signal and a 10-period sinusoidal windowed signal, three potential interface debonding defects are quickly identified. Furthermore, the accurate locations of the three detected potential interface debonding defects are determined with the help of EMI measurements from a number of additional PZT sensors around the three potential interface debonding defects. Finally, the accuracy of the proposed interface debonding detection method is verified with a destructive observation by removing the local steel tube at the three detected interface debonding locations. The observation results show that the three detected interface debonding defects are two mimicked interface debonding defects, and an unexpected debonding defect occurred spontaneously due to concrete shrinkage in the past one and a half years before conducting the test. Results in this study indicate that the proposed method can be an efficient and accurate approach for the detection of unknown interface debonding defects in existing CFST members.

Flexural Strength and Ductility of a Concrete Filled Steel Tube Beam with Different Layouts

Concrete-filled steel tube (CFST) is a composite member consisting of a steel tube filled with concrete, resulting in an enhanced structural element used in various types of construction. This research investigates the flexural strength of CFST members with varying steel tube thicknesses of 1.5 mm and 2.0 mm and different shapes: square, rectangular, and circular. The study aims to determine the flexural strength of each shape. Fifteen beams with different cross-sections and plate thicknesses were tested experimentally in the lab. The results indicated that 2.0 mm thick CFSTs, regardless of shape, exhibited superior strength and deformation resistance compared to thinner and hollow beams. This underscores the significance of using thicker plates and concrete to enhance structural integrity and durability. Notably, rectangular CFSTs demonstrated a 91.84% increase in strength, while circular beams showed greater deflection resistance, highlighting the importance of careful material selection and design choices in structural engineering to optimize performance and resilience under stress.

A Concrete Core Void Imaging Approach and Parameter Analysis of Concrete-Filled Steel Tube Members Using Travel Time Tomography: Multi-Physics Simulations and Experimental Studies.

Concrete-filled steel tube (CFST) members have been widely used in civil engineering due to their advanced mechanical properties. However, internal defects such as the concrete core voids and interface debonding in CFST structures are likely to weaken their load-carrying capacity and stiffness, which affects the safety and serviceability. Visualizing the inner defects of the concrete cores in CFST members is a critical requirement and a challenging task due to the obvious difference in the material mechanical parameters of the concrete core and steel tube in CFST members. In this study, a curved ray theory-based travel time tomography (TTT) with a least square iterative linear inversion algorithm is first introduced to quantitatively identify and visualize the sizes and positions of the concrete core voids in CFST members. Secondly, a numerical investigation of the influence of different parameters on the inversion algorithm for the defect imaging of CFST members, including the effects of the model weighting matrix, weighting factor and grid size on the void's imaging quality and accuracy, is carried out. Finally, an experimental study on six CFST specimens with mimicked concrete core void defects is performed in a laboratory and the mimicked defects are visualized. The results demonstrate that TTT can identify the sizes and positions of the concrete core void defects in CFST members efficiently with the use of optimal parameters.

Interface debonding defect detection for CFST columns based on EMI measurements: Experiment, numerical simulation and blind inspection in practice

Concrete-filled steel tube (CFST) members have been widely used in skyscrapers and long-span bridges. Non-uniformly distributed hydration heat during curing, inadequate compaction during construction and unavoidable shrinkage and creep of mass concrete in service most likely lead to interface debonding defects between concrete core and steel tube, which has been a common concern. The development of effective non-destructive inspection methods for interface bonding condition of existing CFST members is critical. In this study, an interface debonding defect detection method based on electromechanical impedance (EMI) measurement using surface-mounted piezoelectric-lead-zirconate-titanate (PZT) sensors is proposed at first. Then, experimental study on the feasibility of the proposed approach is carried out with scaled CFST specimens with artificially mimicked interface debonding defects. EMI of surface-mounted PZT sensors at different frequency bands are measured and compared to verify the detectability of the proposed approach for the mimicked interface debonding defects. Thirdly, a multi-physics CFST-PZT coupling finite element model (FEM) with interface debonding defects is established, and EMI of surface-mounted PZT sensors at different locations is simulated. The influence of interface debonding defects on EMI measurements is illustrated. Finally, a blind inspection on the interface bonding condition of selected CFST columns in an existing skyscraper in service using the proposed approach is carried out. Both bonding and debonding defect in the tested CFST columns are detected correctly and validated with drilling hole observation. Experimental, numerical and engineering application results show the proposed approach is effective for interface debonding detection of CFST members and sensitive to minor debonding defect of 0.3 mm in CFST members.

Effect of dynamic constitutive differences in materials on the impact performance of CFST via secondary development of ABAQUS

To investigate the effect and adaptability of dynamic constitutive models on the impact performance of concrete-filled steel tubes (CFST), a numerical study on the dynamic properties of CFST under impact loading is conducted. By analyzing the impact response, including impact force, mid-span displacement, energy absorption, and mid-span bending moment, the effect of dynamic constitutive differences in materials on the impact performance of CFST is assessed quantitatively. Moreover, the HJC dynamic model was developed and validated by secondary development in ABAQUS. The findings show that the material model without considering strain rate effects would significantly underestimate the impact performance of CFST members. In contrast, the impact resistance of CFST is overestimated by using the J-C model and the C-S model with biased elastic conditions. The results obtained using the DIFavg model are most in agreement with the results of the actual model. The impact resistance of CFST is only slightly influenced by variations in the dynamic constitutive model of concrete. The impact performance of CFST is accurately predicted by using the VUMAT subroutine to program the HJC dynamic constitutive model.

Impact force curve feature and failure mode identification method of concrete-filled steel tube members under lateral impact

A collision accident caused by derailed trains is a potential safety hazard for the structure adjacent to high-speed rail lines. Concrete-filled steel tube (CFST) columns are frequently utilized in modern high-speed railway stations, however CFST station columns may suffer severe damage from train collisions because of their high impact energy. Therefore, the method of identifying CFST members' failure modes under large impact energy was investigated in this paper. Adopting numerical simulation by LS-DYNA, the corresponding relationship between the local indentation and impact process was researched, the characteristic impact force time-history curve was proposed, and the effects of impactor and CFST specimen parameters on impact force feature were analyzed. The plateau proportion was established and quantified, then the mapping relationship between plateau proportion and failure degree was acquired. Research results indicated that local indentation developed primarily at impact force plateau stage. CFST specimens showed bending deflection when plateau proportion was below 67.5%, experienced fracture failure when it was above 77%, and exhibited crack failure when it was in between respectively. Finally, the empirical formulas for plateau proportion and maximum deflection were established, and a simplified approach for predicting the failure modes was put forward. The research provides suggestions for identifying damage patterns and strengthening CFST columns subjected to lateral impact loads.

Ultra-high strength concrete filled steel tube members: Classification, experimental database, and design

Current design codes do not endorse the use of ultra-high strength steel and concrete materials (Fy ≥ 690 MPa and f’c ≥ 120 MPa) for concrete filled steel tube (CFST) members. This paper makes efforts to address this gap using a four-step method. In the first step, the classification of CFST members based on material strengths and tube slenderness ratio is introduced. In the second step, an experimental database comprised of 805 ultra-high strength concrete filled steel tube (UCFST) specimens under various loading conditions is compiled, and the research gaps in the database are identified. In the third step, the possibility of extending Eurocode 4 and AISC 360–22 to UCFST members are evaluated using the experimental database. Finally, a research program including experimental tests, finite element (FE) parametric studies and new design equations for UCFST rectangular members is introduced.

Shear behavior of square ultra-high performance concrete-filled high-strength steel tube members

High-strength steel and ultra-high-performance concrete (UHPC) can increase structural resistance and reduce material consumption efficiently. However, current design specifications do not permit the use of them in concrete-filled steel tube members due to the lack of research. To address it, this paper experimentally investigates the shear behavior of square ultra-high performance concrete-filled high-strength steel tube (referred to as CuFTh hereafter) members. A total of 20 CuFTh specimens were tested, considering the effects of the span-to-depth ratio, width-to-thickness ratio of the steel tube, yield stress of steel, and fiber volume content of UHPC. The test results showed that: (i) the failure mode was governed by the shear span-to-depth ratio (a/H), including shear failure (a/H = 0.2 or 0.5) and combined shear-flexural failure (a/H = 0.8 or 1.0); (ii) the shear strength increased with decreasing shear span-to-depth ratio and the width-to-thickness ratio of the steel tube; (iii) increasing the yield stress of steel and fiber volume content of UHPC improved the shear strength. The applicability of current specifications for estimating the shear strength of square CuFTh members was evaluated. It was shown that CECS 28 (CECS 2012) had the most reasonable estimation.

United composite strength calculation method for special-shaped CFST columns with multiple cavities

The present study investigates the method of calculating the composite strength of special-shaped concrete-filled steel tube (CFST) members with multiple cavities. Based on the research findings of CFST unified theory and some tests, a quantitative method to evaluate the concrete constraint effect is proposed for cross-sectional shapes ranging from regular triangles, squares…regular octagons to circles. The cross-sectional composite strength of regular polygonal CFST members is determined by the way dividing the concrete into effective and ineffective constraint regions, discounting constraint coefficients, and applying unified formulas. Additionally, a regression analysis is conducted to establish the relationship between interior angles and constraint effects. Subsequently, using quadrangle CFSTs as an example, this study investigates the relationship between cross-sectional regularity and constraint effect, and proposes a composite strength calculation method related to it. Finally, the special-shaped multi cavity cross section is separated into several simple polygonal cross sections, taking into account shape efficiency and regularity, to calculate the composite strength of each cavity CFST member and summarize them for determining the total axial compressive bearing capacity. The research findings indicate that a linear relationship exists between the interior angle of the cross section and the initial tangent line angle of the second-degree curve, which serves as the boundary between effective and ineffective constraint regions in regular polygonal steel tube confined concrete. The proposed unified composite strength calculation method, specifically designed for special-shaped CFST columns with multiple cavities, can accurately predict their actual strength.

Flexural behavior of high-strength square concrete-filled steel tube members subjected to cyclic loadings

This paper investigated the flexural behavior of high-strength concrete-filled steel tube (HSCFST) members subjected to cyclic loadings. Nine cyclic tests of square HSCFST members were conducted. The investigated parameters were the height of steel tube, thickness of steel tube wall, yield stress of steel, and compressive cylinder strength of concrete. A finite element (FE) model was developed and validated. Results from the experimental tests and FE analyses indicated that: (i) the failure modes of HSCFST members were characterized by local buckling and fracture of the steel tube, both occurred earlier with increasing height or yield stress of steel, or with decreasing thickness of steel tube wall, but were delayed when higher strength concrete was used; (ii) using high-strength steel significantly improved the flexural strength, had no obvious influence on the flexural stiffness, but significantly reduced the ductility; (iii) using high-strength concrete improved the flexural strength, stiffness, and ductility; and (iv) the limits by AISC 341-16 on the slenderness coefficient is conservative for HSCFST members subjected to cyclic flexural loadings.

Nonlinear cyclic behavior of steel tube in concrete-filled steel tube members including local buckling

This paper presents an experimental and analytical study on the nonlinear cyclic behavior of steel tube in concrete-filled steel tube (CFST) members. An innovative CFST coupon was proposed and tested under axial loads to acquire the nonlinear cyclic response of the steel tube in CFST members including local buckling. The failure mode, hysteretic response, skeleton curves, strength and stiffness degradation of CFST coupon were studied comprehensively. The results showed that the failure modes of CFST coupon were local buckling and ductile fracture of the steel tube. The hysteretic curve exhibited negative stiffness induced by the local buckling of the steel tube. The tensile strength of the CFST coupon exhibited slight degradation throughout the whole loading process. The compressive strength decreased significantly when the local buckling emerged at the steel tube. The tensile unloading stiffness decreased slightly and the compressive unloading stiffness dropped greatly with the increasing loading level. The post-local buckling and tensile stiffness dropped linearly. The refined continuum finite element (CFE) model was proposed, which could capture the material and local geometric nonlinearities of steel tube. The nonlinear cyclic behavior of CFST coupon was comprehensively studied. Parametric studies of the CFST coupon were conducted, considering different width-to-thickness ratios and cyclic plasticity properties of steel tube.