Concrete-filled Steel Tube Members Research Articles

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.

Experimental study of the cyclic behavior of steel tube in concrete‐filled steel tube members

AbstractA concrete‐filled steel tube (CFST) specimen was tested under uniaxial loading to acquire the nonlinear monotonic and cyclic response of the steel tube in CFST members including inelastic cyclic local buckling. The failure mode, hysteretic response, envelope curves, strength, and stiffness degradation of the CFST specimen were comprehensively studied. The observed failure modes of the CFST specimen were local buckling and ductile fracture of the steel tube due to ultra‐low‐cycle fatigue. The effective stress‐strain curve of the steel tube exhibited negative stiffness due to local buckling. The compressive resistance of the CFST specimen decreased significantly when local buckling progressed at the steel tube. The tensile unloading stiffness slightly decreased relative to the elastic stiffness of the CFST specimen; conversely the compressive unloading stiffness experienced severe deterioration upon increased cyclic loading. The tensile stiffness decreased gradually during the whole loading process, while the post‐capping stiffness remained stable.

Evaluation and improvement of shear-strength expressions for circular concrete-filled steel tubes

The accuracy of the shear resistance design equation for concrete-filled steel tubes (CFSTs) is crucial to ensure the safety of CFST structures. However, the current design equations underestimate the test data, and cannot accurately predict the relative shear resistance of the steel and concrete fill. To solve this problem, this paper developed an advanced finite element model in the LS-Dyna program, which was validated using experimental results of CFST members failing in shear. A systematic parametric study was then conducted. The result showed that the contribution of the steel increased significantly with the strain-hardening ratio (F_u/F_y) while the contribution of concrete fill was greatly influenced by the internally reinforced ratio (ρ_int) and axial load level (P/P_0). These predictions were combined with the experimental results to develop a more accurate and reliable design expression for the shear strength of circular CFSTs.

Experimental and analytical study on the dynamic response of rectangular concrete-filled steel tube under transverse impact loading

Rectangular concrete-filled steel tubes (CFSTs) have been widely employed as prominent structural members in modern constructions. During their life cycles, rectangular CFST members may suffer from impact loads, leading to a need for investigations into their dynamic performance. In this context, 23 rectangular CFSTs subjected to transverse impact loading were experimentally investigated using a drop weight testing system. The instantaneous impact force responses and motion states of the specimens were recorded at high frequencies. The aspect ratio, the steel ratio and the clear span of the specimen, as well as the impact energy conditions, were the key parameters in the test scheme, and their influence on the dynamic response of the rectangular CFST specimens was analyzed. A finite element (FE) model was developed to predict the impact response of rectangular CFSTs, in which the strain rate effects of steel and concrete were well considered, and good accuracy was achieved. Then, an equivalent relationship between rectangular and square CFSTs under transverse impact was proposed and verified, which achieves access for rectangular CFSTs to existing design methods.

Analysis of dumbbell-shaped CFST members subjected to combined compression and torsion loadings

In a study examining the mechanical behavior of dumbbell-shaped concrete-filled steel tubes (CFST) under combined compression and torsion, four concrete-filled steel tubes were fabricated and tested. The test results revealed that after the completion of loading, the specimens exhibited significant torsional deformation but no apparent signs of damage, allowing them to continue loading in subsequent stages.The dumbbell-shaped CFST model was analyzed using the finite element method, and the accuracy of the finite element results was confirmed by comparing them with the existing test results. Based on this, the impact of various factors on ultimate torque was investigated, including steel tube strength, concrete strength, axial compression ratio, slenderness ratio, and steel ratio. In conjunction with the analysis of finite element parameters, a straightforward calculation formula for the compression-torsion correlation curve of dumbbell-shaped CFST was proposed.

Dynamic compressive behavior of concrete-filled steel tubes under single and multiple impacts

In this study, the dynamic compression behavior of concrete-filled steel tubes (CFSTs) under single and multiple impacts was investigated using the split Hopkinson pressure bar (SHPB) apparatus. Analyses of the failure patterns, dynamic stress-strain relationship and dynamic compressive strength of CFST members were conducted, and the results for CFSTs were compared with those for plain concrete samples. Moreover, the confinement mechanism of the CFST specimens during the axial impact was investigated, and the influences of strain rate, steel tube thickness, concrete strength and height-to-diameter ratio on the dynamic increase factor of the CFST (DIFCFST) were explored. The experimental results indicate that the CFST specimens had excellent impact resistance under single and multiple impacts, and the dynamic compressive behaviors of CFSTs exhibited significant strain-rate dependence. Larger loading gas pressure can strengthen the confinement from steel tube to concrete, leading to an increase in the dynamic compressive strength of the CFST. Dynamic compressive strength reduction generally occurred during multiple impacts, while the strength enhancement could be observed at lower impact level due to insufficient single impact energy. The DIFCFST decreased with increasing steel tube thickness and concrete strength, while the height-to-diameter ratio had less effect on the DIFCFST. Based on the experimental test data, an empirical formula was proposed for the DIFCFST considering the strain rate, the confinement factor and concrete strength to predict the dynamic compressive strength of CFST members under axial impact.

Study on Mechanical Properties of Concrete-Filled Steel Tubular Members under Axial Tension

At present, the concrete-filled steel tube structure has been widely used in various practical projects. Due to the low tensile strength of the core concrete of round steel tube concrete (CFST) specimens, the axial tensile performance of CFST specimens is far from superior to its compressive performance. However, in practical projects, the concrete-filled steel tube members bearing tensile load often appear. In order to study the axial tensile properties of CFST specimens, the axial tensile tests of 5 CFST specimens and 1 pure steel tube specimen were carried out with steel tube diameter and concrete strength as variation parameters. The results show that the bearing capacity of CFST specimens is increased by 7.5%–16.3% compared with that of pure steel tube specimens with the same cross-sectional area, mainly because the core concrete limits the circumferential shrinkage of the outer steel tube. The larger the cross-sectional area of CFST specimens is, the higher the bearing capacity is. In this paper, the stress-strain relationship and the overall failure mode of CFST members under tensile force are studied, and the deformation characteristics and stress of steel pipe and core concrete are analyzed, which is expected to provide a reference for the application of CFST specimens in practical engineering.

Experimental Study on Ultrasonic Tomographic Method to Detect Compactness of Concrete filled Steel Tube

When the concrete filled steel tube member is poured, due to the large volume, the internal concrete is difficult to vibrate, easy to produce defects, not easy to observe, and the internal defects will have a non-negligible impact on its mechanical properties, therefore, it is particularly important to detect the internal defects of the concrete filled steel tube column. In this paper, the internal defects of CFST are studied based on the principle that acoustic wave propagates along the shortest acoustic path in CFST. For square concrete-filled steel tubular column, when the measurement method is adopted, the theoretical sound time calculation and sound velocity distribution map imaging are carried out for different measurement point densities. It is found that the defect identification effect is better when the number of measurement points is 5.For circular concrete-filled steel tubular column, when the measurement point spacing is determined to be 30° arc length, the measurement point arrangement method in the specification cannot be combined with CT imaging method, so the measurement point arrangement method of one-point emission and five-point receiving is proposed, and it is found that it has good identification effect on internal defects.

Size effect of circular concrete‐filled stainless steel tubular short columns under axial compression

SummaryConcrete‐filled stainless steel tube (CFSST) members combine the advantages of stainless steel materials and concrete‐filled steel tube (CFST) members. Therefore, it has a broad range of applications than CFST members in the marine environment and other scenarios requiring great durability and corrosion resistance. However, there are limited researches on the large‐sized CFSST members. In this paper, 30 circular CFSST members with varying steel ratios (3.7% ≤ α ≤ 10.3%), diameters (500 mm ≤ D ≤ 900 mm), and strength of concrete ( = 40 MPa, 50 MPa) are studied on the size effect under axial compression. For peak axial stress, peak axial strain, and composite elastic modulus, size effects are investigated. According to the results, the peak axial stress and peak axial strain of the members increase with the increase in diameter. The modulus of composite elasticity essentially stays constant as the diameter increases, showing that there is no obvious size effect on the composite elastic modulus. The size effect of peak axial stress and peak axial strain is influenced by the steel ratio. Increasing the steel ratio tended to decrease the size effect. According to the generated data, it was found that the current codes of Chinese and European underestimate the ultimate bearing capacity of CFSST short columns significantly. To this end, the resistances of the large‐sized austenitic CFSST columns with a low steel ratio are well predicted by the proposed design model after being modified, based on GB 50936‐2014 and EN 1994‐1‐1 design codes.

Size effect of square concrete-filled stainless steel tubular short columns under axial compression

Concrete-filled stainless steel tube (CFSST) members combine the advantages of stainless steel materials and concrete-filled steel tube (CFST) members. Therefore, it has a broad range of applications than CFST members in the marine environment and other scenarios requiring great durability and corrosion resistance. However, there are limited researches on the large-size CFSST members. In this paper, 40 square CFSST members with varying steel ratios (4 % ≤α≤ 10 %), widths (500 mm ≤B≤ 900 mm) and strength of concrete (fcu = 40 MPa, 50 MPa) are studied on the size effect under axial compression. The size effect on the peak axial stress, peak axial strain, composite elastic modulus and ductility coefficient are investigated. Then this paper adopted the two-way analysis of variance test (ANOVA) to analyze the data. According to the results, the peak axial strain, composite elastic modulus and ductility coefficient have size effect, but the size effect in peak axial stress does not significant. It was found that the steel ratio will also affect the size effect of CFSST members, and the size effect decreases with the increase of steel ratio. Compared with the current CFST codes, the size effect coefficient included steel ratio is introduced can better predict the ultimate bearing capacity of square CFSST short columns with low steel ratio of range of 4–10 %.

Impact resistance and simplified evaluation method for square CFST members subjected to transverse impact

Recently, the impact behavior and anti-impact design of concrete-filled steel tubes (CFSTs) have attracted increasing attention; the existing analytical theories have mainly focused on equivalent static methods. Thus, this study presents numerical and analytical analyses of simplified evaluation methods for square CFST members subjected to transverse impacts, by adopting dynamic impact action (i.e., impact mass and impact velocity) to evaluate the impact resistance. A finite element model was first established and benchmarked based on 38 test results collected from existing studies with a wide range of test parameters. The impact process, distribution and development of the internal force, and development of the stress, strain, and strain rate were comprehensively analyzed. The dynamic overall buckling of the CFST members under coupling actions of the axial load and transverse impact was investigated and compared with the control groups for CFST members without an axial load. Based on the parametric analyses, the yield strength of steel, section width, steel ratio, and axial load level (n) are important factors affecting the impact resistance. A simplified formula was therefore proposed to evaluate the impact resistance of CFST members with n ≤ 0.5. A machine learning method was also employed to predict the impact resistance of CFST members covering a wider range of axial load levels (n ≤ 0.9).

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

This paper experimentally and numerically investigates the shear behavior of high-strength square concrete-filled steel tube (HS-SCFST) members. A total of 20 specimens were first tested. The test parameters were the shear span-to-depth ratio, width-to-thickness ratio of the steel tube wall, yield stress of steel, and compressive strength of concrete. Detailed finite element (FE) models were then developed and benchmarked. The benchmarked models were then used to conduct a total of 87 parametric studies. It was shown that (i) the shear strength and stiffness increased with decreasing shear span-to-depth ratio or width-to-thickness ratio, and (ii) the increase in yield stress of steel and compressive strength of concrete improved the shear strength, but had a minor effect on the stiffness. Results from the experimental tests and finite element analyses were also used to evaluate the applicability of several international design provisions (e.g., AASHTO BDS, Eurocode 4, WSDOT BDM and CECS 28) for estimating the shear strength of HS-SCFST members. It was shown that CECS 28 provided the most reasonable estimation of the shear strength, but the contribution of steel tube and concrete infill was not accurately calculated. A new design equation was therefore proposed, considering the effect of shear span-to-depth ratio and the contribution of the steel tube and concrete infill. • Investigates the shear behavior of high-strength CFST members. • Conducts experimental tests and FEM analysis. • Discusses the shear transfer mechanism. • Evaluates the applicability of several international design provisions. • Proposes a design equation considering the effect of shear span-to-depth ratio.