Circular Concrete-filled Steel Tube Research Articles

Interpretable machine learning model for shear strength estimation of circular concrete‐filled steel tubes

Interpretable machine learning model for shear strength estimation of circular concrete‐filled steel tubes

Experimental investigations into stocky composite columns of concrete-filled circular S690 steel tubes under compression

Composite columns of concrete-filled steel tubes (CFSTs) are very efficient in providing large structural resistances and rigidities. However, high strength S690 steel tubes are not covered in current design codes. Therefore, a series of experimental investigations into stocky composite columns with circular CFSTs under compression were conducted. A total of 21 stocky composite columns with different tube diameters and thicknesses made of Grades S355, S460 and S690 steel and Grade C60/75 concrete were tested under different loading conditions. Moreover, extensive measurements on the surface strains of the steel tubes were made during tests. Data analyses on the test results reveal that all composite columns exhibit enhanced section resistances and ductility at large deformations, indicating that the use of S690 steel and C60/75 concrete in these composite columns are highly efficient. A simple method for determining confining pressures provided by the steel tubes to the concrete cores in these composite columns is proposed, which involves a systematic analysis on the measured data and the use of von Mises yielding criterion without any complicated plasticity formulation. Based on the proposed method, the resistance contributions of both the steel tubes and the concrete cores are successfully quantified. Comparison between the measured and the predicted resistances using EN 1994–1-1 of these composite columns reveals that while the section resistances of these composite columns are safely predicted, the resistances of the steel tubes are underpredicted while those of the concrete cores are overpredicted correspondingly. The findings highlight the need to improve the predictability of the current design rules, and to give increased resistances of the steel tubes when high strength S690 steel tubes are used.

Structural performance of circular CFST columns with circumferential gap imperfections

The present study investigated the influence of circumferential uniform gap imperfections (≤ 1.0%) on the structural behavior of circular concrete-filled steel tube (CFST) columns under axial compression. The study involved testing 20 CFST columns with circumferential uniform gaps with varying gap ratios (0.2%–1.0%), diameter-to-thickness ratios (18–43), and concrete strength (68.5 MPa and 114 MPa). The presence of uniform gaps (0.2%–1.0%) reduced the confinement effect of steel tube on core concrete, and peak load capacity decreased by 9.0%–24.0%. The normalized concrete strength in high-strength concrete specimens with gap ratios ranged between 0.89 and 0.99, showing no interaction between steel tube and core concrete due to the less deformable nature of high-strength concrete. Core concrete exhibited a combination of vertical splitting and diagonal cracks distributed across the height of specimens with an increase in gap ratio, and local steel tube bulging was dispersed around the edge regions. A theoretical procedure was developed to assess the impact of uniform gaps on the stress state of steel tube and core concrete by determining their respective load contributions. The suggested approach reasonably estimated the gap ratio's nominal influence on the behavior of steel tubes and its considerable influence on the behavior of core concrete. Considering the experimental limitations, gap ratios were broadened through finite element modelling to investigate the gap ratio effect further. After examining the confinement mechanism of CFST columns and finite element results, an analytical formulation to predict the peak load capacity of CFST columns with circumferential uniform gaps was proposed and verified.

Axial Compression Tests for Circular Concrete-Filled Steel Tube (CFST) Columns with Notch Imperfection

Axial Compression Tests for Circular Concrete-Filled Steel Tube (CFST) Columns with Notch Imperfection

Self-compacting concrete filled steel tube specimens with RHA and M-Sand replacement subjected to axial compression

ABSTRACT Because the cement content is inexorably growing, CO2 emissions from the cement industry are becoming unavoidable. The use of rice husk ash (RHA) makes the system environmentally benign. This paper investigates the effect of RHA and manufactured sand (M-Sand) on the axial capacity of circular Concrete Filled Steel Tube (CFST) specimens subjected to axial compression. Thirty-two CFST specimens infilled with self-compacting concrete (SCC) with a replacement of RHA and M-Sand to cement and fine aggregate in 0%, 15%, 20% and 25% proportions are subjected to axial compression. Results show that use of RHA beyond 20% decreased the compressive strength of concrete and eventually decreased the axial capacity of CFST and occurred higher local buckling irrespective of their L/D ratios. Failure modes, strength index, confinement along with steel and concrete contribution on axial capacities of CFST specimens are studied. The axial capacities of the specimens were compared with various design code results such as: American concrete institute, Australian Standards, Chinese code, Eurocode, and Japanese code. Among all the codes, Chinese code overestimated the test results and hence, the axial capacity equation is altered and compared the results with test data along with 278 circular CFST data available from literature and found the results to be accurate.

Experimental and numerical study on axial compression behavior of slender CFST columns with localized pitting corrosion damage

In order to investigate the axial compression behavior of slender circular concrete-filled steel tubes (CFSTs) with localized pitting corrosion damage, an experimental and numerical study was performed in this paper. In the experimental study, mechanically drilled cylindrical pits were employed to simulate localized pitting corrosion damage. Seven specimens were tested, with six specimens containing corrosion pits located at mid-height and one intact specimen. The experimental results showed that the localized pitting corrosion damage degraded the axial compression behavior of the specimens. Numerical models were developed and validated by the experimental results and used to consider the random characteristics of corrosion pits. A parametric analysis was then performed based on the validated numerical models. It was revealed that the ultimate strength of CFST was influenced by the location and dimensions of the pitting corrosion. Considering the random nature of localized pitting corrosion damage located at the midheight, practical calculation models for predicting the ultimate strength of CFSTs were put forward based on the methods provided in existing codes. On account of the obtained data pool, the predictions were verified against the results from the tests and parametric analysis. In addition, considering the random locations of the pitting region on the surface of the member, a method based on the controlling section was proposed to determine whether the corrosion should be considered and to calculate the ultimate strength.

Parametric analysis on dilation and confinement of axially loaded circular concrete-filled steel tubes using mesoscale modeling approach

Mesoscale modeling can more realistically describe concrete heterogeneity and phase interaction than macroscopic homogeneous simulation. A mesoscale model was developed in this study to investigate the dilation and confinement behavior of circular concrete-filled steel tube (CFST) columns. The validity of the model was demonstrated by comparing it with experimental stress-strain curves and dilation ratio curves. Parametric analysis revealed that the dilation was non-uniformly distributed along the circumference and the height of the specimen, while an increase in confinement factor inhibited dilation development and promoted confining pressure growth. Based on the parametric analysis, a law for hoop-axial strain relationship and confining pressure versus hoop strain curve were proposed. Additionally, an investigation into the behavior at peak load showed that the dilation ratio of circular CFST at peak load was approximately 0.92. The hoop stress ratio of steel tube decreased from 0.29 to 0.19 as the confinement factor varied from 0.2 to 1.5, while the axial stress showed an opposite growth trend. With increasing confinement factor, confining pressure nonlinearly increased, whereas the load-sharing ratio of between steel tube and concrete showed a linear growth trend. Finally, corresponding formulas were proposed.

Mechanical properties of seawater volcanic scoria aggregate concrete-filled circular GFRP and stainless steel tubes under axial compression

Abstract In this study, the properties of seawater volcanic scoria aggregate concrete (SVAC)-filled circular stainless steel (SFCST) and glass fibre-reinforced plastic (GFRP) tubes (SFCGT) were investigated. Ten groups were considered and 30 specimens were prepared, including four different parameters: the concrete type (SVAC and ordinary concrete [OC]), outer tube type (GFRP and stainless steel tubes), concrete strength (C30 and C40), and tube thickness (0, 3, and 4 mm). The typical influences of the SVAC and outer tube on the mechanical properties of specimens were then analysed. The research findings show that the strength and ductility of the SFCGT and SFCST are significantly higher than those of plain SVAC. The peak strain and strength enhancement factor of the SFCGT and SFCST increase with an increase in the tube thickness, and the concrete strength has a detrimental impact on the toughness of the specimen. Unlike in the confined OC specimens, a sudden decrease is observed in the stress–strain curves of the SFCGT and SFCST owing to the changes in the deformability of the SVAC. Generally, the strengths of the SFCGT and SFCST specimens are 10.3% lower and 4.1% higher than those of the confined OC specimens, respectively. Finally, analytical models of the strength and stress–strain curves considering the influences of the SVAC and passive confinement were established, and numerical simulations were performed to provide a basis for the practical application of the SFCGT and SFCST.

The Effects of Steel Fibres and Expansive Agents on the Compressive Behaviour of Circular Concrete-Filled Steel Tube Columns (CFSTCs)

Concrete filled steel tube columns (CFSTCs) are commonly used in large-span bridges and high-rise buildings. As the shrinkage of concrete may lead to significant degradation in the mechanical performance of CFSTCs, expansive concrete or fibre reinforced concrete is usually employed to improve the mechanical performance of CFSTCs. This study presents an experimental analysis of the combined effects of expansive agents and steel fibres on the axial compressive behaviour of circular CFSTCs. 13 CFSTCs were designed and manufactured using concrete mixes with varying expansive agent rates and steel fibre contents. The time-dependent expansive behaviours of the specimens were monitored during the concrete curing period. Then, the 13 specimens were tested under axial compression load. The failure modes, compression load-axial strain relationship, hoop strain and axial strain relationship, and ductility were investigated. The results demonstrated that the combined addition of expansive agents and steel fibres would substantially improve the compressive strength and ductile behaviour of CFSTCs.

Experimental research and engineering application on bearing capacity of concrete-filled steel tube circular arch

The bearing property and deformation failure law of the CFST circular arch were investigated in order to provide direction for the optimization of the concrete-filled steel tube (CFST) support. The flexural bearing capacity test of the 194 mm × 10 mm CFST was performed initially in this study. It was primarily explored how bearing capacity and deformation of circular arches were affected by the slip of the arch foot and anti-bending strengthening, and the neutral layer migration rule and anti-bending strengthening mechanism of circular arches were examined. The bearing performance of the circular arch under the condition of the concentrated load of the vault and the usual uniform load of the entire span is then simulated in an orthogonal test by numerical simulation in order to extend the experimental investigation. The effect of variables including the steel tube’s outer diameter, thickness, slip of the arch foot, and the ratio of the arch rise to span on the bearing capacity of the circular arch is examined. The study demonstrates that under focused stress on the arch top or full-span normal uniform loading, the arch foot slip is the most important element impacting the CFST support’s bearing capacity. In steel tubes with the same outer diameter and wall thickness, the vector span ratio has a bigger influence than anti-bending strengthening. When the span ratio is the same, the influence of the outer diameter and wall thickness are equal and outweigh the influence of bending strengthening. The circular arch with an outside diameter of 168 mm, a wall thickness of 10 mm, and a rise-span ratio of 0.207 has the best performance characteristics under the influence of concentrated load, and the ultimate failure mechanism is local waist drum deformation failure. Based on the aforementioned study findings, the circular CFST support’s buckling deformation and drum deformation are examined, and a composite support consisting of “anchor net spray + CFST support” is directed for the north three deep track downhill of Yangcheng Coal Mine. Two failure types expose the failure mechanism and provide a technique for optimization. The support had contributed to the circular CFST arc arch’s best bearing performance in the event of waist drum deformation and failure.

Axial compressive behavior of high-strength concrete-filled steel tube with multiple confinement effects

To study the axial compressive behavior and the confinement mechanism of high-strength concrete-filled circular steel tube with multiple confinement effects, five large size samples with horizontal stiffeners, longitudinal stiffeners, multilayer reinforcement cages, inner steel tube and separation steel plates were designed. The influences of these structures on the bearing capacity, ductility, stiffness, and residual deformation were then analyzed. Combining the strain analyses and finite element model analysis, the confinement mechanism of different structures and the coupling mechanism of different confinement effects were investigated. The results showed that the confinement effect of the horizontal stiffeners was similar to that of the stirrups. The longitudinal stiffeners did not provide a significant confinement effect on the concrete. The multilayer reinforcement cages had an effective confinement effect on the concrete, where the confinement effect increased from the outside to the inside, while the reinforcement cages had a slight influence on the concrete outside the reinforcement cages. The inner steel tube had an effective confinement effect on the core concrete and had a slight influence on the concrete outside the inner steel tube. The multicell steel tube showed the best confinement effect compared to the other structures and showed the best axial compressive behavior. In the confinement mechanism analyses, the calculation method of the stress–strain relationship of concrete with multiple confinement effects was proposed. The calculated F-Δ curves showed a good agreement with the test results.

Performance of axially loaded slender circular CFSTs with random local corrosion

The performance of axially loaded slender circular concrete-filled steel tubes with random local corrosion was investigated in this paper through experimental and numerical study. Specifically, the randomness of corrosion included the random locations along the column height, irregular corrosion profile, and uneven corrosion surface. In the experimental study, machining defects are employed to simulate local corrosion, and five slender CFSTs were tested, with four containing the defect located at different member heights and one intact specimen. Additionally, a numerical study is conducted to consider the random nature of local corrosion in terms of random locations, irregular profiles, and uneven surfaces. A parametric analysis is then conducted, with varying parameters such as defect dimensions, locations, material strengths, steel ratio, and slenderness ratio. The analysis finds that the ultimate strength of the slender member is influenced by the location and dimensions of the local defect. Some sightly corrosion occurring in the column end has negligible influence on the performance of slender members. A method based on the controlled section is proposed for calculating the ultimate strength of slender circular CFSTs with random local corrosion, yielding accurate results.

Experimental research on load separation of concrete-filled circular steel tube short columns

In recent years, scholars from various countries have conducted extensive experiments and analytical studies on the axial compressive behavior of circular concrete-filled steel tube (CFST) columns. However, the existing research has been based on overall loading analysis of circular CFST columns, making it difficult to accurately measure the loads borne by the steel tube and the core concrete separately. In order to better study the axial compressive behavior of circular CFST columns, this paper adopts the load separation device proposed by Hong-Song Hu and prepares 16 specimens with different concrete strengths for axial compression tests, directly measuring the load components of circular CFST columns under axial compression. The feasibility of the superposition principle is revealed at the experimental level. Furthermore, the compressive strength of the concrete and the compressive strength of the steel tube in circular steel tube concrete columns are analyzed. The results indicate that there is a certain discrepancy between the calculated steel tube strength based on existing formulas and the experimental values. However, for safety considerations, a reasonable estimation of the steel tube compressive strength can be made. Utilizing the measured stress-strain values of concrete and steel, the existing stress-strain models are evaluated, and a more reliable stress-strain model is established.

Lateral impact behaviour of CCFT columns with spherical-cap gap imperfection: Numerical modelling and design method

To clarify the influence of gap imperfection on the impact behaviour of the concrete-filled steel tube (CFT) column, this paper performs numerical modelling on the mechanical responses of the circular CFT columns holding spherical-cap gaps (CCFT-SG) under lateral impact load. The effects of gap ratio, axial compression ratio, impact position, diameter/thickness ratio, boundary condition and material strength on the energy dissipation (Ea), the maximum (Fm) and equivalent impact forces (Feq), the peak (δm) and residual lateral deflections (δr) are analyzed after establishing and validating the finite element (FE) model. The typical failure pattern, time-history curve and contact response between the tube and concrete of the laterally impacted CCFT-SG column are revealed as well. The result declares that the overall failure morphology of the CCFT-SG column imposed with lateral impact showing as overall bending, is similar to that of the common CFT column, but distinct local depression is obtained because of the existence of spherical-cap gaps. With the gap ratio increasing, the Feq, Fm and Ea decrease gradually, while the δm and δr rise slowly. The calculation formulae for predicting the dynamic impact force and resistance of the CCFT-SG column are presented and validated against the existing test and numerical results.

Performance of circular concrete-filled steel tube beams under monotonic and cyclic loadings

This study explored the performance of concrete-filled steel tube (CFST) beams under monotonic and cyclic loadings. Experiments were performed on nine simply-supported 140-mm-diameter CFST beams, which were classified into three groups by the diameter-to-thickness (D/t) ratios of 56.0, 46.7, and 40.0, conforming to Eurocode 4. The experimental results indicated that CFST beams exhibited high ductility. The buckling of steel tubes governed the plastic behavior and the ultimate state of CFST beams. The deformation and curvature concentrated in the middle region of the beams, forming a plastic hinge. The yield load, ultimate load, and yield and plastic stiffness slightly increased when D/t decreased from 56.0 to 46.7. However, by further decreasing D/t to 40.0, the ultimate and yield loads were significantly increased by ∼100%, the elastic stiffness by up to 58.2%, and the plastic stiffness by at least 187.5%. With a decrease in D/t ratio, the yield deflections slightly increased, whereas the ultimate deflection significantly decreased by up to 56.3%. The strength degradation was more pronounced for CFST beams with a D/t ratio of 40.0. Decreasing D/t ratio increased the plastic-to-yield stiffness ratios. The absorbed energy increased linearly with the increase in deflection in the plastic range. The cyclic loading effect caused an average degradation of 1.7%/cycle for both loading and unloading stiffness, whereas it marginally impacted on strength degradation. A decoupling concept and simplifications were employed to propose a simple model for rapidly estimating the moment capacity of CFST beams, which can be useful in designing and evaluating CFST beams.

Seismic behavior of thin-walled stirrup-confined circular concrete-filled steel tube piers: Experimental, numerical, and restoring force model analysis

An investigation was conducted using pseudo-static testing to analyze the seismic behavior of thin-walled stirrup-confined circular concrete-filled steel tube (TSCCFT) piers. In addition, a finite element model (FEM) was developed, incorporating concrete cracks and steel ductile damage. Moreover, based on the FEM and test outcomes, a refined restoring force model (RFM) for these piers was proposed, considering pinching effect coefficients and inflection points. The research results demonstrate that TSCCFT piers outperform counterparts lacking stirrup confinement regarding seismic performance. This enhancement is evident in significant increases of 30.3 % and 31.4 % for yield and horizontal peak load, respectively, along with notable increases of 9.1 % and 5.3 % concerning cumulative energy dissipation and elastic stiffness. Furthermore, the refined RFM demonstrates excellent precision, closely aligning with test and FEM results. This refined RFM holds the potential to serve as a reference for the seismic design of TSCCFT piers, streamlining the design process.

Torsional Behavior of Concrete-Filled Circular Steel Tubes Strengthened with CFRP.

In order to study the torsional performance of steel tube concrete after reinforcement with a carbon-fiber-reinforced polymer (CFRP), the mechanical properties of 18 specimens were studied from both experimental and finite element perspectives. The T-θ curve and τ-γ curve of the specimen were measured in the experiment, and the failure mode of the specimen was analyzed. Subsequently, a reasonable finite element model was established using ABAQUS software, and the variation in various parameters surrounding the performance of the specimen was analyzed. Based on the experimental and finite element results, a formula for calculating the bearing capacity of the specimen was established. According to both the experimental and numerical results, the torsional bearing capacity of C-CF-CFRP-ST, defined as the torque endured by the specimen with maximum shear strain, was determined to be 15,000 με, together with its corresponding calculation formula. After the test, it was demonstrated that the main components of the concrete-filled CFRP-steel tube composite material-the steel tube and the concrete-could be used as reusable resources.

Reconsideration of shear strength expressions for CFSTs: Rectangular tubes

Concrete filled steel tubes (CFSTs) are typically circular or rectangular. Circular CFSTs are mostly used as vertical components of deep foundation systems; rectangular sections are more commonly used in building construction such as square CFSTs for columns. Prior work has focused on circular CFSTs with far fewer studies on square or rectangular sections. In addition to being more limited, prior research on square CFST sections has focused on flexural and axial response; there have been far fewerstudies on the shear response. As such, the accuracy of shear-strength equations is uncertain. A research study was undertaken to understand and improve design expressions for the shear design of rectangular CFSTs. First, the research team collectedexperimental data on shear resistance of rectangularCFSTs. Selected experimental data was used to develop and validate an advanced nonlinear finite element model (FEM) to predict the measured response and observed damage of rectangular CFSTs controlled by shear behavior. The validated model was then used to conduct parameter studies to investigate important design parameters of rectangular CFSTs responding primarily in shear including: 1) concrete and steel material properties, 2) cross-sectional aspect ratio, and 3) impact of enhanced bond capacity/composite action through the introduction of a single shear key at each end of the member. The results were used to better understand the importance of each parameter on the shear response and strength. A database was assembled, combining the experimental data with the strength predicted by each FEM, to evaluate existing design equations. The shortcomings of the existing equations included (1) only accounting for the steel or the concrete fill contribution, and (2) overestimating the total shear strength because the behavior of the CFST in shear is not well understood. This paper provides a new approach to assessing the shear strength as the individual contributions of the steel tube and the concrete fill are computed for each model to better understand their interaction. This also permits the development of a new design equation to remedy the shortcomings of existing codified models. In particular, the proposed model accurately estimates the individual contributions of steel tube and concrete fill to the shear resistance for salient design parameters.

Axial compressive behaviour of composite steel elements incorporating rubberised alkali-activated concrete

This study presents an experimental and numerical investigation into the axial compressive behaviour of steel tubes infilled with rubberised alkali-activated concrete. An experimental programme involving circular and square concrete filled steel tubes with different length-to-diameter or length-to-width ratios and concrete infill mix designs with varying rubber contents, of up to 60% crumb rubber replacement of natural aggregates, is firstly described. A detailed account of the experimental results, including the axial capacity, stiffness, toughness, ductility, stress-strain response, and failure patterns, is given. The numerical study is performed in ABAQUS/CAE and the concrete compressive behaviour is modelled using the Concrete Damaged Plasticity model with a modified function for the compressive behaviour. The numerical results are validated against the experimental results, and a parametric study involving 315 finite element models is carried out to cover a wide range of concrete and steel material properties and different steel tube dimensions. The results show that an increase in rubber content in the concrete infill leads to a reduction in the axial capacity; however, this reduction is lower than that observed for unconfined specimens. The results also illustrate an increase in ductility with higher rubber content, which is mainly noticeable for members with circular sections as compared to those with square sections. The experimental and numerical results are used to examine the axial capacity prediction approaches in Eurocode 4 and AISC 360, with particular focus on assessing the confinement effects. It is shown that codified prediction equations for square concrete filled steel tubes give reasonably accurate results. Both codes, however, result in poor predictions for circular concrete filled steel tubes, with Eurocode 4 leading to unsafe predictions while AISC 360 gives overly conservative estimates. Modifications to Eurocode 4 and AISC 360 axial capacity approaches for circular tubes are proposed and are shown to offer significant improvement in terms of safety and accuracy of design.

Equivalent stress-strain models of components in high-strength concrete-filled circular steel tube columns

Nine circular concrete-filled steel tube (CFST) columns with 109 MPa concrete were tested under eccentric axial loading to derive the equivalent axial stress-strain models of the steel tube and concrete. Based on the measured strain data and the theory of elasticity and plasticity, the progressions of the axial and hoop stresses at different locations of the steel tube section were obtained. For the compression side of the steel tube, the hoop stress was developed at or after yielding; then, the ratio of the axial stress to the effective stress, σsa/fe, gradually decreased to a value of around 0.77. For the tension side of the steel tube, σsa/fe continued to increase after yielding and kept constant when σsa/fe reached a value of approximately 1.15. Using the developed axial stress-strain model of the steel tube, the equivalent axial stress-strain relationship of concrete was determined by matching the axial load-vertical displacement curve obtained from the fiber model analysis to that from the test. The confinement index, ξ, mainly influences the peak compressive stress and strain of concrete, whereas the eccentricity ratio has some influence on the descending branch of the stress-strain curve. The fiber model with the proposed stress-train models of steel and concrete can well simulate the behavior of circular CFST columns with high-strength concrete under compression-flexural loading when 0.35 ≤ ξ ≤ 0.85, 0.14 ≤ e/D ≤ 0.68, and 3.8 ≤ Le/D (Le is the effective length of a column) ≤ 10.6.