Concrete-filled Steel Tubular Structures Research Articles

Axial compressive behavior of circular concrete-filled steel tubular stubs strengthened with CFRP considering preloading and corrosion

Concrete-filled steel tubular (CFST) structures in service are usually subjected to a combination of loads and corrosion, and they need to be strengthened for continued use. Carbon fiber reinforced polymer (CFRP) is characterized by high strength, corrosion resistance, and convenient construction. Experimental and numerical investigations are conducted to study the axial compressive behavior of CFRP-strengthened circular concrete-filled steel tubular stubs considering preloading and corrosion (C-CFRP-PCCFST). Four C-CFRP-PCCFST specimens and one un-strengthened specimen are tested under axial compressive load. Finite element (FE) models of the C-CFRP-PCCFST stubs are established and validated by experimental results. Then, a parametric study on the yield strength of C-CFRP-PCCFST stubs is mainly carried out in terms of different corrosion parameters, material properties, and preloading conditions. The results show that the preloading has a negative effect on the yield strength of C-CFRP-PCCFST stub columns, while its effect on the ultimate strength can be neglected. The yield strength of C-CFRP-PCCFST stub columns decreases with the increase of corrosion parameters. The effects of corrosion and preloading on the yield strength of C-CFRP-PCCFST stubs are reflected by introducing strength reduction factors Re1 and Re2, respectively. Based on the results of the parametric study, the equation for predicting the yield strength of C-CFRP-PCCFST stubs is proposed.

Experimental and numerical study on shear behaviour of concrete-filled double-skin steel tubular structures

The concrete-filled double-skin steel tubular (CFDST) structures show high strength, large stiffness and convenient construction, which has been adopted in engineering practice more and more widely. However, the nonlinear shear behaviour of CFDST is still lack of research when the shear span ratio is low. This study conducts shear tests on four CFDST deep beams with large diameter of 1500 mm as well as low shear span ratio of 0.33 and 0.83, based on the application of CFDST in a typical nuclear engineering. The failure modes, load-displacement relations, concrete cracks and strains on steel tubes are carefully analysed. Then a refined nonlinear finite element model is built and the shear mechanism of the CFDST is revealed. The shear contribution of various components is obtained including the outer and inner steel tubes, the in-filled concrete, the stiffeners. Parametric analysis is further conducted to find the influence of key design parameters including hollow ratio, material strength and thickness of steel plate. Finally, based on test results and finite element analysis, a shear capacity formula considering the superposition of concrete, steel tube and stiffeners is proposed. It is demonstrated that the CFDST beams show typical shear failure for shear span ratio of 0.33 while shear-flexure failure for shear span ratio of 0.83. The shear strength is high with satisfied ductility. The proposed formula gives accurate prediction which is verified by test results and finite element analysis.

Three-dimensional creep calculation model for reliability analysis of concrete-filled steel tubular (CFST) structure

For long-span concrete-filled steel tube (CFST) structure, it is essential to implement efficient and realistic prediction models for the long-term processes of core concrete creep. In order to systematically study the main influential factors in CFST structure creep effects, a probabilistic analysis can be performed. The objective of this paper is to analyse the long-term response of CFST structure by using time-dependent reliability method. To address the problem of creep calculation, a novel and comprehensive presentation was developed. By converting the integral-type creep law to a differential-type form with internal variables, based on the Kelvin chain model, an elastic structural analysis with generally orthotropic elastic moduli and eigenstrains was established. We have compiled a three-dimensional (3-D) finite-element analysis programme that considers the creep Poisson effect of core concrete to analyse the long-term performance of the CFST structure. This is achieved by developing a commercial finite element code, such as Usermat in ANSYS. As calculation examples, several time-dependent analyses of a CFST stub column and long-span CFST arch bridge were performed. The calculated results of long-term behaviour agree well with experimental and measured data. Considering the randomness of various input parameters, such as physical dimensions and materials of the CFST structure, we have analysed the uncertainty using the Latin-hypercube-sampling (LHS) stochastic finite-element method. The results of creep sensitivity analysis revealed that coefficient of creep model uncertainty, steel and concrete elastic modulus were the three most influential parameters for CFST stub columns’ creep, and load, coefficient of creep model uncertainty, relative humidity, steel and concrete elastic modulus were the five for CFST arch bridges’ long-term deflection. In addition, the use of Monte Carlo (MC) and response surface (RS) methods for structural stochastic reliability analysis demonstrates the ability of these approaches to assess the reliability index of long-span CFST arch bridges. This provides a valuable reference for the design and performance evaluation of CFST structures.

Trussed concrete-filled steel tubular hybrid structures subjected to axial compression: Performance and design calculation

This paper presents a study on the axial compression performance of trussed concrete-filled steel tubular (CFST) hybrid structures. A total of 26 specimens were tested, including 15 circular and 11 square cross-sectional specimens. The effects of cross-sectional types, yield strength of steel of CFST chord, web types, number of chords, and equivalent slenderness ratio were investigated to clarify the mechanical behavior of trussed structure. The analyses of the full-range response, the typical failure mode, and the distribution of the transverse deflection and strain of the structures were carried out. A finite element analysis (FEA) modelling of trussed CFST structure was established and validated against the experimental results. Through the numerical simulation, the analysis of the internal force and stress was conducted, and the hybrid effect between the web and CFST chord was revealed. Based on parameter analysis, the design calculation was proposed, and the trussed CFST hybrid structure was equivalent to a single CFST to calculate its load-bearing capacity by introducing equivalent slenderness ratio proposed.

Active learning methods for strength assessment of circular CFST under coupled long-term axial loading and random localized corrosion

In harsh marine environments, the complex combination of loading and corrosion severely affects the performances of concrete-filled steel tubular (CFST) structures. Therefore, accurately assessing their strengths becomes both complicated and vital for structural safety design and maintenance. Previously, the random localized corrosion along the steel surface was often simplified as uniform corrosion, which failed to reflect its inherent randomness and uncertainty. This study proposes an efficient modeling technique that simulates the coupled effects of long-term loading and random localized corrosion. The developed finite element models are validated against a series of test data. By combining numerical simulation and literature collection, databases of CFST with non-corrosion, uniform corrosion and random localized corrosion are constructed, comprising a total of 4038 samples. Seven machine learning methods are employed to predict the strength of CFST under the above three corrosion scenarios, where the active learning approach is adopted to select a few yet valuable data for training models. A hyperparameter optimization method, considering both structural mechanism and prediction accuracy, is proposed to ensure that the mechanical mechanisms are accounted for. The results show that CFST columns with random localized corrosion exhibit considerably lower strengths compared to counterparts with uniform corrosion. Among the established machine learning models, extreme gradient boosting (XGBoost) and Gaussian process regression (GPR) perform the best. The active learning method achieves similar model performance using only 63 % of the data as that using the entire data, demonstrating its capability in effectively addressing challenges in the field of structural engineering such as limited data and costly data acquisition.

Experimental and numerical investigation on strengthening mechanism of rib-reinforced round-over-round RCFDST column jointed with steel beam under cyclic loading

This paper involved both experimental and numerical investigations on the behaviour of twelve types of recycled-concrete filled double-skin steel tubular (RCFDST) structures subjected to the cyclic loading with reinforced ribs at the column–steel beam joints, of which the damage pattern and seismic performance were analysed by varying the ratio of beam–column flexural linear stiffness, axial compression ratio, hollow ratio, type and strength of the concrete, and the length of ribs. The results indicate that the specimens primarily exhibited the damage pattern of the plastic hinge at both ends of the beams, and that the horizontal load–displacement hysteresis curves with full loop at the top of the column were observed. Moreover, the structures composed of recycled concrete from 40 MPa to 60 MPa presented no reduction in ultimate load as well as the joint ductility to those of the ones with normal concrete. Regarding the performance of RCFDST, the optimal ratio of beam–column flexural line stiffness (ki) is found in the range of 1.0 to 1.5. This study demonstrated that the recycled concrete made from the industrialized aggregate formed by crushing waste concrete from road bases has potential application prospects in Concrete-filled double skin steel tubular (CFDST) structures due to its low cost and good seismic performance, including high ultimate load capacity and low energy consumption.

Axial deformation of early-age concrete-filled steel tubular columns under multi-stage loading

The continuous construction scheme for concrete-filled steel tubular structures in practice is time-saving and cost-effective in comparison to the conventional moulding construction methods, whilst this also results in complex stress conditions and working mechanisms in the early-age concrete-filled steel tubes (CFSTs) during construction. This investigation aims to explore the influence of continuous construction process on the axial compressive behaviour of early-age CFSTs as subject to multi-stage loading schemes. Eight short square early-age CFST columns are designed and tested. The axial deformation evolutions of CFSTs are analyzed for different load parameters including the initial stress of steel tube, loading time interval and load magnitude. The test results show that the deformation of early-age CFSTs varies significantly with various load parameters. Excessive deformation can be observed during the multi-stage loadings due to the local buckling of the steel tube. Based on the shrinkage and creep model of concrete, a theoretical algorithm for calculating the relationship between the deformation of early-age CFST and the age of concrete is developed. Furthermore, a simplified calculation method for the deformation of early-age CFST under multi-stage loading is proposed that can realistically describe the deformation of early-age CFSTs under continuous construction process. The research findings lay a foundation for the deformation control and safety assessment of CFST under continuous construction scheme.

Trussed square concrete-filled steel tubular hybrid structures subjected to axial compression

This paper introduced the mechanical performance of trussed square concrete-filled steel tubular (CFST) hybrid structure subjected to axial compression. A refined finite element analysis (FEA) modelling was established and verified against the experimental results. Based on the FEA modelling, the hybrid effect and the working mechanism of the structure were revealed through the comprehensive analysis of the internal force distribution, the axial compression force (N) versus average longitudinal strain (ε) relation, the typical failure modes, and the stress distribution. The N-ε relation was classified into five types according to the failure modes and critical factors elucidated. The criterion for determining the ultimate limit state of the trussed square CFST hybrid structure was established by investigating its load-bearing capacity and peak strain. An interface contact stress model was proposed to describe the hybrid effect of the structure. Based on parametric analysis, critical parameters used to characterize the hybrid effect were identified, thereby the equivalent slenderness ratio for straight trussed CFST hybrid structure and that for a curved one considering the influence of the initial curvature ratio were proposed. The quantitative relationships between the stability factor and equivalent slenderness ratio were established and validated so as to facilitate the calculation of the load-bearing capacity for both straight and curved trussed square CFST hybrid structures.

Prediction of the mechanical performance of compressed circular CFST columns with circumferential debonding gap

Concrete-filled steel tubular (CFST) structures have been widely used in high-rise buildings and long-span bridges for the high strength and ductility due to the confinement effect of the concrete core provided by the steel tube. This beneficial composite action could be damaged by circumferential debonding gap (CDG), which is nearly unavoidable in real CFST structures. Reliable formulations to predict the influence of CDG on the mechanical performance of compressed circular CFST columns are needed. In this paper, the experimental parameters and results of compressed circular CFST columns with CDG are summarized to discuss the deficiencies of the parameters chosen in the tests. Detailed finite element simulation procedures for compressed circular CFST columns with CDG, based on ABAQUS, are then introduced, verifying their accuracy by test results. Based on the validated finite element models, a database of compressed circular CFST columns with and without CDG, including strength, ductility and stiffness of specimens is established; it is obtained by covering suitable parameter ranges for CDG and CFST structures, in accordance with real applications. Successively, new formulations able to predict the ultimate load capacity, the reduction factor, the ductility and the stiffness of specimens are derived by the Evolutionary Polynomial Regression (EPR) methodology, demonstrating good accuracy and low complexity.

Numerical investigation of concrete-filled double skin steel tubular (CFDST) structure subjected to underwater explosion loading

With the increasing applications in the offshore industry such as oil and gas jackets, submarine pipelines and wind turbine foundations, concrete-filled double skin steel tubular (CFDST) structures are encountering the ever-increasing risk of threats to underwater explosions (UNDEX). This study presents a systematical investigation on the structural behaviors and design recommendations of the CFDST structures subjected to UNDEX loadings through finite element analysis (FEA) approaches. Finite element models have been developed, where the non-linear material properties of the constitutive steel and concrete parts and the composite actions in-between have been considered. The FEA models are verified against the experimentally determined shock wave pressure history, the deformation shapes, and the residual strength. The full-range analyses were firstly carried out on the structural responses of CFDST structures, including the typical damage patterns and residual strength of the specimen after UNDEX. Then, the parametric studies show that the cross-section hollow ratio, charge weight, and explosion distance play great roles in determining the residual strengths. Thereafter, damage indexes considering the parameter of the hollow ratio and the scaled explosion distance has been formulated, and design recommendations have been suggested accordingly.

Shrinkage Compensation Technology of Concrete Filled Steel Tubular Structure of Large Cross-Sea Bridge

In view of the poor accuracy and unsatisfactory effect of shrinkage compensation for concrete-filled steel tubular structures of large cross-sea bridge (CSTS-LCSB), a new shrinkage compensation technology for CSTS-LCSB is proposed. The shrinkage mechanism of CSTS-LCSB is analyzed, including plastic shrinkage, dry shrinkage, autogenous shrinkage, and carbonation shrinkage. The proportion of dry shrinkage and autogenous shrinkage in ordinary concrete and high-strength concrete is determined. The CSTS-LCSB has different shrinkage forms in different environments, and the shrinkage strain state of CSTS-LCSB in different environments is calculated according to the difference. The shrinkage area of the concrete structure is determined, and the expansion agent is used to compensate for the shrinkage area of CSTS-LCSB. The experimental results show that the proposed compensation technology has a good effect on shrinkage compensation and has certain feasibility.

Time-dependent non-linear buckling of 3D CFST arch structures with hybrid random interval uncertainties

Limit point and bifurcation buckling loads are critical concerns in structural stability design. With the inevitable viscoelastic effects of creep and shrinkage in concrete, such critical buckling points may vary due to the time-dependent change of equilibrium configuration. Furthermore, the intrinsic uncertainty and natural randomness in the geometry and material characteristics would affect the structural stability performance significantly. The present study provides a new robust method, called the generalized Chebyshev surrogate model-based sampling approach, in assessing the time-dependent nonlinear buckling behaviour of 3D concrete-filled steel tubular (CFST) arch structures when both random and interval uncertainties are involved. In the proposed approach, the relationships between the uncertain parameters and the critical nonlinear limit and bifurcation buckling loads are formulated using Chebyshev surrogate model strategy combined with finite element method. The extreme bounds of the statistical features, including means, standard deviations, of the critical nonlinear buckling loads are furnished by using Monte Carlo method and Quasi Monte Carlo simulation method. Finally, the applicability and the validity of the proposed approach are illustrated with a series of numerical investigations.

Ultimate flexural strength of rectangular concrete‐filled steel tubular beam‐columns using high‐strength materials

AbstractThe ultimate flexural strength of concrete‐filled steel tubular (CFST) beam‐columns is calculated using the full‐plastic strength as per the “AIJ Recommendations for Design and Construction of Concrete Filled Steel Tubular Structures”. However, the full‐plastic strength might overestimate the ultimate strength of rectangular CFST beam‐columns, when the width‐to‐thickness ratio is significant and/or high‐strength materials are used. The calculation of the ultimate flexural strength when it exceeds the applicable range of AIJ CFT recommendations is based on a method that considers the stress reduction on the compression side of the steel tube or outermost edge strain of the concrete. However, both stress reduction and outermost edge strain should be considered when high‐strength materials are used in CFST for accurate ultimate strength formulas. In this study, a simple evaluation method for the ultimate flexural strength of rectangular CFST beam‐columns was proposed using the maximum strain at the extreme concrete compression fiber and stress reduction on the compression side of the steel tube. In addition, the appropriate strain at the extreme concrete fiber needed to accurately calculate the ultimate strength was discussed.

Utilizations of Steel-Reinforced Concrete-Filled Steel Tubular (SRCFST) Constructions

In recent times, there has been a significant amount of research dedicated to traditional concrete-filled steel tubular (CFST) structures. This research has been broad and includes both practical testing and theoretical analysis. As of late, scholars have redirected their attention toward introducing innovations in the domain of steel-concrete composite systems. This piece delves into investigations involving experiments and simulations, as well as the underlying mechanisms and interplays of constraints. The focus is on rectangular columns made of Steel-Reinforced Concrete Filled Square Tubular (SRCFST), subjected to combined compression and torsion forces. By building upon previous investigations, the article not only provides a summary but also evaluates the pathways that research on SRCFST columns has taken. Additionally, it offers a glimpse into the potential directions for future studies. This comprehensive strategy not only serves as a point of reference for upcoming research endeavors but also furnishes practical insights for their real-world implementation.

Temperature field of intumescent coating protected concrete-filled steel tubular columns under fire

In the fire protection design for concrete-filled steel tubular (CFST) structures, the analysis of temperature field usually acts as a preceding step to further study the mechanical performance in fire. However, there is still a lack of systematic research on the temperature field of CFST sections with intumescent fire coating (IFC). In this study, fire tests of four hollow steel tubular specimens and twenty CFST specimens lasted for 180 min ∼240 min were carried out under ISO-834 standard fire curve and all the specimens were protected with IFC. A technical process to determine the equivalent thermal conductivity of IFC applied to CFST columns was proposed. A verified finite element analysis (FEA) model to predict temperature field of IFC protected CFST section was established and the size effects of CFST sections on the temperature field were studied. It was found that the intumescent coating can reliably work together with CFST specimens without overall crumbling during the long-time fire exposure up to 240 min. The temperature rise of the steel tube of CFST column can be significantly delayed with the effective insulation of IFC together with the heat absorption of the core concrete. The main factors affecting temperature rise of the IFC protected CFST section under fire are the core concrete, dry film thickness (DFT) of IFC and sectional diameter.

Improving strength and deformation characteristics of concrete-filled steel tubular elements in bending

The article is devoted to the investigation of the type of monolithic reinforced concrete structures, known as Concrete-Filled Steel Tubular (CFST) elements. They are mostly used in compression and rarely in bending, due to their high deformability. However, there is still a trend in the development of CFST structures with both compressed and bent elements. While bending the reactive lateral pressure transmits from the steel tube to the concrete core creating a favorable condition - volumetric stresses in the concrete, which increases the flexural strength of the material significantly.
 Experimental studies have been carried out to determine the bearing capacity and deformability of steel-reinforced concrete (composite) structures filled with fiber-reinforced concrete (FRC) of different compositions. The optimal composition of the concrete core for CFST elements in bending was selected, it helped to increase the strength and deformation characteristics of the element. To choose this optimal composition of the concrete core among others, a series of specimens were prepared and tested in compression, tensile and bending tests, and together with that, the modulus of elasticity and adhesion between the concrete core and the metal casing were studied. As the optimal solution for the concrete core – a mixture of concrete modifier (CM) and metal fibers were selected, due to an increase in the adhesion between the concrete core and the metal casing.
 As a result, of experimental studies, graphs of load-deflection dependencies were built and presented. The research results proved that with an increase in the physical and mechanical properties of concrete together with the degree of its adhesion to steel, transverse deformations decrease. For further research development, numerical modeling might be created to describe and foresee the behavior of such elements with different parameters in more detail, which will lead to a better understanding of the combined operation of the tube and core.

Design of concrete-filled steel tubular columns using data-driven methods

By leveraging the merits of structural steel and concrete materials, concrete-filled steel tubular (CFST) structures have been increasingly used in the composite construction of bridges and high-rise buildings. However, their design equations are more complicated than those of steel and reinforced concrete (RC) structures, especially for circular columns under eccentric loading. Therefore, the use of emerging data-driven approaches will help structural engineers ease the design process. This paper explores the use of data-driven design methods as alternatives to conventional mechanics-based design models. Five boosting algorithms, including adaptive boosting (AdaBoost), gradient boosting machine (GBR), extreme gradient boosting (XGBoost), light gradient boosting machine (LightGBM), and categorical gradient boosting (CatBoost), are employed to develop predictive models for four different types of CFST columns (i.e., circular columns, rectangular columns, circular beam–columns, and rectangular beam–columns). These predictive models are trained using the most up-to-date and comprehensive database collected from over 3,200 test specimens. Reliability analysis is conducted to calibrate the resistance reduction factors for three different design frameworks (i.e., the US, Eurocode, and Australian frameworks) to ensure that the newly developed predictive models meet the target reliability indices required by different design frameworks. A web-based design tool is also developed to promote the practical use of data-driven methods for the design of CFST columns.

Percussion-based quasi real-time void detection for concrete-filled steel tubular structures using dense learned features

Concrete-filled steel tubular (CFST) structures have been widely used to provide high load-bearing capacity. Percussions are expected to provide simple and effective means of detecting subsurface voids of CFST structures, caused by poor grouting compactness and harsh operating conditions. Existing percussion data processing methods consist of two steps to deal with signals with high sampling rates: heuristic feature extraction and classification. High-dimensional feature representations tend to be selected to achieve visual interpretability and performance, increasing the complexity and computational cost of the entire process. This research aims at developing a computationally efficient end-to-end framework for the percussion-based void detection of CFST structures suitable for mobile computing devices available in the field, while maintaining the performance and a certain degree of visual interpretability. The proposed approach is based on the dense learned features; First, a classifier that processes raw percussion sound signals is developed using dilated convolutions and global max-pooling. The class activation mapping (CAM) technique is then applied to convert the classifier into the dense feature extractor. Feature visualization for the experimental data shows that (i) the classifier listens to the “echo” of the percussion sound carefully, rather than the impact sound itself, and (ii) the classification results depend on both the vibration frequency and its damping ratio. Finally, the feature extractor is extended to quasi real-time void detection and classification, where the processing time duration for a single 0.1-second (50,000-point) percussion signal is 8.28 ms without leveraging advanced computing capabilities of graphics processing units, and the average testing accuracy for the fivefold cross validation is 99.81%. The results demonstrate the potential of the proposed approach for improving the efficiency of inspections.

Anti-collapse performance of concrete-filled steel tubular composite frame with assembled tensile steel brace under middle column removal

To study the effect of assembled tensile steel brace on the progressive collapse resistance of concrete-filled steel tubular (CFST) structures, two 1/4-scaled two-bay two-story composite frames with CFST columns and profiled steel–concrete composite beams were tested pseudo-statically under a middle column removal scenario. The vertical load–displacement responses and failure modes of two specimens were analyzed. Especially, the effects of assembled tensile steel braces on the anti-collapse performance of the frames were discussed. Experimental results show that the assembled tensile steel braces delayed the failure of the upper beam and significantly improved the structural bearing capacity and ductility of the frame. The horizontal reaction forces of the specimens are compressive forces due to the compressive arch action in the initial loading stage, and they change from compression to tension due to the catenary action. Numerical parametric analysis indicates that the splint bolt connection configuration can effectively improve the bearing capacity of the frame models by increasing the bolt preload and friction coefficient. Furthermore, the collapse resistance of the structure is calculated based on the simplified elastic theory method, considering the effect of the steel braces. It is believed that the assembled tensile brace between CFST columns could be used as a reinforcement method to improve the collapse resistance of the framed structure.

Numerical Analysis on Post-Fire Resistance of High-Strength Circular CFST Stub Column in Axial Compression

Concrete-filled steel tubular (CFST) structures using high-strength materials have been increasingly used in civil engineering due to their exceptional mechanical performance. A comprehensive numerical analysis was performed in this study, where a finite element model was established for a CFST stub column using high-strength materials with consideration of fire and load combinations. The influence of critical parameters to the resistance in axial compression were analyzed based on the verified model, including the fire exposure time, the axial load level, the confinement factor, etc. The results showed that the residual resistance of a high-strength circular CFST (HCFST) column in axial compression decreased with the increase of axial load level and fire exposure time, while it increased with the increase of material strength.