External Steel Tubes Research Articles

Investigation of interfacial bond strength of circular CFST columns based on ordinary and inorganic polymers through advanced machine learning methods

Concrete-filled steel tube (CFST) is a combination of materials, in which the interfacial bond-slip behavior is the prerequisite to reflect the synergistic loading of external steel tube and core concrete. The inorganic polymer concrete within the concrete-filled steel tube (IPCFST) compensates for the drying shrinkage of the concrete and enhances the adhesion at the interface between the steel tube and the concrete. However, the current code is too simple for the bond strength of CFSTs, and the existing theoretical calculation methods are not universally applicable. This paper aims to develop a practical artificial neural network tool for predicting the interfacial bond strength of ordinary CFSTs and IPCFSTs. An efficient prediction model for the bond strength of CFST interfaces was developed using a radial basis function network (RBFNN), with concrete strength, steel tube size, bond length and other factors as the main parameters. Bond performance test data was collected for a sample database of 322 circular CFST columns, including 56 ordinary CFSTs and 263 IPCFSTs. The results show that the Artificial Neural Network (ANN) algorithm can effectively address the bond strength problem of CFST structures and improve the accuracy and density of prediction in the comparative analyses with the measured values and the results of the semi-theoretical formulations. Based on the prediction results of ANN model, a sensitivity analysis of the parameters using Garson's algorithm shows that the diameter-to-thickness ratio and concrete strength significantly affect bond strength for both ordinary CFSTs and IPCFSTs. This study provides a more reliable prediction method and technical support for the engineering design of CFST structures.

Axial compressive behavior of FRP-confined square CFST columns reinforced with internal latticed steel angles for marine structures

This paper investigates the mechanical response of axially loaded FRP-confined square CFST columns strengthened with internal latticed angles (F-SRCFST) through experimental and analytical methods. The influence of three parameters on the performance is examined, including dimensions of the specimens, configuration of latticed steel angles, and number of CFRP layers. Experimental results demonstrate that concrete crushing in the specimens confined by CFRP exhibits a higher degree of uniformity, with cracks appearing more densely. The latticed steel angles can prevent the development of concrete cracks into the core concrete, while noticeable local buckling of the steel angle between the battens is observed. The enhancement coefficient of steel angles on the load-bearing capacity and energy dissipation ability of F-SRCFST columns is more pronounced when compared to CFRP. The utilization of CFRP and latticed steel angles can effectively improve the problem of inconsistency in square steel tube deformation under axial loading, and both parts exhibit similar effects on the dilation behavior of square CFST columns. A composite confined model was developed to consider the triple constraints of CFRP, external steel tube, and latticed steel angles, and a predictive formula was developed to estimate the load-carrying capability of F-SRCFST columns. The comparative study shows that the predictive formula agrees well with the ultimate axial strengths of F-SRCFST columns.

Fire test and simulation of circular steel tube confined steel-reinforced concrete columns

Steel tube confined steel-reinforced concrete (STCSRC) columns are novel steel-concrete composite columns where the steel tubes are disconnected at beam-column connections. Few researches have been conducted on the fire behaviour of STCSRC columns by now, though there are some relevant studies on the fire performance of steel-reinforced concrete-filled steel tubular (SR-CFST) columns. However, the mechanical behaviour of STCSRC columns under fire exposure differs from that of SR-CFST columns due to the discontinuous external steel tube. It is essential to carry out a study on the fire performance of STCSRC columns. The fire tests of four axially loaded STCSRC columns were carried out in this paper, and the failure mode, distribution of cross-sectional temperature, fire resistance, and lateral deflection were discussed. A finite element model was developed and validated with test results, and then a parametric analysis of the STCSRC stub and slender columns under fire exposure was performed. The load ratio, steel tube strength, the area ratio of steel tube to concrete, and slenderness ratio have an adverse effect on the fire resistance of STCSRC columns, whereas cross-sectional dimension, area ratio of steel section to concrete, and strength of steel section have a favourable impact. Concrete strength has a different influence on the fire resistance of STCSRC stub and slender columns. Finally, based on the specifications JGJ/T 471-2019 and EN 1994-1-2, simplified calculation methods for assessing the fire resistance of STCSRC columns were proposed.

Experimental study of energy‑absorbing and support characteristics of glass microsphere-filled steel tube columns under uniaxial compression.

An innovative energy-absorbing and bearing structure was proposed, which incorporated the coupling of glass microspheres with a metal tube. Glass microsphere-filled steel tube (GMFST) column, consisting of external steel tube and inner glass microspheres, was expected to give full play to the energy-absorbing and load-bearing capacities of the particle while restricting particle flow from collapsing, thereby enhancing the overall structural strength. Four groups of steel tubes and the GMFST specimens were designed and subjected to axial compression tests at four different loading rates to investigate the performance of the structure. These tests aimed to analyze the deformation mode, mechanical response, and energy absorption capacity of the GMFST columns under quasi-static to low-speed compression conditions. The results indicated that the deformation process and failure mode of GMFST columns were similar to those of hollow steel tubes, albeit with a different post-buckling mode. Filling the steel tubes with glass microspheres reduced the load fluctuation range, moderated load-displacement curves, and exhibited a strain rate strengthening effect. The GMFST columns demonstrated superior energy absorption capacity, with significant increases in crush force efficiency, the averaged crush force, and the total absorbed energy, particularly in terms of subsequent support capacity. The load-increasing reinforcement properties enabled GMFST columns to overcome the limitations associated with the unstable post-buckling path of energy‑absorbing damping structure, exhibiting outstanding load-bearing performance and stability in the later stages. The results provided valuable guidelines for designing and engineering high-performance GMFST columns, serving as a new type of energy-absorbing and supporting structure.

Influence of corrosion on loading capacity of circular concrete-filled steel tubular column

Concrete-filled steel tubular (CFST) columns are commonly used in engineering, especially in load-bearing structures. This study conducts an in-depth examination of how the axial resistance of CFST columns varies with random corrosion. A finite element model for circular CFST columns is developed and its accuracy is verified using available experimental data. The interaction between the outer steel tube and the concrete column is explored, along with the introduction of a strength reduction coefficient concept. Additionally, the concept of an axial load-bearing capacity reduction coefficient is introduced to illustrate variations in the axial resistance of circular CFST columns during different corrosion stages. The degradation patterns of reduction coefficients for uniform and random corrosion depths in the steel tube and CFST columns with varying mass loss rates are analyzed, and formulas are provided to predict the axial resistance in different corrosion stages. Furthermore, the study investigates the influence of variables such as material strength, component dimensions, and the thickness of the external steel tube, resulting in adjustments to the original expressions for various conditions.

Experimental Investigation on the Axial Loading Performance of Grooving-Damaged Square Hollow Concrete-Filled Steel Tube Columns

Under the influence of material defects, structural grooving, environmental corrosion, and other factors in engineering, concrete-filled steel tubes incur local defects on their external surfaces that affect their structural integrity and service life. This work conducts axial compression tests on 10 grooving-damaged square hollow concrete-filled steel tube (SHCFST) columns to investigate the effect of grooving damage on their axial compressive ultimate bearing capacity and the effect of steel tubes on concrete confinement. It explores the effects of three parameters, namely, the length of grooves, presence of slots in internal and external steel tubes, and orientation of grooves, on structural static performance. This study analyzes the loading, failure mechanisms, and axial compressive ultimate bearing capacity of grooving-damaged SHCFST columns. Results indicate that grooving weakens the steel tube’s confinement effect on the concrete core, reducing the axial compressive ultimate bearing capacity of specimens. On the basis of this experimental research, a method for calculating the axial compressive ultimate bearing capacity and axial compressive stiffness of grooving-damaged SHCFST columns is proposed. The calculation results closely align with experimental outcomes, providing valuable insights for related scientific research and engineering applications.

Experimental and theoretical analysis of steel tubed geopolymer concrete columns under axial compression

To alleviate the consumption of resources and promote the further application of geopolymer concrete in practical engineering, the mechanical performance of steel tubed geopolymer concrete (STGC) columns under axial compression was first explored in this study. The impact of experimental parameters including concrete types, configurations of steel tubes, diameter-to-thickness ratios (D/t), and slag contents were analyzed in detail. The test results illustrate that after grooving at the two ends of columns, the axial deformation of the external steel tube becomes smaller compared with the core concrete and the confining pressure of the external steel tube is improved significantly, resulting in a remarkable improvement of the compressive strength and corresponding strain at both peak and residual state of the specimens. Although little influence of concrete types on the steel stress evaluation is detected, the ductility of the STGC columns is inferior to that of the specimens with ordinary concrete due to the obvious brittle property of geopolymer concrete. Moreover, the ratio of axial stress to yield strength of the external steel tube at the peak load of STGC columns is increased with the decrease of diameter-to-thickness ratio and concrete strength. Finally, a new model for predicting the steel axial stress in steel tubed concrete (STC) columns at the peak state is established and a load capacity model incorporating the steel axial contribution is also developed for STC columns. Compared with extensive existing test results, the developed model can provide satisfactory accuracy.

Eccentric compression capacity of circular CFST columns under random pitting corrosion

This study explores how random pitting corrosion affects the ultimate eccentric load-carrying capacity of corroded CFST columns at various corrosion levels. The research examines how the load-carrying capacity of circular steel tube concrete columns changes under eccentric loading conditions due to variations in the quality loss rate of the external steel tube caused by corrosion. The established finite element model is validated using existing experimental data, showing a high degree of accuracy. Additionally, the study investigates influencing factors, including corrosion thickness (tc/t), both constant and random corrosion (tc/t), eccentricity (e), corrosion pit diameter (Dc), material strength, and component dimensions. The findings reveal how these parameters affect the reduction of eccentric compressive load-carrying capacity. To visually illustrate this effect, the concepts of load reduction factors Rcs′ and Rc′ are introduced, representing the ratio of load-carrying capacity before and after corrosion. Based on extensive random finite element analyses, we propose a load-carrying capacity calculation formula for the eccentric load reduction factor applicable to corroded circular CFST columns. The study also demonstrates that axial compression can be considered a special case of eccentric loading with an eccentricity of 0. Consequently, by incorporating stability and eccentricity reduction factors, adjustments are made to existing axial load-carrying capacity prediction formulas, resulting in the calculation formula for the reduction factor Rc′. By measuring the local quality loss rate of the external steel tube of corroded circular CFST columns, it is possible to predict both axial and eccentric load-carrying capacities.

Study on axial behaviour of concrete filled steel tubular columns

Concrete-filled steel tube (CFST) columns have gained wide acceptance in the construction of high-rise buildings due to their exceptional ability to withstand axial load. They are commonly employed as columns in large-span high-rise buildings, bridges, and piers. This research aims to investigate the performance of concrete-filled steel tube (CFST) columns when subjected to axial load. One key advantage is that the steel tube serves as a permanent confinement for the concrete. The concrete filling inside the tube prevents local buckling and enhances the lateral stability of the member. Moreover, the external steel tube acts as a formwork. Eight composite columns were constructed, comprising four CFST columns and four CFDST (Concrete-filled Double Skin Tubular) columns, and were subjected to axial loading for evaluation. The experimental study was conducted to determine the relationship between load, axial displacement, and lateral displacement, as well as the ultimate load-carrying capacity. Furthermore, finite element analysis using Abaqus was performed to investigate the composite columns analytically. A parametric study was also carried out, considering the diameter-to-thickness ratio and grade of concrete. Confinement significantly enhances the load-carrying capacity, and the presence of concrete infill reduces the lateral deflection of long columns.

Compressive behavior of geopolymer recycled brick aggregate concrete confined by steel tubes

Geopolymer recycled brick aggregate concrete (GRBAC) is an environmental-friendly material and its strength and ductility can be improved by employing steel tube for confinement. However, there is little research on the mechanical performance and constitutive model of GRBAC under the confinement of steel tube in the past. In this paper, the axial compressive behaviors on 25 stub columns of GRBAC confined by steel tubes were experimentally studied, and the test variables were reference compressive strength (45 and 65 MPa), recycled brick aggregate (RBA) replacement ratio (0, 30%, 50%, 70% and 100%) and thickness of steel tube (2.5, 3.7 and 5 mm). The characteristics such as failure mode, load–deformation curve, ultimate strength, compressive stiffness, ductility and peak strain of the stub columns were presented and analyzed. The results demonstrate that the adverse effect of RBA content on the ultimate strength is weakened due to the confinement effect of the external steel tube, and the compressive strength of confined GRBAC is 1.73–3.45 times that of unconfined specimen. With the increase of RBA replacement ratio, the axial compressive ultimate strength and stiffness of the steel tube confined GRBAC decrease, and the ductility increases. Compared with the specimen without RBA, the axial compressive ultimate strength and stiffness of the specimens with 100% RBA replacement ratio decrease by 21.8%–33.3% and 31%–40%, respectively. Based on the existing conventional core concrete model, considering the characteristics of GRBAC, a finite element constitutive model and a fiber constitutive model suitable for GRBAC confined by steel tubes were proposed.

Axial compression performance of concrete filled double skin (SHS outer and CHS inner) steel tubular columns strengthened by CFRP

This paper reports the compressive behavior of concrete-filled double skin steel tubular (CFDST) columns strengthened by CFRP under axial loading. The outer skin was made of square hollow sections (SHS), while the inner skin was made of circular hollow sections (CHS). Twelve CFDST columns strengthened by CFRP and three CFDST columns were tested. The failure mode, load-displacement relationship, axial load-carrying capacity and initial stiffness are given and analyzed. The results showed that the CFDST columns strengthened by CFRP have better mechanical properties than the column without CFRP reinforcement (i.e. the CFDST columns). The CFDST columns strengthened by CFRP eventually failed due to CFRP rupture or buckling of the external steel tube near the end and cracked concrete damage from the outer steel tube. The stiffness and axial load bearing capacity of the column can be improved as the number of CFRP layers increases. A simplified design formula of CFDST columns strengthened by CFRP is proposed, which is in good agreement with the experimental results.

Confinement effectiveness of bamboo scrimber-filled steel tube columns under axial compression

Bamboo scrimber-filled steel tube (BSFST) columns, consisting of external steel tubes and inner bamboo scrimber, are expected to improve the strength and deformation ability of bamboo scrimber while maintaining excellent performance characteristics, such as light weight, high strength and environmental friendliness. Thirty bamboo scrimber columns and thirty BSFST columns were tested under axial compression to investigate the confinement effectiveness of BSFST columns. During the test, only the core bamboo scrimber bears the axial compression, and the main parameters to be considered are specimen size and steel tube thickness. The results show that the stress‒strain relation curve of the BSFSTs can be divided into an elastic stage and an elastic‒plastic stage, and the BSFSTs showed obvious stress hardening behavior in the elastoplastic stage. With increasing steel tube thickness, the bearing capacity, ultimate stress and ultimate strain of the specimens increase greatly. Finally, the calculation equations for the ultimate stress and ultimate strain of BSFST and the complete stress‒strain curve models of BSFST were proposed. The proposed model can accurately predict the complete stress‒strain behavior for BSFST columns.

Axial compression behaviour of round-ended recycled aggregate concrete-filled steel tube stub columns (RE-RACFST): Experiment, numerical modeling and design

Round-ended recycled aggregate concrete-filled steel tube (RE-RACFST) column is an innovative composite member, which holds the advantage of round-ended concrete-filled steel tube (RE-CFST) and expands the application of RAC in structural members. This paper presents the testing and numerical investigation on the axial compression behaviour of RE-RACFSTs. A total of 18 stub columns were tested to determine the failure mode, axial load versus strain response, vertical and circumferential strains of the round-ended steel tube, and the confinement effect from external steel tube in different positions. The effects of cross-sectional aspect ratio, coarse recycled aggregate (CRA) replacement and steel ratio were analyzed. In general, the ductile behaviour of the specimens reduced with increasing aspect ratio and decreasing steel ratio. The confinement effect during the entire compression process was investigated based on the vertical and circumferential strains. Finite element (FE) models were then developed, and a constitutive model suitable for core RAC in RE-RACFST columns was proposed. Parametric analysis was then carried out to evaluate the effects of the aspect ratio, steel ratio, CRA replacement level, and material strength on the axial compression response. Finally, several existing design provisions for rectangular CFST and RAC-FST members were assessed the feasibility of their application in RE-RACFSTs by using the equivalent method.

Axial compressive behavior of concrete-filled steel tubes with GFRP-confined UHPC cores

The steel–concrete-GFRP-UHPC (SCGU) composite column is a new type of column consisting of steel, concrete, ultra-high performance concrete (UHPC) and glass fiber reinforced polymer (GFRP). The sectional form of the SCGU column is a circular steel tube as the outer layer, a GFRP-wrapped UHPC column as the core, and concrete between the core column and the steel tube. In this paper, axial compression tests were conducted on twenty-four SCGU composite columns, and their damage morphology, load–strain curves and influencing factors were analyzed. The results show that the damage modes of the specimens present multiple convex curves of the external steel tube, corresponding to the fragmentation of its plain internal concrete, the fracture of GFRP and the large axial crack of the UHPC core column; that the load–displacement curves of the composite columns can be roughly simplified to elastic, elastoplastic and plastic flow stages; that the composite columns still have high residual bearing capacity after compression damage; and that effect of tube thickness, number of GFRP layers and UHPC core column diameter of the specimen on the load bearing capacity and ductility of composite columns. Based on the mechanical analysis of each component of the SCGU composite column, a formula of axial compression bearing capacity of the SCGU composite column was presented, and agreement between the formula predicted results and the experimental results was obtained. In addition, finite element (FE) models that can simulate the SCGU composite column are developed.

Numerical analysis of circular tubed high-strength concrete columns with embedded steel tubes under axial compression

This paper presents an in-depth investigation into the compressive behavior of the circular tubed high-strength concrete with embedded steel tubes (THSC-ST) columns using a finite element (FE) approach. The composite action of the columns is comprehensively explored. It can be detected from the numerical results that the confinement provided by the external steel tube in the THSC-ST column is significantly enhanced since the external steel tube does not carry the axial load directly. Besides, increasing the diameter of the internal steel tube can effectively improve the axial stress of confined concrete. Moreover, more effective confining stress of the external tube is observed after wrapping FRP jackets, leading to stiffer the post-peak behaviors for the core and sandwich concrete simultaneously. An extensive parametric analysis is also performed based on the validated FE model to explore the effects of the internal steel tube location, the number of FRP layers, the thickness and yield strength of the steel tube, and concrete compressive strength on the structural behaviors of the THSC-ST columns. Finally, a new formula is proposed to estimate the axial load capacity of the THSC-ST columns and high predicted accuracy is found by the comparisons between the theoretical and experimental results.

Compressive strength of rammed earth filled steel tubular stub columns

Rammed earth (RE) is a potential sustainable building material applicable for modern composite construction. To expand its potential application scenarios, this paper proposes RE-filled steel tubular (REFST) columns: using RE as the infill confined by external steel tubes. Uniaxial compression tests on both material and member levels are conducted. At the material level, cement-stabilized RE cylinder samples with two RE compositions and four levels of cement content are designed. Addition of cement significantly enhances the axial capacity of RE. A method to estimate the compressive strength of RE with considering the added cement content is proposed. Following the same RE composition and cement content at the member level, 15 REFST columns are axially compressed. The comparative study results show that confinement effect needs to considered in estimating maximum axial compressive strength of REFST columns and the strength ratio between the steel tube and infill RE is suggested to be no larger than 10 to fulfill the confinement effect. Finally, a method is proposed to predict the maximum compressive strength of REFST columns.

Mechanical properties of CFDST column under eccentric compression after exposed to fire

This paper investigates the experimental behaviour of ten square concrete-filled double skin steel tube (SCFDST) columns subjected to eccentric compression after 60 min of fire time. The test parameters included slenderness ratio, eccentricity and hollow ratio. It was found that the slenderness ratio and eccentricity have significant influence on the residual deformation and bearing capacity. However, when the hollow ratio (χ) was 0.72, slenderness ratio has little effect on the bearing capacity. The accuracy of finite element analysis (FEA) models were verified by the presented test results, and they were in good agreement. A FEA model was established to conduct a parameter analysis of CFDST members, and the working mechanism was analyzed in depth. Based on the parametric analysis, it shows that nominal steel ratio, eccentricity and strength of external steel tube have significant influence on the member’s residual bearing capacity under eccentric compression after fire: The residual bearing capacity decreases with the increase of nominal steel ratio. When the eccentricity increased from 0 to 0.6, the bearing capacity can decrease about 42 % after fire. The hollow ratio and concrete strength have little effect on the bearing capacity after fire. With the increase of fire time from 0 min to 120 min, the proportion in bearing capacity of internal and external steel tube gradually increases 16.8 %. Finally, compared with the relevant codes, the results of the T/CCES 7–2020 code were in good agreement with the FEA results.

Cyclic behavior of circular CFST stub columns with different inner constructions

Circular concrete-filled steel tube (CFST) columns, which are widely used in high-rise and super high-rise buildings, have good mechanical performance. Low-cyclic loading tests were conducted on eight large-sized specimens with different shear-to-span ratios to investigate the seismic performance of circular CFST stub columns with different inner constructions (an internal circular steel tube, built-in partitions, vertical stiffeners, circumferential stiffeners, and a built-in reinforcement cage). The results show that built-in partitions coordinated the common deformation of the internal and external steel tubes, providing the specimen with the best seismic performance. By setting an internal circular steel tube in circular CFST stub columns, a significant confining effect on the core concrete was generated and the deformation of the external steel tube was well developed, which significantly improved the seismic performance of the specimens. Setting circumferential stiffeners had a significant confining effect on the radial deformation of the steel tube, and placing vertical stiffeners between the circumferential stiffeners had an obvious effect on preventing local buckling and improving the flexural capacity of the steel tube. The energy dissipation capacity of the specimens with different inner constructions was better when the shear-to-span ratio was smaller. A method for calculating the peak bearing capacity of such circular CFST stub columns is proposed, which can accurately predict the peak bearing capacity of the columns and provide a basis for the calculation of their application in practical engineering.

Compressive behavior of circular steel tubed high-strength concrete columns with embedded steel tubes

This paper presents an experimental study on circular steel tubed high-strength concrete columns embedded with steel tubes under axial compression. A total of 18 columns were tested in this study to investigate several parameters covering the continuity conditions for external steel tubes, diameters of internal steel tubes, thicknesses of external and internal tubes, and the number of CFRP layers. Test results show that the strength of specimens is improved to different levels due to the discontinuous external tubes and embedded internal tubes, and a significant enhancement can be achieved in terms of ductility and residual strength. Moreover, the concrete core as well as the sandwich concrete exhibits superior compressive behavior as the internal steel tube diameter increases under a constant steel ratio. Besides, when the external tubes are wrapped with CFRP jackets, more effective confinement can be provided for the inner concrete, while the effect of the position of internal steel tubes become less obvious. Finally, several existing models are selected to predict the axial stress-strain responses of the concrete in tubed columns. It is found that only two models can provide relatively reasonable predictions for the average axial stress-strain curves of concrete, but considerable overestimations of the ultimate stress are still observed for these two models, which indicates that further investigation is needed to develop a more accurate model.

A study on the compressive strengths of stiffened and unstiffened concrete-filled austenitic stainless steel tubular short columns

Concrete-filled steel tubular columns are currently used in offshore structures, through which the external steel tubes become at risk due to aggressive ocean climate. This has recently led to the introduction of the concrete-filled stainless steel tubular (CFSST) columns, which make use of stainless steel as a superior material resistant to metallic corrosion. However, concrete-filled stiffened stainless steel hollow tubular (CFSSST) columns have not received much attention, although recently they have shown great performance under compression. Accordingly, this paper is devoted to addressing the advantages of using CFSSST columns in practice. Currently, a comparison is made between CFSSST columns and CFSST columns that have the same cross-sectional areas for both the stainless steel and concrete components. A nonlinear finite element (FE) analysis is first generated, using ABAQUS software, for CFSST columns. Then, FE results for CFSST columns are compared with the available experimental results for CFSSST columns. The results show that CFSSST columns provide significantly higher strengths compared to unstiffened columns, which correlates with the significant increase in the strength of the confined concrete compared with equivalent CFSST columns. Furthermore, both strengths of the two column types are compared to EC4 predictions, the results of which appear to be suitable for CFSST columns but conservative for CFSSST columns. For the case of CFSST short columns, a modified version of the continuous strength method is suggested, where a more accurate value of the buckling factor of stainless steel plate is used when it is in contact with concrete. Additionally, a formula is proposed for the lateral confining pressure provided by the stiffened austenitic stainless steel tubes to the concrete core, and used in a suggested design model for CFSSST short columns, for the first time in the literature.