Concrete-filled Steel Tubular Research Articles

Mechanism-based unified design formula for axially loaded CFST columns with various profiles

Concrete-filled steel tubular (CFST) columns with various profiles are increasingly applied. Nevertheless, current design codes mainly focus on circle or square/rectangle profiles, and there lacks accurate and reliable design method for other profiles. Motivated by the research gap, this study developed a mechanism-based unified design formula suitable for CFST columns with various profiles. This study was based on a theoretical analysis of circle profiles, through which the relationship between confining stress and cross-section geometric/material parameters was established. The non-circle profiles were converted into equivalent circle profiles in this study, and the effects of corner radius and aspect ratio on equivalent confining stress were investigated based on the discussion of effectively and ineffectively confined areas. The loads shared by concrete and steel tubes due to variations in confining stress were analytically evaluated. Thus a mechanism-based unified design formula was established. The formula was validated using a database of 258 specimens from 21 independent studies. The average ratio of the predicted to experimental load-bearing capacity was 0.979. Additionally, the proposed model showed a similar accuracy for square/rectangular sections and other profiles. The inconsistency in the prediction results for different profiles according to Eurocode 4 or ACI 318–19 was addressed.

Dynamic Compression-Bending Performance of Concrete-Filled Steel Tubular Columns under Lateral Impact

In this paper, a finite element model of concrete-filled steel tubular (CFST) columns under compression and lateral impact is developed and validated against previous experiments. After analyzing the influence of axial compression on the impact performance of CFST columns, the effects of eccentricity, material strength, and steel ratio on the dynamic compression-bending performances of CFST columns subjected to lateral impact are discussed. The simulated results show that at different axial compression ratios, CFST columns show overall bending failure under lateral impact. The axial force ratio below 0.2 shows a positive effect on the impact resistance of CFST columns, otherwise the axial force would degrade the impact resistance of CFST columns. Eccentricity has a negative effect on the dynamic compression-bending performance of CFST columns. The increase in the concrete strength has little effect on the dynamic compression-bending performance of the CFST columns under lateral impact and eccentric compression. The increases in steel strength and steel ratio can improve the dynamic compression-bending performances of the CFST columns under lateral impact and eccentric compression. Even though the prediction formula for the dynamic compression-bending performance of CFST columns shows good fitness with the simulated results, it is modified to have sufficient strength reserves for design applications.

Simplified Method for Calculating the Bearing Capacity of Slender Concrete-Filled Steel Tubular Columns

Concrete-filled steel tubular (CFST) columns are one of the most effective reinforced concrete structures, and improving their calculation is a critical task. The purpose of this study was to develop a simplified method for calculating slender CFST columns, taking into account the effect of lateral compression. The idea of the method is to use the equation of a reinforced concrete column’s longitudinal bending, without taking into account the effect of lateral compression. To take into account the lateral effects, the cross-sectional stiffnesses are corrected based on the analysis of the stress–strain state in the cross-sectional plane using the finite element method. The developed method was implemented by the authors in the MATLAB environment. The approbation of the proposed method was carried out on experimental data for centrally compressed columns of a circular cross-section, as well as eccentrically compressed columns of a circular and square cross-section, presented in two papers. For the centrally compressed columns, we conducted a study on the influence of initial imperfections in the form of eccentricities and initial curvatures on the value of the ultimate load. For the eccentrically compressed columns of the circular and square cross-section, the area of their effective operation was determined.

Load introduction and transfer mechanism of K-type CFST circular section connections

This paper investigates the mechanism of load introduction and transfers for K-type concrete-filled steel tubular (CFST) circular section connections experimentally and numerically. Six K-type CFST connections were tested. Three-dimensional finite element (FE) models were then developed and validated against the test results, where the degradation and failure of the direct bond interaction were considered explicitly. The longitudinal strain distribution along the circumferential direction of chord-wall demonstrated that the non-uniform force transfer in the chord was caused by the one side load introduction through braces. The effects of the chord length, cross-sectional slenderness and interfacial interactions on the force transfer of tube-concrete interface were evaluated: 1) the chord length above the connecting region has a positive influence on the force transfer; 2) for the K-type CFST connections in this study, the material strength of concrete in the chord with non-compact and slender sections could not be fully utilized due to the insufficient force transfer; 3) the direct shear interaction dominated the force-transferring process from chord-wall to concrete for the compact section chord with reinforcing plates. Furthermore, the test and FE result confirmed that the load introduction length of the CFST chord with braces included the chord above the connecting region and the full connecting region. In addition, the equation of effective load introduction length for the CFST chord of the K-type connections was proposed.

A comparative experimental study on blind‐bolted beam connections to square steel tubular columns filled with polyurethane foam or concrete

AbstractIn the last two decades blind‐bolted connections have been developed to allow bolting of open section beams (e.g, I‐beam) to steel hollow section (SHS) columns where access from all sides of the column is not possible. Filling the tubular column with concrete in the vicinity of the beam‐to‐column connection has been shown to improve the anchoring mechanism of the blind‐bolted connection thereby increasing the strength and stiffness of the connection and reducing the column face deformation, however it adversely affects the joint ductility. This study introduces polyurethane foam filled steel tubular columns in combination with a new anchoring arrangement employing hollo‐bolts to develop a novel connection between the SHS columns and open section beams that provides a better combination of stiffness, strength and ductility. To investigate the behaviour of the proposed arrangement, monotonic pull‐out tests of blind bolted T‐stub connections anchored to foam‐filled steel tubular (FFT) and concrete‐filled steel tubular (CFT) columns using holloBolts were conducted. Benchmark tests on unfilled SHS columns were also conducted for comparison. The FFT in conjunction with the proposed anchoring mechanism was shown to provide a good stiffness as the CFT columns, and almost similar ductility to that of the unfilled steel tube blind‐bolted connection. Hence it is concluded that employing foam instead of concrete as an infill leads to favourable combination of strength and ductility, thus rendering FFT an attractive light alternative to CFT sections when ductility of the joint is an important design consideration.

Study on headed shear studs influencing the interfacial bond behaviour in CFST columns

AbstractThe performance of concrete‐filled steel tubular (CFST) columns with headed shear studs under push‐out loading has been investigated. The primary objective of the research programme was to investigate the bond behaviour between the steel tube and the infill concrete core of the square CFST column. A numerical investigation has been carried out using the commercially available finite element software. For the simulated numerical models, they have been initially validated with existing experimental results. The validations were made in terms of failure modes, bond strength‐slip curves and the initial stiffness. The validated models were further used to carryout parametric studies. The parameters studied include, vertical spacing of shear studs, horizontal spacing of shear studs, and CFST cross‐section slenderness. From the investigation, it is observed that with reduced vertical spacing of studs, the bond strength increases significantly. Specimens having horizontal spacing of studs as width/4 has more bond strength than specimen with horizontal spacing of width/2 and 0. For specimens with reduced vertical and horizontal spacing of studs though leads to higher bond stress, but also drops suddenly due to loss of contact between the stud and concrete. Therefore, 70% of the peak stress value may be considered as the achieved bond stress, as this stress value sustains for higher slip values. Lastly, as the column cross‐section slenderness reduces, there is a trend in increase in interfacial bond strength.

Robustness analysis for composite multi‐storey modular buildings subjected to wind loads.

AbstractIn this research, an attempt has been made to evaluate the structural response of multi‐storey composite modular buildings when subjected to lateral wind loads. Finite element (FE) models were developed in Abaqus for a general 10 storey composite modular building, with and without the consideration of lateral load resisting mechanisms of composite shear walls. Concrete filled steel tubular (CFST) columns and composite shear walls were proposed in place of conventional elements to improve the structural resistance, modularity, and construction timeframe. It was observed that under the effect of wind loads, the forces between the modules are transferred primarily by horizontal axial forces. When the effect of sudden module removal is considered along with the wind, the vertical shear forces in the horizontal inter‐module connection increase and the connection becomes critical in shear. Even though the probability of occurrence of critical wind loads and a sudden module loss is rare, it was concluded that the wind forces do not influence the overall stability of the structure in the event of progressive collapse by sudden module removal if an effective lateral load resisting mechanism has been designed.

Burnout resistance of concrete‐filled steel tubular (CFST) columns under realistic fire conditions

AbstractIn this paper, we investigate the structural behaviour of concrete‐filled steel tubular (CFST) columns under various fires. Our contribution is based on two fire scenarios in modelled FDS with subsequent 3D FE numerical analysis (Abaqus), where material properties are defined for heating and cooling using user subroutines. Our goal is to examine the burnout resistance of CFST columns under different realistic fire scenarios involving varying number of burning cars.The loss of compressive strength of concrete due to high temperature is irreversible. Hence, the burnout resistance is affected by the field of the highest temperatures reached inside the column. Moreover, the pre‐existing load affects the deformation of the column during heating and influences the post‐fire behaviour. The analysis is performed in several steps: first, results from the CFD models are extracted and appropriately mapped, then the heat transfer analysis between fire and solid is performed, and the resulting temperature history of the CFST column is calculated. Subsequently, a mechanical analysis is performed, where (1) the column is loaded, (2) the analysis of the column's structural fire behaviour is carried out, (3) if the column sur‐vives burnout, then in the last step, it is loaded until failure.

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.

Influence of anchored orbicular rings on the performance of sandwich concrete‐filled double‐skin steel tube columns

AbstractSince recent years, double‐skin tube column sections formed from two parallel, concentrically spaced steel casings, with the sandwiched area between the casings packed with concrete infill, have been employed extensively. The cross‐sectional shapes of the inner and outer casings might be the same or different, such as round, square, octagonal, etc. These concrete‐filled double‐skin steel tubular (CFDST) columns act similarly to concrete‐filled steel tubular (CFST) sections but have less self‐weight and better stiffness. The present study evaluated the response of CFDST column stiffened with orbicular rings under axial compression. It consists of inner and outer steel tubes stiffened by orbicular rings near both ends of columns. The orbicular rings were provided at different locations as L/3, L/4, and L/5, and a series of tests were conducted to obtain the performance of CFDST columns stiffened with orbicular rings with regard to the maximum load‐carrying capacity, load–strain behavior, and load‐deformation behavior. Furthermore, a parametric study was conducted by varying the diameter to thickness ratio, hollowness ratio, the thickness of orbicular rings, yield strength, and location of orbicular rings. The results showed that CFDST column stiffened with orbicular rings have higher ultimate load capacity than unstiffened CFDST. Stiffened CFDST with orbicular rings showed more ductile response than CFDST column and helps to improve the outer steel tube's restraint on sandwiched concrete. In order to estimate the load‐bearing capacity of CFDST columns stiffened with orbicular rings under axial compression, an empirical equation was proposed. The experimental results and findings of the suggested equations were found to be in good agreement.

Parametric study of steel-beam to flat-rectangular CFST column connections with small-size diaphragms

This paper studies the influence of various design parameters on the hysteretic behavior of beam-to-column connections between steel beams and flat rectangular concrete filled steel tubular (CFST) columns with small-size external diaphragms. Finite element models for the corner and exterior connections in a frame structure are developed and validated using test results. The influence of the parameters including the axial compression ratio, the thickness and flatness of the rectangular steel tube, and the size of the diaphragm on the hysteresis, load capacities, ductility, and energy dissipation capacities of the connections are particularly evaluated. Based on the evaluation, recommendations on the optimum design of the parameters are provided to ensure reliable hysteretic behavior of the connections for the seismic design of steel structures. This study provides the knowledge for the use of small-size external diaphragms beyond the limitation of existing design specifications for beam-to-column connections, which is meaningful to best exploit the unique properties of flat rectangular CFST columns and widen their applications in residential buildings.

Numerical study on the anti-impact performance of novel concrete-filled steel tubular joint with corrugated web and cover plates

To improve the impact resistance of the traditional concrete-filled steel tubular (CFST) bolt-welded hybrid joint, a novel connection configuration with the corrugated web and the cover plate was proposed in this study. The failure modes and bearing mechanism of the novel connection under vertical dynamic impact load were studied in detail based on the finite element model validated against the previous test results. The falling-debris impact on top of the joint column can be simplified as the drop hammer of the non-deformed discrete shell rigid body. The effect of the design parameters including the dimensions of the cover plate and corrugated web was analyzed numerically. Results show that the existence of the cover plate increases the bending capacity of the steel beam and delays the fracture of the connection. The fracture and local buckling of the web are delayed due to the folding effect of the corrugated web. The energy consumption of the joint with the cover plate and the corrugated web is 20.2%–52.8% higher than that of the traditional joint, in which the corrugated web and bottom flange with cover plate contribute 6.6%–8.3% and 16.3%–32.2% respectively. The time history curves of the internal force of the joints are mainly consisted of the beam mechanism stage, the catenary mechanism stage and the failure stage. Through the parameter analysis, it is suggested that the long diameter of the bolt hole is 0.08–0.09 times of the beam height and the thickness of the cover plate is 0.42–0.50 times of the flange thickness to reach the optimum impact resistance. Furthermore, a simplified calculation method for the impact resistance considering the effect of the corrugated web and the cover plate is proposed. Impact energy can be absorbed by the bending effect and the catenary effect of the joint. The relative errors of the predicted impact resistance and energy absorption by theoretical formulas are within +10% compared with the experimental and numerical results.

A REVIEW ON CONCRETE FILLED DOUBLE SKIN STEEL TUBULAR COLUMNS

Concrete-filled double-skin steel tubular (CFDST) Columns considered as a new type of Concrete Filled Steel Tubular (CFST) Columns. It consists of an inner and outer steel tube with the annulus between the skins filled with concrete. The concrete is properly compacted and filled in between the steel surfaces. CFDST are high performance composite columns that are increasingly being used in bridges and high-rise buildings as well as to reinforce CFST columns. CFDST has many advantages over CFST such as improved stability, section modulus, lighter weight and better damping characteristics. The inner tube increases their strength and ductility as compared to CFST. The hollow steel tubes can be chosen with any type of cross section, commonly circular and square tubes are preferred.

Modal analysis of concrete filled steel tubular structure under lateral impact loading

In this paper, deformation mode and the large deflection of the concrete filled steel tubular (CFST) structural member with fixed ends under lateral impact loading was studied by rigid-plastic modal approximation method and finite element analysis by ABAQUS. Using the yield criterion and beam-on-foundation model, the modal analytical solution for plastic behavior of CFST member under lateral impact is obtained considering the interaction of bending and axial stretching. The rigid-plastic modal solution captures the FE results and the deformation mode is accordance with the available experimental results. It shows that lateral deformation of CFST member can be effective predicted by the modal solution consisting of geometric and physical parameters variations. The influence of steel tube thickness, cross-section diameter, effective length, strength and density of concrete and steel, and initial impulsive are discussed in detail. The results show that deflection of the CFST member increase with the increase of the length to diameter ratio, initial impulse and smaller thickness to diameter ratio. Finally, the relationship between lateral deformation and geometric variations are established by optimal sets of modal solution.

Experimental studies on the effect of nano silica modified novel concrete CFST columns

Under axial compression, the present article analyses and compares the behaviour of Plain Concrete filled and Nano Silica modified concrete filled with light gauge steel rectangular columns. In 18 work samples, plain and nano silica-modified concrete was tested on light gauge steel sections. Investigations were conducted into several essential variables, including the geometry of the cross-section, the tubular thickness of light gauge steel, the depth-to-thickness ratio, and the type of concrete infill used. From the experimental results, effects of flat width-to-thickness ratio (w/t), axial load-end shortening, axial load-deflection, axial load-strain characteristics, and failure modes have been studied under the linear stage. To evaluate their confinement effects, this study compared plain and silica-modified nano concrete tube columns with light gauge steel hollow hollow hollow columns. The experimental results are compared using BS5400, EC4, and ACI codes to determine the ultimate sectional capacity. Test outcomes showed that tested nano-silica modified concrete filled steel tubular (CFST) columns exhibited higher strength and outstanding ductility than the plain CFST and hollow light gauge steel columns. The nano silica-modified concrete in-filled columns showed a good yield plateau under the non-linear stage compared with the other columns.

Structural Performance of Nanosilica-Blended Square and Circular CFST Stub Columns

The study focuses on the structural behavior of concrete-filled steel tubular (CFST) stub columns with conventional cement concrete and nano-silica blended (doses of 4%, 6%, 8%, and 10% nano-silica by weight) concrete core infill of M25 grade under a pure axial compressive load. The strength and deformation capacities, as well as failure modes, of the CFST columns were assessed with reference to steel hollow and reinforced concrete (RC) stub columns. At optimum replacement of cement with nano-silica, the increase in strength from filling the hollow steel columns is about 1.6 and 1.5 times for square and circular sections, respectively. The addition of 6% nano-silica to the cement concrete core of CFST columns showed 2.55 and 1.19 times more strength as compared to RC stub columns with square and circular sections, respectively. As a result, adding nano-silica to the concrete mix increases the core’s axial compressive load carrying capacity and strengthens the nano-silica blended CFST stub column. The addition of the optimum doses of nano-silica increases the higher rate of hydration and rapid formation of hydrated products in the cementitious matrix due to its higher specific surface area. Due to its higher specific surface area, adding the right amount of nano-silica speeds up the formation of hydrated products in the cementitious matrix and makes the rate of hydration faster.

Seismic performance of concrete-encased hexagonal CFST column base: Design method

This paper presents the parametric analysis and design method for the concrete-encased hexagonal concrete-filled steel tubular (CFST) column base under combined compression and biaxial bending. The corresponding experimental and numerical investigations on the seismic performance of the concrete-encased hexagonal CFST column base under combined compression and biaxial bending have been presented in the companion paper (Yang et al., 2023). The finite element model established in the companion paper is used to study the influence of key parameters on the load bearing capacity and the distribution of internal force. These parameters include those of the CFST column, the outer RC, the plate-anchor component, the axial load ratio and shear connectors. A total of six failure modes are identified, including three types of flexural failure and three types of shear failure. The ultimate capacity and ductility of the column base with flexural failure at the bottom or top of the plate are higher than that of other four failure modes, which are determined as the reasonable failure modes for practical application in the design and analysis of concrete-encased column bases. A novel design approach for the concrete-encased CFST column base is introduced. A simplified method is proposed to predict the flexural resistance of the concrete-encased CFST column base, following up with the shear strength calculation of outer RC to determine the stirrup arrangement. Some constructional recommendations are also proposed.

Seismic performance of concrete-encased hexagonal CFST column base: Experimental and numerical analysis

This paper presents the seismic behaviour of concrete-encased hexagonal concrete-filled steel tubular (CFST) column base under combined compression and biaxial bending. A total of 12 column base specimens are tested in terms of failure modes, load-displacement relationship, strain distribution, strength and stiffness degradation and energy dissipation capacity. The main parameters include the height of the outer reinforced concrete component, the loading angle of lateral load, the axial load ratio, the cross section of the CFST column and the base type. A finite element model is established, validated, and used to study the distribution of internal force. The effects of some key parameters on the internal force distribution of column base are studied, including the second-order torsion, the loading angle, the loading path of the lateral load and the sustained load. Three types of flexural failure modes are identified in both experimental and numerical investigations. The outer RC component as well as the axial load ratio have a significant influence on the failure mode of the column base. The concrete-encased column base tends to fail at the top section of the base plate with a higher axial load level. The outer reinforced concrete not only resists external load with the CFST column and plate-anchor component together, but also improves the strength and ductility of the CFST column.

Numerical Study on the Various Profile Sections of Concrete Filled Steel Tubular Columns Under Compression

The axial load-carrying capacity for a wide range of short concrete-filled steel tubular (CFST) members having different section profiles is evaluated in the presented work. A numerical study has been carried out through Finite-Element based demonstration and it has been accomplished in the ABAQUS package for relevancy of analytically predicted axial load carrying capacity by unified formula as suggested by Yu M. et al. (2010). To validate the results from the unified formula and the experimentally available literature, finite element-based models for hollow and solid sections of CFST columns with circular, octagonal, and square section profiles have been generated. A total of 31 hollow and 24 solid circular columns, 9 hollow, and 9 solid octagonal columns, and in the last 9 hollows and 38 solid square CFST columns are examined for the persistence of the results. After evaluation of obtained results from the modeling existing results are validated, and it is found that the proposed unified formula predicts satisfactory results when compared with the result of established models. Further, it is concluded that displacement in the direction of applied load is not uniform throughout the length of CFST columns thereby using the ring confinement technique for the region of applied force may be reasonable.

Investigation of effective parameters on seismic performance of steel frames with CFST columns

The seismic performance of steel frames with composite columns is evaluated by the seismic performance factors. Despite the essentiality of appropriate coefficient values, there are limitations in the building code regarding seismic performance factors of composite structures. These values have been presented in the regulations based on engineering judgment and comparison with similar steel and concrete structures. However, limited research exists on the determination of seismic performance factors for composite structures, where these coefficients are affected by numerous parameters. This article aims to cover the existing gaps and examine the parameters that have received inadequate attention in regulations and articles, despite their significance in the seismic design of composite structures. The present study conducts a nonlinear static pushover analysis of steel structures with concrete-filled tubular steel columns (CFST) to investigate the influential parameters in seismic performance factors. The variables studied in this research encompass the height of the structure, soil type, lateral force-resisting system, and structural irregularity. The findings demonstrate that changes in structural height influence the value of the response modification factor in all examined soil types for IMFs and SMFs. Also, the response modification factors provided in ASCE 7 prove to be conservative for the studied lateral force-resisting systems excluding SMFs in composite structures. In addition, the seismic performance of the studied models has dropped due to the geometric irregularity of the plan and the irregularity of non-parallel systems. Besides, the response modification factor values for composite structures surpass those specified for similar steel structures, while Eurocode 8 and ASCE 7 consider these values to be equal and similar to steel structures, respectively.