Thin-walled Concrete-filled Steel Tubes Research Articles

Vertical diaphragms for moment connection of thin-walled CFT column to steel beam

A vertical diaphragm scheme is proposed to connect the concrete-filled thin-walled steel tube (thin-walled CFT) column and steel beam, as an alternative to horizontal diaphragm connections: dual vertical diaphragms are installed inside the steel tube, considering poor out-of-plane resistance of the thin tube wall. For verification of the moment connection, two experimental programs were carried out. In the first experiment, an ultimate tension test was conducted for flange-to-column connections, to identify the fundamental behavior of the proposed connection (i.e., strength, failure mode, and out-of-plane deformation). The design parameters, such as the thickness, number, and continuity of the vertical diaphragms, were considered. All the test specimens exceeded the design strengths based on a yield line model (i.e., the tensile capacity-to-demand ratios ranged Tu/TYL = 1.11–1.31), without early fracture of welded joints. In particular, at the design strengths, the tube out-of-plane deformations were more limited with the heavier diaphragms. In the second experiment, beam-to-column connections using prototype I-section steel were tested under cyclic flexure, to verify the joint flexural strength and rigidity as well as the seismic performance. The beam-to-column connections failed by the beam flexural mechanism, whereas damage was marginal at the joint wall (i.e., the flexural capacity-to-demand ratios ranged Mu/Mj = 0.87–1.09). Overall, the dual diaphragms were effective in achieving rigid (full-strength) connections with satisfactory plastic rotation of the beam. On the basis of the test results, design considerations were recommended for the vertical diaphragm connections.

Experimental investigation on residual capacity of steel-reinforced concrete-filled thin-walled steel tubular columns subjected to combined loading and temperature

Steel-reinforced concrete-filled thin-walled steel tube (SRCFST) is gaining popularity in construction, primarily due to its superior structural performance. However, there is still a scarcity of studies investigating the residual capacity of SRCFST columns subjected to a full-range fire. A comprehensive research was conducted in this, involving both experimental testing and numerical simulation, to investigate the residual capacity of SRCFST columns. Six SRCFST columns were tested under combined loading and temperature to achieve the temperature distribution, failure modes, residual strength, and structural response within the cross-section. The developed finite element (FE) model was used to validate the test results. Three different paths, including after exposure to fire and without initial load, the full-range fire, and the actual path in the current test, were considered toevaluate the influence of different time-load-temperature paths on the mechanical properties of SRCFST columns. Results reveal that the region comprised of profiled steel and surrounded concrete forms a composite constrained region, with little loss of strength and stiffness occurring in the core region. The initial load level and heating time ratio were found to have a significant negative influence on the residual load-carrying capacity and ductility of SRCFST columns. In contrast, circular SRCFST columns are more resistant to the effects of fire than square SRCFST columns. Finally, the existing design methods were used and extended to evaluate the residual load-carrying capacity of SRCFST columns subjected to a complete temperature-load-time process.

Eccentric Compression Behaviors of Self-Compacting Concrete-Filled Thin-Walled Steel Tube Columns.

For the sake of solving sustainability issues and analyzing the complicated service force states, eccentric compression experiments on self-compacting concrete-filled thin-walled medium-length steel tube columns with a circular cross-section were carried out in the present study. Thereafter, the influence of the eccentric ratios and the wall thickness factors on the mechanical behavior and failure characteristics of both the eccentrically loaded and axially loaded columns was comprehensively analyzed. Finally, prediction formulas for the ultimate load of the columns under eccentric compression were proposed, and a comprehensive comparison of the ultimate loads between the predicted values and experimental values was also conducted. The results indicated that the typical failure characteristics of the eccentrically loaded columns presented lateral deflection together with buckling, while the axially compressed columns displayed expansion and rupture at local positions. Moreover, the ultimate loads of the eccentrically loaded columns decreased by 43.0% and 34.5% in comparison to the columns under axial compression, with the wall thickness factor decreasing from 116.7 to 46.7, respectively. Meanwhile, the ratios of the ultimate loads calculated using design codes to the tested values were in the range of 0.70~0.90, which demonstrated that the design codes could predict the ultimate loads conservatively. Additionally, the ratios of the ultimate loads calculated using the proposed formulas to the tested values were within the range of 0.99~1.08, implying that the proposed formulas were more accurate than the design codes. At the same time, the initial stiffness of the columns under eccentric compression was correspondingly lower than that of the columns undergoing axial compression. The lateral deflections along the height of the columns were almost symmetrical at different loading levels. This study could provide a meaningful approach for designing columns and facilitate their application in civil industry.

Analysis of ultimate seismic performance of thin-walled concrete-filled steel tube bridge piers under dynamic load

This study entailed the use of a cyclic loading test on a thin-walled concrete-filled steel tube (CFST) bridge pier to investigate its ultimate seismic performance. Additionally, a three-dimensional solid-shell finite element (FE) model of thin-walled CFST piers was established using ABAQUS. Cracks were introduced into in-filled concrete, and ductile damage was imposed on the steel tubes, in order to accurately reproduce the seismic performance degradation and post-earthquake residual displacement variation in thin-walled CFST bridge piers. The FE model was validated using pseudo-static and pseudo-dynamic loading tests performed on thin-walled CFST bridge piers. The analysis indicates that the FE model with concrete cracks can reflect the pinching effect of the hysteresis curve; the ductile damage can reflect the degradation of the bearing capacity during large plastic deformation under cyclic loading and the residual displacement under unidirectional and bidirectional dynamic loading. The seismic response of the full in-filled thin-walled CFST bridge pier was the least under the same intensity seismic wave, the in-filled concrete could improve ultimate seismic capacity. Finally, this study established a formula for the relationship between the residual and maximum displacements at the top of thin-walled CFST bridge piers under bidirectional seismic waves.

Experimental and analytical study of thin-walled stirrup-confined CFST piers under pseudo-static loading

A pseudo-static test study was conducted to investigate the seismic performance of stirrup-confined square concrete-filled steel tube (CFST) bridge piers. Furthermore, a refined finite element model (FEM) that considers the cracks in concrete and the ductile damage of steel was established. Based on this, a refined restoring-force model for stirrup-confined square CFST bridge piers that takes into consideration the pinching effect coefficient was proposed. The research results show that, compared with traditional square CFST bridge piers, stirrup-confined square CFST bridge piers exhibit a superior seismic performance, as their yield load, horizontal peak load, cumulative energy dissipation, and elastic stiffness are higher by 49.3%, 42.8%, 24.1%, and 15.1%, respectively. The axial compression ratio, yield strength of the steel tube, compressive strength of concrete, aspect ratio, and width–thickness ratio have a significant effect on the seismic performance of stirrup-confined square CFST bridge piers, while the yield strength of the stirrup has little influence on their seismic performance. The proposed refined restoring-force model for stirrup-confined square CFST bridge piers that takes into consideration the pinch effect coefficient has high accuracy and is in good agreement with the FEM results.

Experimental Study on Self-Compacting Concrete-Filled Thin-Walled Steel Tube Columns

Concrete-filled steel tubes present excellent structural and constructional performances because they ideally combine the advantage of concrete and steel tube. Thus, they are widely used in civil infrastructures. However, they inevitably suffer from either hard compacting or high costs. Thus, convenient and rapid construction for compacting concrete and cost saving are the urgent challenges for concrete-filled steel tubes. Therefore, this study investigates filling a thin-walled steel tube with self-compacting concrete to solve the challenges presented by traditional concrete-filled steel tube columns, such as poor compacting performance and high costs. This experimental study tests self-compacting concrete-filled thin-walled steel tube (SCCFTST) columns under concentric compression from loading to failure. Effects of wall thickness of the thin-walled steel tube on the failure modes, load-deformation behaviors, and the ultimate loads of the SCCFTST columns are comprehensively investigated. The ultimate loads between experiments and their calculated values in terms of design codes are also compared. The results suggest that buckling on thin-walled steel tube surface is the typical failure mode. The amount of local buckling increases with decreasing wall thickness, and the decreasing rate of the load-deformation curves in the descending branch decreases by increasing the wall thickness, as well as the ultimate load increasing with increases wall thickness. The ACI and CECS are the most conservative and accurate design codes, respectively, for predicting the ultimate load. Therefore, the SCCFTST columns can be used as structural components in civil infrastructures and their peak loads can be calculated using design codes for conventional concrete-filled steel tube columns. However, modification measures must be taken while predicting the ultimate loads of the SCCFTST columns by design codes. The experimental results of this paper can contribute towards the application of SCCFTST columns in practice.

Assessment on seismic performance of circular concrete-filled thin-walled steel tube columns

Concrete-filled steel tubular (CFST) with circular section is a very efficient composite component bearing vertical loads, which takes advantage of the synergy of steel tube and concrete to fully develop the strengths of both materials. To promote the application of circular CFST columns in multi-story buildings or medium-span bridge piers with improved economic efficiency, circular CFSTs using thin-walled steel tubes were studied in this paper. 8 specimens were designed and tested to assess their seismic performance under constant axial compression and cyclic lateral loads. The key test parameters include axial compression ratio, diameter-to-thickness ratio of steel tube, as well as stiffening scheme. The effectiveness of two steel plate stiffeners (longitudinal stiffener, binding stiffener) at the potential plastic hinges of thin-walled circular CFST columns was verified and discussed. Although all specimens failed in a manner characterized by significant local buckling of steel tubes, they still performed a relatively ductile behavior with the ductility coefficients generally exceeding 4.0, and the ultimate drift ratios exceeding 1/40. The steel plate stiffeners did not achieve the expected enhancement in strength and ductility, due to the premature fracture in the steel tube which may be caused by the damage in the welding heat-affected zone. Based on the test results, a numerical model and a simplified hysteretic model were established to predict the seismic responses of the thin-walled circular CFST columns.

Behavior of Corroded Thin-Walled Concrete-Filled Steel Tubular Stub Columns

The behavior of two series of corroded thin-walled concrete-filled steel tubes (CFSTs) was investigated in this paper. One series, called the CT series, consists of fourteen CFST specimens corroded by NaCl solution, while the other, called the AT series, consists of six test pieces corroded in the atmosphere. Outer surfaces of corroded steel tubes were scanned using a 3D scanner machine, and a complete surface profile of each specimen was obtained for corrosion evaluation using different methods in this study. Entire test pieces were compressed to failure, and their behaviors under loading were recorded. It is shown that corrosion has a fractal feature, and it has a significant effect on the ductility of specimens. In addition, FEA models of corroded CFST specimens were established based on the 3D scanning profile results using the ABAQUS program and verified against column test results.

Behaviors of FRP confined rectangular concrete-filled thin-walled steel tubular stub columns using high-strength materials under axial load

Rectangular concrete-filled thin-walled steel tube (CFT) columns using high-strength steel and concrete are susceptible to local buckling, which will weaken the constraint effect and reduce the advantages of high-strength materials. Fiber reinforced polymer (FRP) has favorable mechanical properties, which can be used to delay or even prevent the outward buckling of steel tube. This paper presents the experimental and theoretical study on axial compressive behavior of FRP confined concrete-filled thin-walled steel tube (FRP-CFT) stub columns with high-strength materials. The test variables included the aspect ratio of section, concrete strength and the layers of FRP, etc. Results show that the confinement of FRP delays the local buckling of steel tubes and increases the restraint strength of core concrete. The ductility of high-strength concrete is also improved. Based on the superposition theory and the effective width theory, this paper proposed the design approach to calculate the ultimate compressive resistance, considering the effect of local buckling of thin-walled steel tube and the confinement of the FRP. The design approach was verified with experimental results and can be used in practice.

Seismic performance and damage evaluation of spiral ribbed thin-walled concrete filled and encased steel tube composite columns

As a new type of composite components, thin-walled concrete-filled steel tubes (CFSTs) have some advantages in terms of economy and processing. After the steel tube wall thins, the local buckling performance decreases and the stiffness decreases, which is not conducive to the structural safety. In this paper, combining the advantages of traditional spiral hoops and a stiffener, a new constraint in the form of a screw stiffener was proposed. On this basis, the composite member of thin-walled CFSTs with spiral ribs was put forward. The horizontal hysteretic test was carried out for the new composite column, and the failure mode, hysteretic characteristics, ductility, and energy dissipation capacity were obtained. The results showed that, compared with the traditional form, the seismic bearing capacity of the new composite column was increased by 11% and the ductility was increased by 45%. The deformation capacity was significantly improved. Based on experimental research, the seismic bearing capacity calculation and seismic damage assessment of the composite column were studied, and the practical calculation and the two-parameter damage assessment method considering the interaction between deformation and energy were proposed, which were in good agreement with the test results. This study can provide a technical basis for its engineering application. The composite column has good seismic performance, durability, and fire resistance, and thus has potential for application in practice.

Seismic Behavior and Design of Concrete-Filled Thin-Walled Steel Tube Column-to-Foundation Connections

AbstractThin-walled steel tubes are highly recommended to be used in concrete-filled steel tube (CFST) columns due to their economic benefits. However, the inherent difficulty of connections betwee...

Numerical evaluation of compressive failure behavior of thin-walled concrete-filled steel tubes using coupled RBSM and Shell FEM

Numerical evaluation of compressive failure behavior of thin-walled concrete-filled steel tubes using coupled RBSM and Shell FEM

Axial Compression Performance of Square Thin Walled Concrete-Filled Steel Tube Stub Columns with Reinforcement Stiffener under Constant High-Temperature

This study investigated the axial compressive performance of six thin-walled concrete-filled steel tube (CFST) square column specimens with steel bar stiffeners and two non-stiffened specimens at constant temperatures of 20 °C, 100 °C, 200 °C, 400 °C, 600 °C and 800 °C. The mechanical properties of the specimens at different temperatures were analyzed in terms of the ultimate bearing capacity, failure mode, and load–displacement curve. The experiment results show that at high temperature, even though the mechanical properties of the specimens declined, leading to a decrease of the ultimate bearing capacity, the ductility and deformation capacity of the specimens improved inversely. Based on finite element software ABAQUS, numerical models were developed to calculate both temperature and mechanical fields, the results of which were in good agreement with experimental results. Then, the stress mechanism of eight specimens was analyzed using established numerical models. The analysis results show that with the increase of temperature, the longitudinal stress gradient of the concrete in the specimen column increases while the stress value decreases. The lateral restraint of the stiffeners is capable of restraining the steel outer buckling and enhancing the restraint effect on the concrete.

Research on Compression Behavior of Square Thin-Walled CFST Columns with Steel-Bar Stiffeners

Local buckling in steel tubes was observed to be capable of reducing the ultimate loads of thin-walled concrete-filled steel-tube (CFST) columns under axial compression. To strengthen the steel tubes, steel bars were proposed in this paper to be used as stiffeners fixed onto the tubes. Static-loading tests were conducted to study the compression behavior of square thin-walled CFST columns with steel bar stiffeners placed inside or outside the tube. The effect and feasibility of steel bar stiffeners were studied through the analysis of failure mode, load–displacement relationship, ultimate load, ductility, and local buckling. Different setting methods of steel bars were compared as well. The results showed that steel-bar stiffeners proposed in this paper can be effective in delaying local buckling as well as increasing the bearing capacity of the columns, but will decrease the ductility of the columns. In order to obtain a higher bearing capacity of columns, steel bars with low stiffness should be placed inside and steel bars with high stiffness should be placed outside of the steel tubes. The study is helpful in providing reference to the popularization and application of this new structural measure to avoid or delay the local buckling of thin-walled CFST columns.

Pull-Out Performance of T-Stub End Plate Connected To Concrete Filled Thin-Walled Steel Tube (CFTST) using Lindapter Hollo-Bolts

The application of concrete filled steel tubes (CFSTs) as composite members has widely been used around the world and is becoming popular day by day for structural application especially in earthquake regions. This paper indicates that an experimental study was conducted to comprehend the behaviour of T-stub end plates connected to concrete filled thin-walled steel tube (CFTST) with different types of bolts and are subjected to pullout load. The bolts used are normal type bolt M20 grade 8.8 and Lindapter Hollo-bolt HB16 and HB20. A series of 10 mm thick T-stub end plates were fastened to 2 mm CFTST of 200 mm x 200 mm in cross-section. All of the specimens were subjected to monotonic pull-out load until failure. Based on test results, the Lidapter Hollo-bolts showed better performance compare to normal bolts. The highest ultimate limit load for T-stub end plate fasten with Lindapter Hollo-bolt is four times higher than with normal bolt although all end plates show similar behaviour and failure mode patterns. It can be concluded that T-stub end plate with Lindapter Hollo-bolt shows a better performance in the service limit and ultimate limit states according to the regulations in the design codes.

Pull-out performance of T-stub end plate connected to concrete filled thin-walled steel tube (CFTST) using lindapter hollo-bolts

The application of concrete filled steel tubes (CFSTs) as composite members has widely been used around the world and is becoming popular day by day for structural application especially in earthquake regions. This paper indicates that an experimental study was conducted to comprehend the behaviour of T-stub end plates connected to concrete filled thin-walled steel tube (CFTST) with different types of bolts and are subjected to pullout load. The bolts used are normal type bolt M20 grade 8.8 and Lindapter Hollo-bolt HB16 and HB20. A series of 10 mm thick T-stub end plates were fastened to 2 mm CFTST of 200 mm x 200 mm in cross-section. All of the specimens were subjected to monotonic pull-out load until failure. Based on test results, the Lidapter Hollo-bolts showed better performance compare to normal bolts. The highest ultimate limit load for T-stub end plate fasten with Lindapter Hollo-bolt is four times higher than with normal bolt although all end plates show similar behaviour and failure mode patterns. It can be concluded that T-stub end plate with Lindapter Hollo-bolt shows a better performance in the service limit and ultimate limit states according to the regulations in the design codes.

Pull-Out Performance of T-Stub End Plate Connected To Concrete Filled Thin-Walled Steel Tube (CFTST) using Lindapter Hollo-Bolts

The application of concrete filled steel tubes (CFSTs) as composite members has widely been used around the world and is becoming popular day by day for structural application especially in earthquake regions. This paper indicates that an experimental study was conducted to comprehend the behaviour of T-stub end plates connected to concrete filled thin-walled steel tube (CFTST) with different types of bolts and are subjected to pull-out load. The bolts used are normal type bolt M20 grade 8.8 and Lindapter Hollo-bolt HB16 and HB20. A series of 10 mm thick T-stub end plates were fastened to 2 mm CFTST of 200 mm x 200 mm in cross-section. All of the specimens were subjected to monotonic pull-out load until failure. Based on test results, the Lidapter Hollo-bolts showed better performance compare to normal bolts. The highest ultimate limit load for T-stub end plate fasten with Lindapter Hollo-bolt is four times higher than with normal bolt although all end plates show similar behaviour and failure mode patterns. It can be concluded that T-stub end plate with Lindapter Hollo-bolt shows a better performance in the service limit and ultimate limit states according to the regulations in the design codes.

Analysis and Design of Elliptical Concrete-Filled Thin-Walled Steel Stub Columns Under Axial Compression

To analyze the axial compressive performance of elliptical concrete-filled thin-walled steel tube (ECFTST) columns, finite element (FE) modeling was built up by ABAQUS software in this paper. The material nonlinearity and the complex interaction between steel tube wall and core concrete in an elliptical section were considered in this numerical analysis. Failure modes and interaction mechanism of ECFTST stub columns were also studied. Moreover, a modified equivalent circular diameter approach for ellipse feature was proposed to substitute for the constitutive relation of ECFTST columns. The prediction accuracy of this method was verified by the comparison of the FE and test results in terms of failure modes, axial compressive capacities and axial force-axial displacement relation curves. Further, the influence of diameter-to-thickness ratio, cross-section slenderness and scale effect etc. on the performance of ECFTST stub columns were estimated under axial compressive loading. The results of numerical analysis showed that the axial force-axial displacement curves of the ECFTST stub columns were obviously affected by the confinement factor. Lastly, based on the simple superposition approach and the unified theory approach, two simplified design methods were proposed to predict the axial compressive capacities of ECFTST columns.

Seismic performance of FRP-reinforced concrete-filled thin-walled steel tube considering local buckling

The objective of this study is to investigate the seismic performance of fiber-reinforced polymer-reinforced concrete-filled thin-walled steel tube (CFTST). Twelve specimens with different fiber-reinforced polymer types (glass fiber-reinforced polymer and carbon fiber-reinforced polymer) and reinforcing modes were tested under constant axially compressive load and cyclic lateral load. The failure mode and lateral load versus displacement relationship for each specimen were recorded during testing. The strength, ductility, and energy dissipation capacity were analyzed accordingly. Further, a stress–strain relationship and a restoring force model of the fiber-reinforced polymer confining steel tube with local buckling were proposed. A hysteretic model for the fiber-reinforced polymer-reinforced CFTST was developed subsequently. The results indicate that the seismic performance of fiber-reinforced polymer-reinforced CFTST can be effectively improved by optimizing the fiber-reinforced polymer type and corresponding reinforcing scheme. Carbon fiber-reinforced polymer and glass fiber-reinforced polymer are suitable materials for the confinement and bending reinforcement of the column, respectively. The modeling results show the energy imported into the column is mainly dissipated by the thin-walled steel tube. The energy dissipation proportion of the steel tube, concrete core, and longitudinal fiber-reinforced polymer are >80%, 10%–20%, and <8%, respectively. The energy dissipation value of the steel tube can be improved more than 40% after effectively restraining the local buckling.

Numerical investigation on compressive performance of CFST columns with encased built-up lattice-angles

This paper presents a numerical investigation on the compressive behaviour of thin-walled concrete-filled steel tube (CFST) columns with encased built-up latticed angles. The additional confinement provided by encased angles was considered in the finite element (FE) model, in addition to the contact between tube wall and inner concrete and the effect of tube-wall confinement on concrete model. The established FE model is verified against the existing test results that it can well predict the behaviour of reinforced composite columns under axial compression. The composite action between encased angles and concrete and the efficiency of built-up angle configurations on performance improvement are extensively analyzed through the validated FE models. The influence of three parameters, namely confinement factor (ξ), reinforcement ratio (ρsr) and slenderness ratio (λ), to the angle-contributions on lateral confining pressure and the ultimate strength are evaluated and quantified. It is found the relationship between the strengths of CFST columns and built-up member can be defined as an exponential equation. Finally the design equation based on AISC 360-10 design provisions with consideration of angle-contributions is proposed, and the comparison results show it has a desirable accuracy and satisfactory correlation with both experimental and numerical results.