Concrete-filled Steel Tube Specimens Research Articles

Behavior of stiffened concrete-filled steel tube columns infilled with nanomaterial-based concrete subjected to axial compression

The use of nanotechnology in the field of construction emerged as the utilization of nanomaterials to improve the mechanical properties of concrete. In this study, a novel form of stiffening scheme was suggested, named a catty-cornered propped concrete-filled steel tube (CFST) column. The performance of the suggested stiffened CFST column was analyzed under axial compression. The steel tube of CFST specimens was filled with normal and nanomaterial-based concrete. The three kinds of nanomaterials were utilized viz., nano-silica (NS), carbon nanotubes (CNT), and nano-titanium dioxide (NT). The results of the investigation were collected in terms of ultimate capacity, load vs strain behavior, and load vs deformation response. The ductility index (DI), secant stiffness, composite interaction, and confining effect variation were also discussed further. It was observed that the suggested stiffening scheme was able to increase the ultimate capacity of unstiffened CFST by approximately 14%. The use of nanomaterials in CFST infill concrete also resulted in an approximately 7% increase in load capacity. Further increasing the number of stiffening bars improved the ductility and stiffness of the column section. On the other hand, the inclusion of nanomaterials resulted in a decrease in the ductility index and improved the stiffness of the section. The proposed stiffening scheme resulted in better composite interaction and increased confinement. It was also concluded that the utilization of nanomaterial-based concrete as an infill in the stiffened CFST column could enhance its performance under axial compression loading.

Post-fire load-reversed push-out performance of normal and lightweight concrete-filled steel tube columns: Experiments and predictions

In this study, the push-out experiment with load reversal and up to four load half-cycles was conducted on concrete-filled steel tube (CFST) members to investigate parameters affecting the natural bond capacity (chemical adhesion, micro-locking, and macro-locking) and bond-slip curves before and after heat-induced damage. Variables under study included the exposure temperature, length-to-diameter ratio of steel tube (L/D), diameter to thickness ratio of steel tube (D/t), grade of cementitious materials, and type of concrete (pumice lightweight concrete and normal concrete). The results showed that exposure to heat often had a negative effect on chemical cohesion and friction between the steel tube and concrete core. In this regard, after exposure to 200, 400, and 600 °C, the bond capacity of the lightweight CFST specimens respectively remained relatively unchanged (negligible increase or decrease) and declined by 84 and 94% compared with that at the ambient temperature. An increase in parameter L/D due to an increase in length and a decrease in diameter led to a drop and a rise in the bond capacity, respectively. As the D/t ratio increased (due to a rise in D or a drop in t) in the lightweight CFST specimens, the bond capacity declined after exposure to all the target temperatures. In addition, lightweight concrete specimens with a smaller cement grade showed greater bond capacities compared with the corresponding columns with a higher cement grade for most exposure temperatures. Moreover, the bond capacity of the specimen with normal concrete was significantly higher than the corresponding specimen with lightweight concrete (around 8 times) for the exposure temperature of 600 °C. By comparing half-cycles 1 and 3 of CFST specimens, the share of macro-locking was obtained as 12 and 34% for 600 °C. The initial stiffness and energy absorption of the specimens with the lightweight concrete experience a significant drop after exposure to 600 °C compared with the ambient temperature; however, an inverse trend was seen for the specimens with normal concrete. Finally, two simple models with proper accuracy were proposed to predict the normalized bond capacity of the CFTS specimens with lightweight and normal concrete in terms of temperature.

Flexural behavior of circular concrete filled steel tubes with partially incorporated demolished concrete lumps

For the past decade, researchers have been experimenting with the use of Demolished Concrete Lumps (DCLs) in structural members, as it has been proven to be a promising method to recycle concrete in different field applications. Although there have been several studies on incorporating DCLs in Concrete Filled Steel Tube (CFST) members, there are no studies that evaluate its effect on the flexural performance of CFST beams. Therefore, this paper focuses on studying the flexural behavior of CFST beams with DCLs, where the DCLs are inserted at the center of the cross-section. In total, fourteen circular CFST and two circular Steel Tube (ST) specimens were tested under bending using a four-point loading setup. The specimens are categorized into two different steel tube sections, where two circular sections of D/t = 55 (C1) and D/t = 45 (C2) were considered. The CFST specimens within each steel tube category differ in DCLs particle sizes and DCLs inner occupation area. CFST beams filled completely with normal concrete were also tested as control specimens. In addition, two CFST specimens from the steel section C2 were left partially hollow at the center to study the contribution of DCLs to the overall flexural performance. The results were very promising as the flexural behavior of DCL CFST specimens was very similar to the control CFST specimens. The DCLs’ particle sizes and the inner occupation area had minimal effect on the ductility, stiffness, yielding capacity, and ultimate capacity of the CFST specimens. The maximum percentage reduction in the flexural capacity between DCL CFST and control specimens were less than 2% for both sections whereas the percentage gain was up to 7%. Furthermore, the obtained flexural capacities were compared with nominal predictions from different design codes and models. All codes underestimated the capacities of all CFST specimens, with Han's model being the most conservative, followed by EC4, BS-5400–5, and AISC-LRFD, respectively.

Effect of spontaneous-combustion coal gangue aggregate on axial performance of square concrete-filled steel tube stub columns

This work experimentally studied the effect of spontaneous-combustion coal gangue aggregate (SCGA) on the mechanical performances of the square concrete-filled steel tube (CFST) stub columns subjected to axial loading. The influences of the substitution level of SCGA and the steel tube thickness on the failure mode, axial load–displacement response, ultimate load, elastic stiffness, and ductile performance were investigated. Trial results showed that the reduction in the ultimate compressive load of the CFST stub columns at an SCGA substitution level of 50% and 100% is within 5.96% and 12.10% respectively, compared to the CFST specimens made of natural aggregate (NA). Moreover, the elastic stiffness of the columns made of SCGA declined by 4.58%–16.67% and 22.44%–24.20% and their ductility coefficient by 1.10%–14.76% and 8.04%–18.27% at an SCGA substitution level of 50% and 100% respectively. To extend the parametric range of the experiment, the finite element models were analyzed in detail for estimating the influences of the SCGA substitution level, the concrete strength, the yielding strength of the steel tube, and the diameter of the stub columns utilizing Abaqus software. The results demonstrated that influences of SCGA incorporation on ultimate compressive load of CFSTs with square section could be presented via concrete strength and confinement coefficient. The design equations described in standard GB50936 were comparable to predict the ultimate load of the square CFST stub columns according to the finite element modeling data and verified by experimental results, and could be used for the routine design of CFSTs with SCGA.

Experimental study and design of bond behavior in concrete-filled steel tubes (CFST)

Steel plate-concrete (SPC) and concrete filled steel tube (CFST) push out test specimens are conducted to study the nature of the bond behavior between the steel tube and concrete and the according contribution of each component (chemical adhesion, micro-interlocking and friction). A total of 10 SPC specimens considering lateral normal pressure applied on the concrete and 18 circular CFST specimens with reversed loading have been carried out, where the surface roughness of the steel tube and the length of the specimens have been considered as variables. The chemical adhesion contributions are obtained through the SPC specimens without normal pressures. The micro-interlocking and friction can be calculated through both the SPC and CFST specimens. The micro-interlocking coefficient concept has been proposed and determined together with the friction coefficient through SPC test specimens with normal pressures. Both the micro-interlocking and friction component contribution to the bond behavior can be determined through the steel tube hoop strain from the CFST specimens and the obtained coefficient accordingly. The bond mechanism of CFST specimen has been systematically and quantitively explained by analyzing the full-history developments of the chemical adhesion, micro-interlocking and friction components. A bond-slip constitutive model considering both the physical significance and prediction accuracy has been proposed and verified, which may promote refined numerical analysis.

Distributed detection of internal cavities in concrete-filled steel tube arch bridge elements

Internal cavities in structural elements, such as in concrete-filled steel tubes (CFSTs) of arch ribs reduce confinement, bearing capacity, and the durability of the arch bridges. Formation of internal cavities may be either materials related, which affects proper consolidation, or construction related, especially in terms of proper timing and delivery of concrete slurry to horizontal and inclined structural members. Nondestructive test methods such as ultrasonics have been employed for detection of internal cavities. Despite their success, point-by-point inspection of structural members becomes time-consuming and inefficient, especially when entire structures need to be inspected. Infrared thermography (IR) provides a more reasonable global approach for detection of anomalies. The accuracy and resolution of IR depend on the local ambient conditions affecting convection and external thermal flux prior to detection by IR. The research described herein pertains to the development of a hybrid thermographic approach by using Brillouin fiber optic sensors (BFOSs) for direct detection of thermal convection at the surfaces of steel tubes. Accurate quantitative detection of internal cavity locations and their dimensions required development of a specific machine learning-based approach through which the distributed thermographic data was converted to a series of grayscale images. A Faster Region-based Convolutional Neural Network (Faster R-CNN) based on the improved VGG-16 (IVGG-16) network architecture was utilized for this purpose. The experimental work involved design and fabrication of a distributed temperature sensing sheet (DTSS) for simultaneous transmission of thermal energy into CFST specimens and acquisition of distributed thermographic data by the BFOS. By using the proposed approach, it was possible to detect the embedded internal cavities during the experiments.

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.

Concentric and eccentric compression performance of multiple-cell cruciform CFSTs

Multiple-cell cruciform concrete-filled steel tube is a novel and efficient category of conventional cruciform concrete-filled steel tubes, mainly suitable for structures requiring high resistance and excellent seismic performance. This paper presents a test study of ten multiple-cell cruciform concrete-filled steel tube specimens, including six stub specimens and four slender specimens under compression. The experiment comprises the following key parameters: tube width-to-thickness ratio, length-to-breadth ratio, and eccentricity ratio. The influence of these factors on peak resistance, failure pattern and ductility were investigated by discussing and analyzing the failure procedure, load-displacement relationships, load-lateral deflection graphs and load-strain relationships. The test results demonstrated that: (1) local buckling was deferred, and the peak resistance was improved for specimens with smaller width-to-thickness ratio; (2) specimens with increment in length-to-breadth ratio or eccentricities had lower compression capacities; (3) the configuration of multiple-cell steel tubes improved the constraint effect significantly. Further, the finite element simulation was performed to determine the behaviors of the specimens and was validated with test results. Based on the parametric analysis, the N–M relationships were obtained considering parameters as the width-to-thickness ratio, slenderness ratio, eccentricity, steel grade and concrete strength. The concentric and eccentric compression capacity of multiple-cell cruciform concrete-filled steel tubes was predicted by current design codes. At last, the equations to calculate the compression capacity under concentric or eccentric loads were suggested, which were verified by the experimental and simulation results.

Effect of concrete wet packing density on the uni‐axial strength of manufactured sand CFST columns

AbstractRiver sand (RS) has been the most popular fine aggregate in concrete production. Recently, because of the continuing decline in supply of RS, alternative source of fine aggregates is needed. Manufactured sand (MS), which is produced by mechanical crushing of virgin rock or construction waste, is becoming more popular for replacing RS. Concrete containing MS has higher compressive strength but is more brittle. To promote the application of MS in concrete, concrete‐filled‐steel‐tube (CFST) column is advocated to restore the ductility by composite action. It is believed that the improved strength and ductility of CFST column containing MS is due to the enhanced wet packing density (WPD) of concrete, which has been researched to affect significantly the mechanical performance of concrete. Nonetheless, there was very limited research carried out on the effect of WPD on the behavior of confined concrete and CFST column. Ergo, 15 CFST column specimens with MS replacing RS partially or totally were prepared and tested under uni‐axial compression. It is evident that CFST specimens with concrete containing blended fine aggregates showed different axial load‐strain behavior to those with RS concrete (especially the maximum strength and postelastic slope). More importantly, the normalized strength (maximum strength to squash load) of CFST specimens can be correlated positively to the WPD. This article thus provides a new insight into the interdependence between the uni‐axial strength of CFST columns and the WPD of the in‐filled concrete.

Influence of parameters on the strength and behavior of concrete filled steel tube specimens subjected to axial compression and cyclic loading

The strength and behavior of Concrete Filled Steel Tube (CFST) columns subjected to uni-axial compression and cyclic loading are presented from the available literatures. Based on the experimental test results of 230 circular, 163 rectangular and 113 square specimens subjected to axial loading and 33 circular, 18 rectangular and 23 square specimens subjected to cyclic loading, the parameters affecting the strength and behavior of CFST columns are outlined. The effect of relative slenderness ratio (λ), confinement (ξ), Length to Diameter (L/D), Length to Breadth (L/B), Diameter to thickness (D/t), Breadth to thickness (B/t) on the axial capacity of columns are examined. The effect on ductility ratio (μ) due to axial load level (n), steel ratio (α) and member slenderness (ƛ) are also presented. The ultimate axial loads (Ne) of the columns are compared with the design values (Nc) predicted using various codes namely, Eurocode-4 (EC4), Australian standards (AS5100), Chinese code (DBJ13-51) and American code (AISC 360–10). Results showed that relative slenderness ratio and Length to Diameter of circular CFST specimens showed direct effect on the strength and behavior of the column. Circular sections with high steel ratio reflected high ductility compared to rectangular and square sections. Steel ratio and member slenderness show significant changes in ductility ratio as the values are increasing for all three types of sections. Axial load predictions of EC4 agrees well with the experimental test results and are conservative. While, results predicted by AS5100, DBJ13-51 and AISC 360 – 10 are unconservative.

Structural performance of geopolymer-concrete-filled steel tube members subjected to compression and bending

Materials research has shown geopolymer concrete has the potential to significantly improve the sustainability of concrete construction. In this study, the structural performance of geopolymer concrete-filled steel tube members to take advantage of the beneficial confinement from the outer steel tubes is investigated experimentally and numerically. Experiments on eleven geopolymer concrete-filled steel tube specimens with either square or circular sections subjected to compression, flexure or combined loading conditions are presented. The experimental results, including the ultimate strength, load-deformation responses and failure modes, are obtained and also used as the basis for the validation of a finite element model which is subsequently used to perform parametric studies on the structures with varying cross-sectional dimensions, steel yield strength and member slenderness ratios. The experimental and numerical results are also used to evaluate the applicability of existing design standards for geopolymer concrete-filled steel tube members under combined compression and bending. The evaluation shows that the design approaches developed for ordinary Portland concrete-filled steel tubes in existing standards provide conservative strength predictions for the structures with the geopolymer concrete infill and can therefore be directly applied for safe structural design.

Evaluation of the ultimate eccentric load of rectangular CFSTs using advanced neural network modeling

In this paper an Artificial Neural Network (ANN) model is developed for the prediction of the ultimate compressive load of rectangular Concrete Filled Steel Tube (CFST) columns, taking into account load eccentricity. To this end, an experimental database of CFST specimens from the literature has been compiled, totaling 1224 individual tests, both under concentric and under eccentric loading. Except for eccentricity, other parameters taken into consideration include the cross section width, height and thickness, the steel yield limit, the concrete strength and the column length. Both short and long specimens were evaluated. The architecture of the proposed ANN model was optimally selected, according to predefined performance metrics. The developed model was then compared against available design codes. It was found that its accuracy was significantly improved while maintaining a stable numerical behavior. The explicit equation that describes mathematically the ANN is offered in the paper, for easier implementation and evaluation purposes.

Bond behavior of lightweight concrete-filled steel tubes containing rock wool waste after exposure to high temperatures

Evaluating the concrete-steel composite action and bond behavior in structures with concrete-filled steel tube (CFT) members is an essential part of the design phase of these structures, especially for exposure to fire as a potential risk in the service life of any structure. In this research, the natural bond strength and bond stress-slip behavior of lightweight concrete-filled steel tubes (LCFTs) containing rock wool waste following thermal loading at elevated temperatures were investigated. For this purpose, 40 CFT specimens consisting of circular steel sections filled with lightweight concrete containing several volume contents of rock wool waste (0, 2.5, 5, 7.5, and 10%) were made and thermally loaded at 20, 200, 400, and 600 °C. Afterward, all the lightweight concrete-filled steel tube (LCFT) specimens were subjected to up to four loading cycles in the load-reversed push-out test. The first loading cycle was conducted to investigate the heat-induced bond strength loss of the LCFT specimens and the impact of rock wool waste on this parameter, while the remaining cycles were conducted to assess the macrolocking component (after the loss of chemical adhesion and microlocking components). The findings demonstrated that raising the temperature to 600 °C decreased the bond strength of the LCFT specimens by up to 97% relative to that at the room temperature, while the presence of rock wool relatively compensated for this severe loss. The incorporation of rock wool into the lightweight concrete mix increased the bond strength values of the LCFT specimens after thermal loading at 200, 400, and 600 °C by up to 20.2, 18.2, and 7.2%, respectively, relative to that of the reference specimen (lacking rock wool). In the macrolocking assessment, the first cycle was compared with the third one, and the second cycle was compared with the fourth one. It was observed that at the room temperature, the bond strength of the LCFT specimens in the third cycle decreased by 40–52% relative to that in the first cycle (with the average macrolocking contribution of 53%), while this reduction reached 21–27% in the fourth cycle relative to the second one. Furthermore, by increasing the temperature to 200 °C, the bond strength values in the second to fourth cycles increased by up to 71.2% relative to the corresponding values at the room temperature, while for the target temperatures of 400 and 600 °C, the corresponding bond strength values dropped by up to 96.6 and 98.9%, respectively. In the end, a prediction model for the bond strength of LCFT specimens containing rock wool waste was proposed as a function of temperature.

Stone prism encased concrete-filled steel tube columns subjected to axial compression

A new type of concrete-filled steel tube (CFST) structural member is proposed by replacing the central portion of the infilled concrete with a stone prism. The mechanical behavior of the stone prism encased concrete-filled steel tube (SCFST) members under axial compression was studied through experimental testing. A total of twenty-four stub columns, including eighteen SCFST specimens and six CFST specimens, were subjected to axial compression until failure occurred. The primary parameters considered were the size of the stone prism (s = 75 mm, 100 mm or 125 mm), wall-thickness of the steel tube (t = 6.0 mm, 8.0 mm or 12.0 mm), and concrete strength (fcu = 62.3 MPa or 107.0 MPa). Test results demonstrated the composite actions between the three parts of the member, which significantly enhanced the load-carrying capacity of the SCFST column. The effect of the parameters on the failure mechanism, load-carrying capacity, load versus deformation response, and ductility of the SCFST specimens was researched. Meanwhile, the experimentally obtained axial compressive strength of each SCFST specimen was compared with predicted strengths obtained by adopting the formulae from currently available design codes for conventional CFST columns. These design codes could not precisely predict the load-carrying capacity of SCFST stub columns, and an improved model was proposed.

Understandings on the Performance of Concrete-Filled Steel Tube with Different Kinds of Concrete Infill

In recent years, concrete-filled steel tube (CFST) has gained attention in the construction sectors due to its unique static and earthquake resistant properties. The steel-concrete composite member is formed by filling the concrete inside the hollow steel tube of various shapes. The experimental and numerical investigations were carried out for CFST members filled with different types of concrete of the same grade. The types of concrete used for this study are conventional concrete, steel fiber-reinforced concrete, geopolymer concrete, and expansive concrete, respectively. The concretes were cast in the form of cubes followed by curing underwater and self-curing conditions. The specimens cured for 7 and 28 days of age were subjected to axial compressive strength testing. The effect of curing conditions on the strength of concrete was evaluated by comparing the obtained results. Self-curing, the only possible way of curing infilled concrete in the case of CFST columns, was found to have no influence on the strength of concrete, and the same curing method was adopted for the CFST columns tested in this study. A total of 24 CFST specimens were cast by different types of concrete infill, and their load carrying capacity under axial compression and their bond strength were determined through experimental investigations. The characteristic strengths measured for the CFST columns with different types of concrete infill were compared, and the results are presented. The analytical investigations were carried out by using Eurocode-4 to predict the load-bearing capacity of CFST columns under axial compression. Finally, the load carrying capacities of CFST columns under axial compression obtained through experimental and simulation studies were compared and the results are presented.

Circular concrete filled thin-walled steel tubes under pure torsion: Experiments

This study experimentally explored the torsional behaviour of 33 specimens, which comprises of 18 concrete filled steel tube (CFST), 9 steel tube (ST) and 6 concrete (C) specimens. The length to diameter (L/D) ratio was 7.14 while the ratios of diameter to thickness (D/t) were 60.9, 46.7 and 40.0. The compressive strength of the infill concrete was 24.2 MPa (normal) and 33.3 MPa (relatively high). All specimens were tested to failure and the obtained data was analysed. The results indicated that buckling failure of CFST specimens was prevented due to the presence of concrete infill. Diagonal cracks of concrete infill observed on CFST specimens after cutting steel tubes are the evidence for the collaboration between steel tube and concrete. This evidence also reveals that the concrete infill contributes to increase the torsional moment capacity of CFST and helps steel tubes to work effectively. The superposition method fails to apply for CFST members under torsion because it does not take into account the mutually beneficial collaboration and interaction between steel tubes and concrete infill which resulted in the improvement on the failure mode, mechanical properties and behaviour of CFST specimens under torsion. Details of the test results and above-mentioned findings are reported in this paper.

Effect of axial loading conditions and confinement type on concrete-steel composite behavior

This paper aims to analytically study the effect of loading conditions and confinement type on the mechanical properties of the concrete-steel composite columns under axial compressive loading. The axial loading is applied to the composite columns in the two ways; only on the concrete core, and on the concrete core and steel tube simultaneously, which are called steel tube-confined concrete (STCC) and concrete-filled steel tube (CFST) columns, respectively. In addition, the confinement is investigated in the three types of passive, short-term active and long-term active confinement. Nonlinear finite element 3D models for analyzing these columns are developed using the ABAQUS program, and then these models are verified with respect to the recent experimental results reported by the authors on the STCC and CFST columns experiencing active and passive confinements. Axial and lateral stress-strain curves as well as the failure mode for qualitative verification, and compressive strength for quantitative verification are considered. It is found that there is a good consistency between the finite element analysis results and the experimental ones. In addition, a parametric study is performed to evaluate the effect of axial loading type, prestressing ratio, concrete compressive strength and steel tube diameter-to-wall thickness ratio on the compressive behavior of the composite columns. Finally, the compressive strength results of CFST specimens obtained via the finite element analysis are compared with the values specified by the international codes and standards including EC4, CSA, ACI-318, and AISC, with the results showing that ACI-318 and AISC underestimate the compressive strength of the composite columns, while EC4 and CSA codes present overestimated values.

A Study on Effect of Steel Tube in Axial Behavior of CFST Stubs

Concrete filled steel tubular (CFST) columns are becoming popular due to its advantages from both steel and concrete in resisting loads. CFST members are made up of steel hollow section of rectangular or circular sections filled with plain or reinforced concrete. CFST columns can be used effectively in high raised structures and in bridges. The reduced cross sections of columns can be achieved by the composite action of steel and concrete. The steel tube provides confinement to the concrete and helps to improve load carrying capacity. The concrete core offers higher resistance to axial compression. The inward buckling of steel tube will be avoided due to concrete and the out buckling resistance may be improved due to higher stiffness of the column due to higher gross moment inertia with combined effect of steel and concrete. This experimental work made an attempt to understand the effect of thickness of steel tube in behavior of CFST stubs.In the present experimental study, a total of 24 cylindrical concrete filled steel tube specimens were prepared to evaluate the uniaxial compressive behavior and to understand the effect of thickness and aspect ratio (h/D) on the compressive strength. Two different thickness of steel tubes 3 mm and 5 mm and four aspect ratios (h/D) 1, 2, 3 and 4 are considered in the present study. M30 grade of concrete is used. From this study, as the aspect ratio increases the buckling loads decreases. The decrement is up to 37% in aspect ratio 4 when compared to aspect ratio1.

Analytical post-heating behavior of concrete-filled steel tubular columns containing tire rubber

This research focused on analyzing the post-fire behavior of high-performance concrete-filled steel tube (CFST) columns, with the concrete containing tire rubber and steel fibers, under axial compressive loading. The finite element (FE) modeling of such heated columns containing recycled aggregate is a branch of this field which has not received the proper attention of researchers. Better understanding the post-fire behavior of these columns by measuring their residual strength and deformation is critical for achieving the minimum repair level required for structures damaged in the fire. Therefore, to develop this model, 19 groups of confined and unconfined specimens with the variables including the volume ratio of steel fibers, tire rubber content, diameter-to-thickness (D/t) ratio of the steel tube, and exposure temperature were considered. The ABAQUS software was employed to model the tested specimens so that the accurate behavior of the FE-modeled specimens could be examined under test conditions. To achieve desirable results for the modeling of the specimens, in addition to the novel procedure described in this research, the modified versions of models presented by previous researchers were also utilized. After the completion of modeling, the load-axial strain and load-lateral strain relationships, ultimate strength, and failure mode of the modeled CFST specimens were evaluated against the test data, through which the satisfactory accuracy of this modeling procedure was established. Afterward, using a parametric study, the effect of factors such as the concrete core strength at different temperatures and the D/t ratio on the behavior of the CFST columns was explored. Finally, the compressive strength values obtained from the FE model were compared with the corresponding values predicted by various codes, the results of which indicated that most codes were conservative in terms of these predictions.

Behavior of FRP Ring-Confined CFST columns under axial compression

Although concrete-filled steel tube (CFST) columns have been widely accepted in applications, they suffer deterioration due to various reasons. On the other hand, the concrete and the steel in CFST columns subjected to compression are not perfectly bonded at the initial loading stage. To this end, fiber-reinforced polymer (FRP) rings are proposed to strengthen the CFST columns in the present study. By strengthening CFST columns with FRP rings, it is expected that their nominal yield strength is increased and the strengthened CFSTs exhibit a monotonic ascending load–strain behavior under compression. An experimental study on axial compressive behavior of FRP ring-confined CFST (FRCCFST) columns is carried out and the effects of the FRP type, the FRP ring thickness (0, 2, 3, 4 layers) and the FRP ring clear spacing (40, 80, 120 mm) on the axial compressive behavior of FRCCFST columns are investigated in this study. The experimental results show that FRP rings substantially enhance the ultimate load capacity and slightly enhance the yield strength of CFST columns. The ultimate axial strain of polyethylene terephthalate (PET) FRP ring-confined specimens is much better than that of CFRP ring-confined specimens due to the large ultimate tensile strain of PET FRP. The ultimate stress and strain of concrete in CFST specimens are obtained and compared with those provided by existing stress–strain models to examine their accuracy and capability.