Concrete-filled Steel Tube Columns Research Articles

Axial compressive performance of hybrid fiber cementitious composite-encased CFST columns

In order to study the axial compressive performance of hybrid fiber cementitious composite (HFC)-encased concrete-filled steel tube (CFST) columns, eighteen column specimens were prepared and tested experimentally with three factors (i.e., the HFC strength, core concrete strength, and steel tube thickness) considered. The failure modes, load-strain curves, peak load, and ductility of the column were investigated. With the help of the HFC encasement, nearly no crack was observed before the peak load, and the column could still maintain its integrity after failure. The increase of the HFC strength, core concrete strength, and steel tube thickness could enhance the stiffness and the load capacity, while the utilization of high HFC strength reduced the ductility of the HFC-encased CFST column. In comparison with various standards, the theoretical models in CECS188–2005 and GB50936–2014, which take the confinement effect into account, gave the best prediction. In addition, the finite element model was also established and validated by the experimental results. The performances and stress distributions of all components of the HFC-encased CFST column during the axial loading process were analyzed and discussed by using the finite element model.

Tube-in-tube rigid beam to CFT column connection in moment-resisting frames: An experimental study

Using concrete-filled steel tube (CFT) columns in high-rise buildings has attracted considerable attention in recent years. Aside from its advantages, rigid beam to CFT column connections are followed by some challenges. Difficult construction and implementation, sophisticated details of connection, and most importantly, placing and welding internal stiffeners (continuity plates) are among the common challenges. In the present study, a novel connection called “Tube-in-Tube (TIT) connection” has been proposed. In TIT connection, an inner tube and a pair of vertical straps are utilized in the panel zone area. To investigate the proposed connection, two full-scale specimens were tested using cyclic reversal loading. The two specimens were identical in dimensions and detailing, except for their strap width and the diameter of inner tube. Specimen 1 and Specimen 2 were detailed with inner tube diameter equal to half and one-third of the column dimension, respectively. The inter-story drift ratio, joint's shear deformation, plastic hinge formation, and energy dissipation capacity were analyzed. Moreover, mechanical parameters such as yield and ultimate strengths, and ductility of the specimens were calculated. Specimen 2 showed 3% and 8% increase in the peak flexural strength and cumulative energy dissipation in comparison with Specimen 1. The results showed that the panel zone remain elastic during the test. In order to further investigate the proposed connection, finite element (FE) analysis was conducted on the specimen. The results of FE analysis confirmed the elastic behavior of the tube and straps.

Seismic behavior of composite shear walls with boundary columns using rectangular and U-shaped concrete-filled steel tubes

Steel-concrete composite shear walls with boundary concrete-filled steel tube (CFST) columns have drawn the attention of researchers due to their good seismic performance such as high strength, ductility and energy dissipation capacity. The reported study proposes a novel composite shear wall design that aims to complete the majority of the work in the factory, leaving only concrete pouring, steel welding, and formwork removal to be done on-site, so that the constructional time and cost can be saved. Two types of CFSTs were used for the boundary columns of composite shear walls, one is conventional rectangular CFSTs, another one is U-shaped CFSTs. In order to study the seismic behavior of the novel composite shear walls, cyclic loading tests were conducted on four full-scale specimens. The bearing capacity, energy dissipation, skeleton curves, hysteretic loops, deformation capacity and failure mode are analyzed. The experimental results have demonstrated that the bearing capacity and stiffness of shear walls with U-shaped CFST columns are slightly higher than that of shear walls with rectangular CFST columns, whereas the latter has better deformation capacity. In addition, the reported study investigated the methods of calculating the shear capacity of the novel composite shear walls and provided recommendations on the calculation method to be used for design and structural analysis.

Analysis and calculation method for concrete-encased CFST columns under eccentric compression

This paper presents a computational method for concrete-encased concrete-filled steel tube (CFST) columns under eccentric compression, which adopts the form of equivalent rectangular stress integrated by material constitutive curve. When establishing finite element model via ABAQUS, confinement provided by stirrup was considered to modify the constitutive law of encased concrete, so that the numerical modeling was in good agreement with tests. By analyzing the simulation results, the applicability of basic assumptions, such as plane section assumption, was discussed, and a database of 207 models with various parameters was created. The developed method took into account the influences of geometric nonlinearities of section shape, and its accuracy was verified against various collected tests and the database. It turned out that the formula presented was applicable to a wide range and had sufficient precision, while the predictions of N-M interaction curves of concrete-encased CFST columns in EC4, AISC and CECS were conservative due to the ignorance of the exact integration about irregular section-shape.

Axial compression performance of CFST columns reinforced by ultra-high-performance nano-concrete under long-term loading

Abstract The addition of nano-silica to ultra-high-performance concrete (UHPC) to increase its toughness has been proposed to obtain ultra-high-performance nano-concrete (UHPNC). This work mainly studies the reinforcement effect of UHPNC on concrete filled steel tube (CFST) columns under long-term load. Ten CFST columns strengthened with UHPNC were selected and reinforced with UHPNC. The influences of different thicknesses of UHPNC reinforcement layer and different nano-silica contents on the axial compression properties of specimens were mainly studied, by loading specimens in two steps: long-term load and ultimate load. This study discussed the failure modes, compressive toughness, ultimate bearing capacity, initial stiffness, and ductility coefficient of the specimens. The results show that the outsourced UHPNC reinforcement method is an effective method to improve the performance of CFST columns during service period. With the increase in the thickness of UHPNC reinforced layer, the ultimate bearing capacity of CFST column increases greatly. The compression toughness is increased with the increase in nano-silica content and UHPNC reinforcement layer thickness. The decrease rate of initial stiffness increases with the increase in nano-silica content.

Experiments and design of concrete-filled steel tubes with timber chips under axial compression

This study includes experimental and analytical sections, which discuss the structural response of concrete-filled steel tubes (CFST). In the experimental section, the authors investigate the CFST columns with different proportions of recycled redwood timber chips embedded inside the concrete. An analytical investigation was also carried out in this paper, which puts together equations proposed by four design codes of Eurocode-4, AISC-LRFD, the Chinese code of DL/T, and ACI to predict the axial capacity.The idea of embedment of timber inside aggregates was inspired due to the significant compression strength of timber compared to concrete. In addition, environmental and sustainability concerns advocate substituting conventional aggregates with recycled natural material. In this study, 18 stub columns (in two groups with different steel tubes) were tested under pure axial compression with various compositions of the aggregates. The failure, capacity, weight, and energy absorption of the specimens were studied to evaluate the effect of the weight reduction as a result of timber embedment into the concrete. The results showed that embedding waste timber inside concrete is promising in upcycling wood residue into a commercially viable product. An equation is developed in this paper that takes into account the effect of confinement based on the assumptions that have been made in the literature. 100 test samples were extracted from the literature with a relatively wide range of geometry and material features, and then the capacity of these columns was calculated by the four standards and the proposed equation in this study. The results of the comparisons were analysed and it was found that the proposed equation predicts the capacity accurately.

Experimental study on self-compacting concrete-filled steel tube samples containing shrinkage-controlling admixtures

Once again, the earthquake disaster in Turkey and Syria indicates the significance of aseismic structures. Concrete-filled steel tube (CFT) columns are key components in such structures. In this study, the effect of shrinkage control admixtures on the performance of self-compacting mixes that are often used in CFT column construction was evaluated. They were confined to a 3 mm thick cylindrical hollow steel tube and the infilled concrete shrinkage was monitored with the embedment of a vibrating wire strain gauge. The concrete length of this compound was found to be reduced as the length of the confining tube expanded. However, this change in concrete length was reduced by 70% with the use of a commercial grade liquid mixture that reduces shrinkage in the early stages. A substantial reduction in concrete shrinkage was observed with the use of limestone powder and a compensating admixture for shrinkage. The use of limestone also improved the resistance of concrete to water permeation and chloride ion transport by pore filling effects. Although steel confinement reduced concrete shrinkage by 50% at 28 days, the addition of specialty admixtures is still highly recommended to avoid the effect of debonding, induced by the shrinkage discrepancy between confining steel and the inner concrete mass.

Experimental Study on Ultrasonic Tomographic Method to Detect Compactness of Concrete filled Steel Tube

When the concrete filled steel tube member is poured, due to the large volume, the internal concrete is difficult to vibrate, easy to produce defects, not easy to observe, and the internal defects will have a non-negligible impact on its mechanical properties, therefore, it is particularly important to detect the internal defects of the concrete filled steel tube column. In this paper, the internal defects of CFST are studied based on the principle that acoustic wave propagates along the shortest acoustic path in CFST. For square concrete-filled steel tubular column, when the measurement method is adopted, the theoretical sound time calculation and sound velocity distribution map imaging are carried out for different measurement point densities. It is found that the defect identification effect is better when the number of measurement points is 5.For circular concrete-filled steel tubular column, when the measurement point spacing is determined to be 30° arc length, the measurement point arrangement method in the specification cannot be combined with CT imaging method, so the measurement point arrangement method of one-point emission and five-point receiving is proposed, and it is found that it has good identification effect on internal defects.

Finite Element Analysis of Self-Compacted Concrete Filled Steel Tube Columns Exposed to High Temperatures

Finite Element Analysis of Self-Compacted Concrete Filled Steel Tube Columns Exposed to High Temperatures

The effects of steel fibers and boundary elements on the seismic behavior of high-strength concrete shear walls

Six shear walls using high-strength concrete and high-strength steels were studied by conducting cyclic load tests, including two reinforced concrete shear walls, and four composite walls reinforced by concrete-filled steel tubes (CFSTs). The effects of steel fibers and boundary element forms on the cyclic behavior of the walls were investigated. Research founds that the steel fiber reinforced specimens showed lighter damage degree, lower deformation capacity, and lower proportion of shear deformation than the non-fiber reinforced specimens. Among the three types of specimens with different boundary elements, steel fibers had the greatest influence on the composite walls with concrete-encased CFST columns in boundary zone. Compared the two types of composite walls reinforced by CFSTs which were designed by the principle of “equal vertical bearing capacity of the boundary elements”, the walls with outer CFST columns showed better energy dissipation capacity than the walls with concrete-encased CFST columns. Finally, considering the stress characteristics and the constraint effect of boundary elements of different kinds of shear walls, the simplified calculation model of load-carrying capacity and the finite element analysis model were established and verified.

Axial compressive behavior of square concrete-filled steel tube columns with high-strength steel fiber-reinforced concrete

Adding fibers in the concrete is a practical approach to improving the deformation capacity of square concrete-filled steel tube (CFST) columns with high-strength concrete (HSC). To investigate the axial compressive behavior of square CFST columns with high-strength steel fiber-reinforced concrete (HSSFRC), 23 specimens were tested under monotonic axial compression, among which five specimens used an innovative testing method to directly obtain the load components of the steel tube and concrete infill. The test variables included the concrete compressive strength, the fiber volume fraction, and the yield strength and width-to-thickness ratio of the steel tube. The test results demonstrated that adding steel fiber basically did not change the failure modes of the specimens. In general, better postpeak behavior was observed as the fiber volume fraction increased; however, for the specimens with 130 MPa matrix concrete, adding steel fibers of a volume fraction of 0.75% basically did not bring any improvement. When the confinement index was between 0.29 and 1.43, the compressive strengths of the HSC or HSSFRC in square CFST columns were close to the corresponding unconfined concrete strengths. Based on the test results, an average stress–strain model was developed for the HSC or HSSFRC in square CFST columns.

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.

Estimation of blast-induced peak response of concrete-filled double-skin tube columns by intelligence-based technique

This paper aims to predict the peak response of concrete-filled steel tubes (CFSTs) under blast loads using regression-based machine learning (ML) algorithms considering the variability of input parameters. Twelve regression metrics that are commonly used are utilized to evaluate and compare the efficiency and effectiveness of the proposed ML models. The Monte Carlo approach is used to propagate the variability in the input space to the predicted output. The results showed that the optimal method varies with perspectives. The XGBoost algorithm has the highest final score, and the SVM algorithm exhibits the highest total score for standard deviation. Furthermore, the confidence interval increases with the peak response, and the single model presents a higher accuracy in predicting large displacements. However, ensemble models do the opposite. Sensitivity and uncertainty analyses are performed, indicating that the scaled distance, section size and ultimate moment capacity exhibit the highest contribution to the peak response of the CFST. Finally, a simple formula is developed based on the MLP model, which can cover three input parameters. The findings of this paper can be used for computer-aided dynamic response design of CFST columns subjected to blast loads.

Seismic performance of circular concrete-filled steel tube columns reinforced with inner latticed steel angles

The seismic behavior of circular concrete-filled steel tube (CFST) columns with inner latticed steel angles under combined axial load and reversed cyclic horizontal load was studied in this paper. A total of 8 specimens was tested, including two CFST specimens and six latticed steel angles reinforced CFST specimens. The main parameters studied were the diameter-to-thickness ratio of steel tube, the cross-sectional area of inner latticed steel angles and the axial compression ratio. Firstly, the load-displacement curves and load-strain curves were obtained experimentally; secondly, the failure modes, hysteretic behavior, skeleton curves, stiffness degradation, ductility index, hysteretic energy dissipation capacity were analyzed; thirdly, the seismic performance of the tested specimens were simulated by the established finite element (FE) models, and parametric studies were subsequently performed; finally, the modified calculation method for the horizontal bearing capacity was proposed. The research results showed that the failure of the composite columns was caused by the local buckling of the latticed steel angles and steel tube, and the latticed steel angles could effectively participate in the overall loading process. It was also found that latticed steel angles are able to improve the energy dissipation capacity.

Confinement mechanism and compressive stress-strain behaviours of FRP-interlayer-steel confined concrete tubes

A fiber-reinforced polymer (FRP)-interlayer-steel confined concrete (FISC) column is formed by bonding the FRP material and a concrete-filled steel tube (CFST) with the grouting material, and it can improve the corrosion resistance and bearing capacity of CFST columns and avoid the early cracking of the epoxy resin in FRP confined concrete-filled steel tubes (CCFT). However, there are few researches on the compressive behaviours of FISC columns and the dual confinement on core concrete from the FRP tube and steel tube has not been expounded clearly. In this paper, twelve FISC specimens were tested under axial compression. The ratio of FRP and steel confining indexes and the concrete strength were the main parameters and the loading conditions (core CFST loading and whole section loading) were included. Failure patterns, load–axial shortening curves, and the strain development were analysed. In addition, the confining stresses of the FRP tube and steel tube were obtained, and it was found that the confinement on concrete could be divided into three stages of the non-confinement stage, the steel-controlling stage, and the FRP-controlling stage. In the FRP-controlling stage, the FRP confining pressure increased at a constant growth rate while the steel confinement was gradually weakened owning to the restraints of the interlayer grouting material. Furthermore, by regarding the concrete and grouting material as a whole unity, the axial stress-strain model of the confined concrete-grouting was developed, and the proposed model can predict the compressive stress-strain behaviours of an FISC column with good accuracy and simplicity.

Overall stability capacity design of T-shaped irregularly concrete-filled steel tube columns considering axial compression and any directional moments

A slender T-shaped irregularly concrete-filled steel tube column (T-ICFSTC) may be subjected to overall instability failure under combined axial load and moment. But current research is not available about a complete design of T-ICFSTCs subjected to axial compression and any direction bending moments because of a complicated parameter study involved in the analysis. This paper investigates the overall stability capacity performance and design prediction of the T-ICFSTCs that are formed by I-shaped steel, U-shaped steel and several cell infilled concrete. First, the finite element model for analyzing the overall instability performance and failure is established and verified by the previously experimental tests carried out by the authours. Then, the overall stability capacity of T-ICFSTC around its symmetric axis is investigated numerically and accordingly design formulas of φy-λny are established. In particular, interaction influence between initial imperfection direction and applied bending moment direction is studied, thus resulting in a critical initial imperfection direction relatively to applied bending direction. Sequently, the load-bearing capacity and performance under any direction bending moments are explored numerically and the normalized design curves reflecting interaction of N-M and Mx-My at their limit states are formulated by fitting the FE results of examples. Finally, a complete set of overall stability prediction formulas of T-ICFSTCs under combined axial load and any directional bending moment are established and the results obtained agree well with FE numerical investigation results.

A comprehensive and reliable investigation of axial capacity of Sy-CFST columns using machine learning-based models

The literature on predicting the load-carrying capacity of symmetrical concrete-filled steel tube (Sy-CFST) columns using different machine learning methods has mainly focused on a single method or cross-section type in each study. Sy-CFST column has been widely used in the engineering field because of its several benefits such asincreased strength due to confinement generation, better ductility due to high steel ratio, and less construction cost and time as compared to the encased reinforced concrete. This study attempted to evaluate the load-carrying capacity of these columns with circular and square cross-sections based on the simultaneous use of the two gene expression programming (GEP) and artificial neural network (ANN) approaches. The database required for extracting GEP and ANN models was based on the empirical results of 993 specimens. Variables considered here include the compressive strength of concrete (fc), yield stress of steel (fy), cross-sectional areas of concrete (Ac) and steel (As), diameter to thickness ratio of the steel tube (D/torB/t), and slenderness ratio of Sy-CFST columns (λ). Moreover, parametric and sensitivity analyses were conducted separately to assess the contribution of each effective parameter to the axial capacity. To validate the efficiency of the models, prediction values of GEP and ANN were compared with the predictions of existing codes (6 codes) and different studies (8 studies). The results indicated that the developed models provide accurate predictions for the load-carrying capacity of Sy-CFST columns. In addition, the variation of parameters in the proposed models is consistent with experimental trends observed in other studies, which confirms the consistency of the proposed numerical models with physical observations.

Experimental and numerical study on the prefabricated double L-shaped beam-column joint with triangular stiffener

This study proposes an innovative prefabricated double L-shaped beam-column joint with triangular stiffener (double L-shaped joint) for low-rise residential buildings. The double L-shaped joint consists of the L-shaped connector with triangular stiffener, web side plate connection, H-shaped beam, and concrete-filled lightweight steel tube (CFLST) column or square hollow steel tube (SHS) column. Twelve double L-shaped joints are manufactured and experimented. The design parameters include the column types (CFLST or SHS), triangular stiffener height, perforated end plate (with or without), and the web side plate connection (with or without). In addition, the failure modes, hysteretic performances, ultimate capacity, stiffness degradation, deformation capacity, energy dissipation, and strain responses of joint specimens are analyzed. The experimental results show that the failure modes include steel tube local buckling, weld cracking, and bolts fracture in the area of the L-shaped connector. The ultimate moment of the CFLST column joints rises from 4.53% to 13.05% compared to the SHS column joints, while the initial stiffness increases from 2.16% to 26.46%. The perforated end plate grows at the ultimate moment, but the ductility index reduces. The initial stiffness and deformation capacity of the specimens without a web side plate connection decreases significantly. The finite element model is developed, and the numerical simulation is conducted. The experimental results are consistent with failure modes and bending moment versus rotation angle curves obtioned by numerical simulation.

Axial load-carrying capacity of concrete-filled steel tube columns: a comparative analysis of various modeling techniques

Limited research is available in the literature to investigate the presentation of normal strength concrete-filled steel tube (CFST) circular compression elements under compression loading by considering various material and geometric coefficients. Thus, the present study investigates the mechanical behavior of CFST compression elements employing nonlinear finite element analysis (NLFEA), empirical/theoretical modeling, and a newly developed Artificial Neural Network (ANN) model, based on a large experimental database of 1223 samples. The NLFEA modeling is performed in ABAQUS (6.14) with an improved concrete damaged plasticity model for laterally restrained concrete. Various geometric and material properties are considered, for parametric NLFEA estimates preliminary validated toward the available experimental database. A new ANN model for the load-carrying capacity of CFST circular elements was also offered by employing the experimental database. The comparison between the calculations of the NLFEA model, empirical model, and ANN model displayed a close agreement with the database results.

Mechanical properties of CFSST with steel reinforcement cage under biaxial eccentric compression

To study the biaxial eccentric compression mechanical properties of high-strength concrete-filled square steel tube (CFSST) columns (high-strength materials are used for both steel tube and concrete) with steel reinforcement cage, twelve high-strength CFSST columns with the same cross-sectional dimensions were designed and fabricated with the form of the steel reinforcement cage, eccentricity, and the steel tube width-to-thickness ratio as the main parameters. The constraint enhancement mechanism of the concrete by the steel reinforcement cage and the effect of different parameters on the biaxial eccentric ultimate bearing capacity of column specimens was further analyzed by combining with ABAQUS simulation. The results show that the bearing capacity of the specimens decreases sharply with the increase of eccentricity and steel tube width-to-thickness ratio; the steel reinforcement cage can improve the bearing capacity and ductility of the specimen but has little effect on the stiffness; the arrangement of double orthogonal horizontal steel reinforcement can enhance the constraint effect of the steel tube in the middle of the cross-section edge length on the concrete of the specimens within a certain height range and weaken the constraint effect of the corner within the same height range; the enhancement effect of the steel reinforcement cage on the biaxial eccentric ultimate bearing capacity of the column specimens gradually decreases with the increase of eccentricity; when the eccentricity is constant, the effect of the steel reinforcement cage on the improvement of the biaxial eccentric ultimate bearing capacity of the column specimens with different width-to-thickness ratios is basically the same.