Behavior Of Concrete-filled Steel Tubes Research Articles

Long-term effects including shrinkage and interfacial creep on bond behavior of concrete-filled steel tubes (CFST): Experiment and design

Concrete-filled steel tubes (CFST) are widely used in significant projects due to their high performance stemming from full utilization of combined material advantages. The interfacial bond behavior is the foundation of their high performance, but will deteriorate during the long-term service. Due to the difficulties on carrying out long-term experiments and decoupling the multi-constitutive component bond strength including chemical adhesion, micro-interlocking and friction, there is still a lack of research on long-term interface constitutive models, restricting the safe design of CFST structures throughout their full-lifecycle. Here, push-out tests were carried out on CFST specimens considering long-term effects spanning 850 days including concrete age and duration of long-term interfacial load. The steel plate-concrete specimens, reversed push-out tests and interfacial normal pressure calculation were used to decouple the components of bond stress. the CFST interface database considering long-term effects was established and the mechanism of long-term effects have been revealed. As the duration increases, the bond stress decreases at the beginning, and thereafter stabilizes where the friction component dominates. Compared to that without interfacial load, long-term interfacial load accelerates the damage to chemical adhesion and micro-interlocking at initial stage, but the difference of their effects on bond stress is not significant in the later stage. On this basis, a long-term interfacial constitutive model for its full-lifecycle safe design and a simplified numerical method for long-term effects on bond behavior have been proposed.

Bond Behavior of Concrete-Filled Steel Tube with Molybdenum Tailing

Bond Behavior of Concrete-Filled Steel Tube with Molybdenum Tailing

Numerical investigation and design of CFST columns strengthened by CFRP grid-reinforced ECC under eccentric compression

Carbon fiber-reinforced polymer (CFRP) and engineered cementitious composites (ECC) have become universal materials for strengthening structural members, and their effectiveness in improving the mechanical behavior of concrete-filled steel tube (CFST) columns has been verified by experiments in previous studies. However, how the CFRP, ECC and CFST columns work together, and the mechanical mechanism of the strengthened column is still unclear. Therefore, to further reveal its mechanical behavior and working mechanism, combined with the experimental results of the CFST columns strengthened with CFRP textile grid-reinforced ECC, a detailed numerical analysis was conducted based on a series of finite element (FE) models in this paper. The load-displacement curve, N-M curve, lateral displacement, stress distribution, the contribution and influence degree of each part of materials to the load-bearing capacity of the strengthened columns were investigated. Finally, taking into consideration the confinement effects of the CFRP grid and the ECC on concrete, a novel theoretical model was established and verified. As a result, the concrete contributes approximately 60% to the peak load of the strengthened column. Under small eccentricity ratio, the mechanical behavior of the CFRP grid can be fully realized. The yield strength of the steel tube and the compressive strength of the concrete have a highly significant effect on enhancing the load-bearing capacity of the strengthened column. The theoretical model proposed in this study can accurately predict the load-bearing capacity of the strengthened columns under eccentric compression, thus offering valuable insights for the design and research of such strengthened columns.

STensile behaviour of T-stub to hollow section column using elliptical-head one-side bolts

The Elliptical-head one-side bolt (EOB) is a typical representative of the third generation for one-side bolts, relying on its elliptical head and bolt hole to achieve orthogonal anchorage, and the application scenario is mainly the bolted connection of closed-section members. Studies have been reported for the shear performance of EOBs bolted lap connections and the tensile behaviour of concrete-filled steel tubes, but there is still a lack of the tensile performance of EOBs connecting Hollow section columns (HSCs), which is necessary for the application of the component method for the beam-column joints design. Therefore, numerical simulation and theoretical analysis methods were employed in this paper to demonstrate the feasibility of adopting EOBs for this type of connection, including typical failure modes, novel yield line pattern in the HSC wall, comparison with the Conventional high-strength bolt (CHB) connections, analysis of the parameters related to the elliptical bolt head and hole, and the establishment and validation of the methodology for calculating the HSC bearing capacity. The results show that: The novel trapezoidal yield line pattern tended to show in the HSC wall with the smaller bolt pitch distance and larger gauge distance; The yield strength of the EOBs bolted T-stub to HSC connections was reduced by about 15% compared to the CHBs bolted ones; Vertical layout of bolt holes, accuracy of rough assembly, rotation deviation of bolt shank within 20° and avoidance of bolt offsets in bolt hole were recommended for EOB connections; The novel yield line pattern established in this paper predicted the yield strength of the connections, and the accuracy and stability were verified by comparing with finite element modelling (FEM) results.

A Review of Research on the Bond-Slip Behavior of Concrete-Filled Steel Tubes with Varying Confinement Interface Damage

The structural safety of concrete-filled steel tubes primarily depends on the interfacial bonding between the steel tube and concrete, two heterogeneous materials. Simultaneously, varying confinement interfaces have diverse impacts on the bond-slip behavior of concrete-filled steel tubes, leading to a variety of damage and failure modes. This article reviews research on the performance of concrete-filled steel tubes, explores damage at different confinement interfaces, and collates findings on bond-slip behavior under various confinement interface conditions, aiming to provide a reference for subsequent studies.

Behaviour of concrete-Filled steel tube having granulated blast furnace slag as fine aggregate in normal strength concrete after exposure to elevated temperature

The significant growth of steel production in India has led to a considerable increase in the production of granulated blast furnace slag (GBS). To resolve the environmental concerns associated with GBS production, it can be used as a substitute for natural fine aggregate in concrete within concrete-filled steel tube (CFST). However, literatures regarding the behaviour of GBS aggregate added concrete-filled steel tubular columns (GBSCFST) after exposure to fire is very limited. Hence the present research work aims to investigate fill the aforementioned research gap. A total 30 number of GBSCFST specimens having square, circular and rectangular shapes, including both intermediate and stub columns, are tested. GBS is used as fine aggregate for preparing infill concrete. All the specimens are heated to varying target temperatures of 200°C, 400°C, 600°C and 800°C with a temperature holding time of 2 h in a furnace. Afterwards, the specimens were left inside the furnace for cooling purposes. Axial compression load was applied to all the specimens by using a compression testing machine with a strain-controlled procedure. The reduction in the load carrying capacity ranged from 1.07% to 12.68% for stub specimens and 1.05% to 19.35% for intermediate specimens heated upto 600°C. However, the load carrying capacity experienced a significant reduction reaching a maximum percentage decrease of 24% and 34% when heated at 800°C. The primary failure shapes observed were local buckling and bulging in stub specimens and mid-height buckling in intermediate specimens.

Lumped plasticity model for simulating the inelastic earthquake response of CFT columns

A simplified approach for simulating the seismic behavior of concrete-filled steel tube (CFT) columns using a lumped plasticity modeling approach based on a nonlinear spring model is proposed in this paper. Model parameters were calibrated considering plastic deformation, load–deformation during unloading and reloading cycles, and material strength degradation using an experimental database comprising 123 CFT column specimens (47 circular and 76 rectangular). 80% of the calibrated samples were used to develop predictive equations for the model parameters based on six input variables using multiple linear regression. The resulting equations were verified by the remaining 20% of all specimens; the verification results agreed with the experimental data and demonstrated better results compared to that of an existing fiber-based beam-column model. The proposed modeling approach was applied to model three frames with different stories (three, six, and nine stories) to perform nonlinear dynamic analyses. Then, their fragility curves were developed and compared with the curves obtained using existing fiber-based frame models. The proposed modeling approach increased the seismic vulnerability of the frames and reduced the analysis computational time.

Lateral impact test and inertial force of concrete-filled steel tube column

The mechanical behavior of concrete-filled steel tube (CFST) subjected to lateral impact was studied by drop hammer test and finite element method (FEM). Lateral deformation occurred when the midspan of CFST was impacted by a drop weight. Two plastic hinges were formed in the mid-span and near the fixed support, respectively. The numerical and experimental results were compared to ensure that the numerical model was reasonable for investigating the stress development, damage, and inertial force of the CFST column. The effects of the impact velocity, impact mass, cross-sectional steel ratio, and boundary conditions on the inertial force of the CFST column were analyzed. During the impact process, the maximum stress was at the steel tube part of the impact location and the fixed support, while the other parts were still in the elastic or elastoplastic stages. The region under restraint at the fixed end of the CFST column exhibited the first signs of concrete core damage under tension, followed by the region near the location of the impact at midspan. At the peak impact force (Fp), the impact velocity had a considerable influence on the distribution of inertial force of CFST, while the impact mass and cross-sectional steel ratio had no discernible impact on the inertial force at the Fp, and boundary conditions had little impact on the distribution of inertial force. Based on a research of the inertial force at the Fp, a simple mechanical model of the inertial force was built.

Dynamic compressive behavior of concrete-filled steel tubes under single and multiple impacts

In this study, the dynamic compression behavior of concrete-filled steel tubes (CFSTs) under single and multiple impacts was investigated using the split Hopkinson pressure bar (SHPB) apparatus. Analyses of the failure patterns, dynamic stress-strain relationship and dynamic compressive strength of CFST members were conducted, and the results for CFSTs were compared with those for plain concrete samples. Moreover, the confinement mechanism of the CFST specimens during the axial impact was investigated, and the influences of strain rate, steel tube thickness, concrete strength and height-to-diameter ratio on the dynamic increase factor of the CFST (DIFCFST) were explored. The experimental results indicate that the CFST specimens had excellent impact resistance under single and multiple impacts, and the dynamic compressive behaviors of CFSTs exhibited significant strain-rate dependence. Larger loading gas pressure can strengthen the confinement from steel tube to concrete, leading to an increase in the dynamic compressive strength of the CFST. Dynamic compressive strength reduction generally occurred during multiple impacts, while the strength enhancement could be observed at lower impact level due to insufficient single impact energy. The DIFCFST decreased with increasing steel tube thickness and concrete strength, while the height-to-diameter ratio had less effect on the DIFCFST. Based on the experimental test data, an empirical formula was proposed for the DIFCFST considering the strain rate, the confinement factor and concrete strength to predict the dynamic compressive strength of CFST members under axial impact.

A State-of-the-Art Review on Axial Compressive Behavior of Concrete-Filled Steel Tubes Incorporating Steel Fiber and GFRP Jacketing

Several types of fibers have enhanced the structural response of reinforced concrete-filled steel tubes (CFSTs). This article presents a state-of-the-art review of studies done on the axial compressive behavior of steel and glass fiber-reinforced CFSTs. The aim of using fibers is to improve the response of the CFSTs. This research indicates the findings of experimental programs and analytical evaluations of the effects of the fiber incorporation on the behavior of the CFSTs. The results of this research work demonstrate that steel fibers (SFs) have enough evident improving effects on the failure mode and load-carrying capacity of the CFSTs. The SFs greatly increase the ductility of the CFSTs. To enhance the compressive strength and ductility of the CFSTs, adding the SFs by 1% to the concrete mix is more effective than adding by 1.5%. The use of the SFs mixed with expansion agent considerably increases the yield and ultimate loads of the CFSTs. More glass fiber-reinforced polymer (GFRP) sheets reduce buckling and develop the compressive strength of the CFSTs. The implementation of the GFRP jackets not only enhances the load-carrying capacity of the CFSTs, but also increases their ductility. The GFRP reinforcement techniques for the CFSTs are also effective in improving their structural stiffness and energy absorption capacity.

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

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

Eccentric behaviour of square CFST columns strengthened using square steel tube and high-performance concrete jackets

The eccentric behaviour of concrete-filled steel tube (CFST) square columns strengthened with square steel tubes and high-performance concrete (HPC) jackets is investigated. Four types of HPC are used for concrete jackets: ordinary concrete (OC), self-stressing concrete (SSC), steel-fibre-reinforced concrete (SFC), and steel-fibre-reinforced self-stressing concrete (SFSSC). The experimental variables include eccentricities, width-to-thickness ratios, strengths of the concrete jacket, initial stresses and volume percentages of the steel fibre. The eccentric behaviours of columns with various types of concrete jackets are carefully addressed through the failure mode, load-lateral deflection curve, load versus strain behaviour, capacity analysis and ductility. Among the three volume percentages of steel fibre (0.6%, 0.9%, 1.2%) investigated in this study, 0.9% is the most suitable for improving the behaviour of retrofitted columns. The columns strengthened with the SFSSC jacket exhibit higher load-bearing capacities than the other three types of HPC jackets with good ductility. Considering the self-stressing and steel fibre effects, the stress–strain curves of various HPC jackets are presented. Subsequently, a finite-element (FE) model for the retrofitted column is established and verified along with the loads carried by each component. A detailed analysis of the load-lateral deflection curve and the interaction and stresses of the four components is performed in three stages. Finally, the load-bearing capacities of the retrofitted columns under eccentric loads calculated fusing the EC4, GB50936 and AISC 360 codes are compared.

Flexural strengthening mechanism of concrete-filled steel tube beam by welding round steel at soffit of the beam

In the present study, the flexural behavior of concrete-filled steel tube (CFST) beams strengthened by welding round steel at the soffit of the beam has been investigated. First, a total of six specimens were tested under four-point bending load, which includes one reference beam and five retrofitted beams strengthened by welding round steel of different diameters at the soffit of the beams. Then, the corresponding nonlinear finite-element models have been developed and calibrated against the experimental results. By combining indoor experiments and numerical simulations, the bending moment versus mid-span curvature curves, the flexural capacities, flexural stiffness, ductility, vertical deflection curves, strain distribution across the cross-section, and neutral axis offset of the test beams have been systematically analyzed. The results show that the welded round steel at the soffit can improve the flexural bearing capacity and stiffness of the CFST beams significantly. This can be attributed to the CFST beams benefitting from the flexural bearing effect of the round steel and the downward offset of the neutral axis which causes more core concrete to be in compression. Further, the larger the diameter of the round steel, the greater the benefit. Finally, comparisons have been made with the flexural stiffness calculated using the existing design codes, such as AIJ standard, BS5400, Eurocode 4 and AISC specification. The comparisons show that the flexural stiffness of the unstrengthened CFST beam can be predicted by AIJ and BS5400 perfectly, whereas those of the strengthened CFST beams can only be accurately calculated by AISC.

Impact behavior of concrete-filled steel tube with cruciform reinforcing steel under lateral impact load

The objectives of this research are to investigate the behavior of cruciform steel-reinforced concrete-filled steel columns under impact load through experimental and numerical studies. The effects of impact velocity/height, axial pressure, thickness of steel tube and boundary conditions on the impact resistance of composite columns are studied through the tests of 16 specimens. The detailed progressive responses were recorded, including deformation mode, impact force-time history, deflection-time history and axial force-time history. Based on the test results, the impact resistances of steel-reinforced concrete-filled steel tubular columns and concrete-filled square steel tubular (CFST) columns are compared. In addition, FE models of the composite column are established using ABAQUS and validated based on experimental data. The strain rate effect of steel and concrete is considered in the model. The results show that the outer square steel tube effectively protects the internal steel-reinforced concrete. In the impact energy range of 17.02–51.08 kJ, the specimens only suffer minor bending deformation, which shows their excellent impact resistance. The ultimate plastic strain energy of square steel tube (EPT), cruciform steel section (EPS) and concrete (EPC) accounts for 56.4%, 18.2% and 25.4% of the total energy dissipation, which suggests that the steel tube (EPT) is the main energy dissipation mechanism of concrete-filled square steel tube columns.

Seismic Behavior of Concrete-Filled Steel Tube (CFST) Column and Reinforced Concrete (RC) Beam Connections under Reversed Cyclic Loading

Previous studies on the connection between concrete-filled steel tube (CFST) columns and reinforced concrete (RC) beams have shown a loss of joint confinement because the steel tube was completely or partially cut in the joint area. This research presents a new connection system that provides joint confinement through a continuous steel tube. Potential sliding shear at the smooth interface between the columns and beams in the joint face is mitigated using two mechanisms: (i) shear connectors and (ii) longitudinal web beam reinforcement. This study tested two CFST column and RC beam joints to 4.5% drift ratio under combined compression axial load and lateral cyclic load. The experimental results revealed no cracks at the joint zone and the specimens satisfied the ACI 374.1-05 criteria, despite minor sliding at the beam-column interface. The finite element (FE) model showed good agreement with the experimental results.

Axial compression tests on elliptical high strength steel tubes filled with self-compacting concrete of different mix proportions

The compaction state and the paste volume of concrete may significantly influence the compressive behaviour of concrete-filled steel tube (CFST) columns, while limited studies have been conducted to understand the compressive behaviour of CFST columns with different mix proportions. This study presents an experimental investigation to the axial compressive behaviour of elliptical CFST (ECFST) column consisting of a high-strength steel (HSS) tube and a self-compacting concrete (SCC) core with different mix proportions. Eighteen ECFST column specimens, fabricated with high-strength elliptical steel tubes in three different cross-sections and three batches of concrete with different mix proportions, were tested under axial compression. The parameters under investigation are the concrete paste volumes (41%, 37% and 32.5%), the cross-sectional aspect ratios (1, 1.5 and 2) and the compaction conditions (with and without mechanical compaction). The standard cylinder tests reveal that SCC without mechanical compaction has a better filling ability and thus a better self-compaction capability when the paste volume is larger than 37%, while SCC with mechanical compaction has the highest strength when the paste volume is the lowest. The axial compression tests show that the peak axial load capacities of ECFST columns are dependent on the paste and aggregate contents in concrete. The compaction state also affects the performance ECFST columns because compaction usually increases concrete strength especially when the aggregate content is high. In addition, it is found that key performance parameters (e.g., strain enhancement ratio, ductility and strength index) all decrease with the increase of cross-sectional aspect ratio due to the non-uniform confinement provided by elliptical steel tubes.

Axial Load Behavior of Ultrahigh Strength Concrete-Filled Steel Tube Columns of Various Geometric and Reinforcement Configurations

This paper presents the behavior of concrete-filled steel tube (CFST) columns infilled with fiber-reinforced self-consolidating ultrahigh strength concrete (UHSC) subjected to axial concentric monotonic loading to failure. UHSC is expected to improve ease of fabrication, strength, and ductility of CFST columns. Seventeen columns having varying geometric properties such as tube wall thickness, cross-sectional shape (circular, rectangular, and square), and slenderness were constructed and tested by applying load through both steel tube and concrete core. Circular columns were further distinguished by the presence or absence of main and hoop steel reinforcing bars in the core concrete. Axial load-displacement response, axial/transverse strain development, and failure modes were recorded during the loading history to analyze the performance. Experimental confined concrete strength and axial strength of UHSC-filled CFST columns were compared with those obtained from three suggested analytical models and three code-based design procedures including Eurocode 4, Canadian CAN/CSA S16, and American AISC. Analytical models were found to over-predict the confined concrete strength and the axial strength of CFST columns. Canadian and American codes were found to be most applicable for predicting axial strength of UHSC-filled CFST columns while remaining conservative.

Analysis of Concrete-Filled Steel Tube Reinforced Concrete Column-Steel Reinforced Concrete Beam Plane Frame Structure Subjected to Fire

The ABAQUS finite-element analysis platform was used to understand the mechanical behavior of concrete-filled steel tube reinforced concrete (CFSTRC) columns and steel reinforced concrete (SRC) beam plane frames under fire conditions. Thermal parameters and mechanical constitutive model of steel and concrete materials were reasonably selected, the correct boundary conditions were chosen, and a numerical model for the thermal mechanical coupling of CFSTRC columns and SRC beam plane frame structure was established. The finite-element model was verified from related experimental test results. The failure modes, deformation, and internal force distribution of the CFSTRC column and SRC beam plane frames were analyzed under ISO-834 standard fire conditions and with an external load. The influence of beam and column fire-load ratio on the fire resistance of the frame structure was established, and the fire-resistance differences between the plane frame structures and columns were compared. The CFSTRC column-steel reinforced concrete beam plane frame may undergo beam failure or the column and beam may fail simultaneously. The frame structure fire-resistance decreased with an increase of column and beam fire-load ratio. The column and beam fire-load ratio influence the fire resistance of the frames significantly. In this numerical example, the fire resistance of the frames is less than the single columns. It is suggested that the fire resistance of the frame structure should be considered when a fire-resistant structural engineering design is carried out.

Interface Bonding Behavior of Concrete-Filled Steel Tube Blended with Circulating Fluidized Bed Bottom Ash.

The interface bonding behavior between the steel tube and the concrete of concrete-filled steel tube (CFST) blended with circulating fluidized bed bottom ash (CFB-BA) was investigated in this study. A total of 8 groups of CFSTs stub columns were prepared with different dosage of CFB-BA, water-binder ratio (W/B), and interface bonding length. A series of push-out tests were carried out to acquire the data representing the interface bonding behavior. The results show that the dosage of CFB-BA has a direct effect on interface bonding behavior of CFST. CFB-BA can improve the interface bonding behavior of CFST. The highest ultimate bonding load and strength are achieved when the dosage of CFB-BA is 30%. When the dosage of CFB-BA increases to 50%, its interface bonding behavior decreases, but is still better than that of CFST without CFB-BA. W/B has a negative correlation with the interface bonding behavior of CFST. While the W/B increases, the interface bonding load and strength of CFST decreases. The increase of the interface bonding length can improve the interface bonding load, but cannot improve the interface bonding strength.

Long-Term Creep and Shrinkage Behavior of Concrete-Filled Steel Tube.

A concrete-filled steel tube (CFT) combines the advantages of concrete and steel in construction and structural applications. However, research on the time-dependent deformation of the CFT under long-term sustained loading are still limited, particularly for stress transfer between the steel tube and concrete due to creep. This study investigated the creep behavior of CFT over a long period of 400 days. The creep and shrinkage strain of CFT was significantly lower than those of concrete that was not confined within a steel tube. The vertical strains of the steel tube and concrete core were almost identical, and it was shown that they were well bonded and acted as a composite. The vertical stress of steel increased by 32.7%, whereas the vertical stress of concrete decreased by 15.8% at 375 days. The stress transfer is notable and cannot be neglected in CFT design. Moreover, the results of creep and shrinkage were compared to prediction values of the B4 model and B4-TW model to verify their validity.