High-strength Concrete-filled Steel Tube Research Articles

Mechanical performances of thin-walled high-strength concrete-filled steel tube square columns with high-strength reinforced cages under biaxial eccentric compression

In the current research that both steel tube and concrete use high-strength materials, there are few studies on the small wall thickness of steel tube exceeding the standard. In order to make full use of the mechanical properties of high-strength steel and enhance its restraint ability on concrete, this paper designs a steel cage dominated by transverse steel bars. A total of 15 square high-strength steel tube high-strength concrete medium-long column test pieces are tested and studied. The main parameters of the specimens are slenderness ratio, loading eccentricity and the form of internal steel reinforcement cage. After the loading test, according to the test results, the failure mode of the specimen is obtained and the mechanical properties of middle-long concrete-filled steel tubular columns are analyzed. The research results show that the ultra-thin high-strength steel pipe can restrain concrete well, but with the increase of eccentricity, its restraint capacity is slightly insufficient; the built-in steel cage can slow down the bulging of the specimen and help the steel pipe to further strengthen the restraint on concrete, but the steel cage as an indirect constraint cannot avoid the final bulging damage. Due to the existing formula overestimating the effect of ultra-thin steel tubes, this paper proposes a calculation formula for the axial compression of thin-walled high-strength concrete-filled steel tubes with steel cage short columns. Based on this method, the bearing capacity of medium-long columns under biaxial eccentric compression is calculated with good accuracy.

Axial compressive behavior of high-strength concrete-filled steel tube with multiple confinement effects

To study the axial compressive behavior and the confinement mechanism of high-strength concrete-filled circular steel tube with multiple confinement effects, five large size samples with horizontal stiffeners, longitudinal stiffeners, multilayer reinforcement cages, inner steel tube and separation steel plates were designed. The influences of these structures on the bearing capacity, ductility, stiffness, and residual deformation were then analyzed. Combining the strain analyses and finite element model analysis, the confinement mechanism of different structures and the coupling mechanism of different confinement effects were investigated. The results showed that the confinement effect of the horizontal stiffeners was similar to that of the stirrups. The longitudinal stiffeners did not provide a significant confinement effect on the concrete. The multilayer reinforcement cages had an effective confinement effect on the concrete, where the confinement effect increased from the outside to the inside, while the reinforcement cages had a slight influence on the concrete outside the reinforcement cages. The inner steel tube had an effective confinement effect on the core concrete and had a slight influence on the concrete outside the inner steel tube. The multicell steel tube showed the best confinement effect compared to the other structures and showed the best axial compressive behavior. In the confinement mechanism analyses, the calculation method of the stress–strain relationship of concrete with multiple confinement effects was proposed. The calculated F-Δ curves showed a good agreement with the test results.

Flexural behavior of high-strength square concrete-filled steel tube members subjected to cyclic loadings

This paper investigated the flexural behavior of high-strength concrete-filled steel tube (HSCFST) members subjected to cyclic loadings. Nine cyclic tests of square HSCFST members were conducted. The investigated parameters were the height of steel tube, thickness of steel tube wall, yield stress of steel, and compressive cylinder strength of concrete. A finite element (FE) model was developed and validated. Results from the experimental tests and FE analyses indicated that: (i) the failure modes of HSCFST members were characterized by local buckling and fracture of the steel tube, both occurred earlier with increasing height or yield stress of steel, or with decreasing thickness of steel tube wall, but were delayed when higher strength concrete was used; (ii) using high-strength steel significantly improved the flexural strength, had no obvious influence on the flexural stiffness, but significantly reduced the ductility; (iii) using high-strength concrete improved the flexural strength, stiffness, and ductility; and (iv) the limits by AISC 341-16 on the slenderness coefficient is conservative for HSCFST members subjected to cyclic flexural loadings.

High-strength concrete-filled steel tube columns with tie bars: Seismic behavior and design

The current study reports the utilization of tie bars to connect the opposite faces of steel tubes in the plastic hinge regions to strengthen the deformability of square high-strength concrete (HSC)-filled steel tube (CFST) columns. To evaluate the seismic performance of square HSC-filled steel tube columns with tie bars, four CFST columns with tie bars (TBCFST) and four CFST counterparts with concrete strength of about 125 MPa were examined. The test variables included the tie bar spacing, steel tube strength, and axial load ratio. It was found that when the height of the tie bar-confined region was equal to the steel tube width and the tie bar confinement index (λm) was in the range of 0.09 to 0.17, the local buckling of steel tube and concrete crushing were primarily concentrated in the unconfined zone adjacent to the tie bars. Finite element analysis was also conducted and showed that the ultimate drift ratio (θu) of the columns enhanced linearly with the increase in λm and confinement index of CFST (ξs), but diminished swiftly with an increase in the axial load ratio (n). For a given value of λm, θu increased with the increase in the tie bar columns (ntb). The flexural moment capacity ratio (Mtu/M0u) of the TBCFST column to the CFST counterpart increased as λm increased. The ratio Mtu/M0u did not follow a clear trend with the variation of ξs and n, but increased slightly with the increase in ntb. The minimum requirements for the design of tie bars were proposed.

High-strength rectangular concrete-filled steel tube long columns: Parametric studies and design

High-strength steel and concrete materials (i.e., steel with yield stress Fy > 525 MPa and concrete with compressive strength f'c > 70 MPa) have been applied in various types of structures because they can increase the resistance and space efficiency of the structure. However, the AISC Specification (AISC 360-16) does not permit the use of high-strength materials in concrete-filled steel tube (CFT) members due to a lack of adequate research. This paper aims at investigating the behavior of high-strength CFT long columns and evaluating the applicability of current design equations for high-strength rectangular CFT long columns. Firstly, an experimental database including 74 tests of high-strength rectangular CFT long columns was compiled and gaps in the database were identified. Then, finite element (FE) models were developed and benchmarked for parametric studies. The effects of width-to-thickness ratio of the steel tube, yield stress of steel, compressive strength of concrete, and length-to-depth ratio of the member were investigated. Finally, new design equations for high-strength rectangular CFT long columns were proposed based on the modified slenderness reduction factor in AISC 360-16.

Seismic performance of composite jacket-confined square high-strength concrete-filled steel tube columns

Adding external confining jackets in the potential plastic hinge regions 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 seismic behavior of composite jacket-confined square CFST columns with HSC, three jacket-confined CFST (JCCFST) columns and two CFST counterparts with concrete compressive strength of approximately 125 MPa were tested. The test variables included the jacket configuration and steel tube strength. The test results demonstrated that both the jacket-confined region and the adjacent unconfined region could be damaged when the jacket length was 1.25 times the steel tube width. Finite element analyses were also performed to comprehensively investigate the influence of the parameters on the column behavior. It was found that the ultimate drift ratio, θu, increased approximately in proportion to the effective jacket confinement index, λm, and the confinement index of the CFST section, ξs, and decreased rapidly as the axial load ratio, n, increased. The ratio of the moment capacity of a JCCFST column to that of the corresponding CFST column, Mju/M0u, increased as λm increased. The influence of n and ξs on Mju/M0u was not significant. Formulas for designing the confining jacket were also developed.

Numerical study on the behavior of eccentrically loaded square spiral-confined high-strength concrete-filled steel tube columns

Square spiral-confined concrete-filled steel tube (SCCFST) column with high-strength concrete (HSC) is a new type of composite column, which can give full play to high-strength concrete carrying capacity advantages and has good deformation capacity. A refined finite element model was developed to comprehensively investigate the compression behavior of SCCFST columns with HSC, and parametric analysis was carried out. The results demonstrated that the spiral stress developed slowly in the early loading stage but increased rapidly when the load was about 85% of the peak load. As a result, the compressive strength of concrete in the spiral-confined region was improved by about 14% when the compressive strength fc' = 111 MPa. However, the decrease in the axial load of the concrete outside the spiral-confined region offset this improvement. Thus adding the spiral had little effect (within 6%) on the load-carrying capacity of the columns while significantly improving the post-peak deformation performance. This improvement increased with the spiral confinement index increase. For a given spiral confinement index, the effect of decreasing the spiral spacing on improving the ductility of the column was the most significant. When the peak load was reached, except for the region near the neutral axis, the steel tube stress in most of the region reached the yield stress. Design expressions were derived using the plastic stress distribution method and assuming that the concrete strength in the compression zone is 0.8fc' to determine the axial load and flexural capacity of the SCCFST or CFST columns with HSC.

Experimental investigation of FRP-confined HSC-filled steel tube stub columns under axial compression

In this study, the axial compressive properties of fiber-reinforced plastic (FRP)-confined high-strength concrete-filled steel tube (HSCFST) stub columns were investigated, and the effects of the steel tube thickness, FRP type, concrete strength, and number of FRP layers were analyzed. The results reveal that fracturing of the FRP and bulging deformation of the steel tube are apparent when the stub column fails. An increase in the concrete strength, thickness of the steel tube, and number of FRP layers effectively improve the ultimate strength of the stub column. Compared with carbon fiber-reinforced plastic (CFRP), glass fiber-reinforced plastic (GFRP) has a more evident effect on improving the ultimate strength and delaying the stiffness degradation of stub columns. Further, a finite element model that accurately simulates the axial compressive behavior of FRP-confined HSCFST stub columns was established to better understand the stress development of the high-strength concrete (HSC), steel tube, and FRP. In addition, a calculation method for the peak strain and ultimate strength of FRP-confined HSCFST stub columns subjected to axial compression loads was developed; this method can accurately predict the actual test results.

Effect of creep on the strength of high strength concrete-filled steel tubes

High strength concrete filled steel tubes (HSCFSTs) are being increasingly used as columns in tall buildings, bridges, and other critical infrastructure and complex structural systems. Very little research has been done to study the performance of these columns under long-term sustained loads where creep effects take place, and no study exists on the long-term performance of these columns under eccentric sustained loads at the large scale. Six large scale (2700 mm height) HSCFSTs were tested in this paper under sustained eccentric compression for a period of five months. After that, the columns were tested to failure and their performance was compared to another six companion HSCFSTs that were kept unloaded for the same time period. A decrease of about 13% in the load carrying capacity was observed after creep. It was also observed that creep significantly changed the force distribution between steel and concrete. A study of the force redistribution showed that the force carried by the steel increased by >50% of its initial value.

Eccentrically loaded concrete-filled steel tubes made with high strength materials

High-strength welded steel sections filled with high-strength concrete, so called high-strength concrete filled steel tubes (HSCFSTs), are widely used in multi-story buildings and large span bridges to achieve superior structural performance with optimized cost. Many experimental and numerical investigations have been conducted on the behaviour of small and large scale HSCSFTs under centric axial compression. However, very few studies have investigated the response of large scale HSCFSTs under eccentric loads. This paper presents an experimental investigation conducted on five square HSCFSTs that are 2.7 m height under eccentric axial compression. The effects of loading eccentricity and depth-to-thickness ratios on the behaviour of HSCFST columns were investigated. The experimental results reveal that the failure mode of HSCFSTs was governed by local buckling of the steel tube in all cases. A finite element model was also developed in ABAQUS and was validated against the test results. Sensitivity and parametric studies that investigate the effect of various parameters of the concrete damaged plasticity model, imperfections, concrete and steel strengths, load eccentricity and plate slenderness are also presented.

Experimental study on the flexural behavior of square high-strength concrete-filled steel tube beams with different CFRP wrapping schemes

Recently, carbon fiber reinforced polymer (CFRP) materials have been recommended for strengthening concrete-filled steel tube (CFST) members. In this paper, experimental study is presented to further investigate the flexural behavior of square high-strength concrete-filled steel tube beams with different CFRP wrapping schemes. Results reveal that all of the specimens display the failure of longitudinal fibers at the bottom of the tensile zone when reaching the ultimate state. Meanwhile, the performance of CFRP confined CFST (CFRP-CFST) beams is improved significantly due to the transverse confinement and longitudinal reinforcement of CFRP sheets. For example, the moment capacity of CFST beams of fcu = 62.8 MPa and 128.2 MPa increased by up to 22 % and 34 %, respectively, strengthened with the full scheme. Moreover, for CFST beams of fcu = 128.2 MPa, the half scheme achieved the similar enhancement effect (29 %) in comparison with the full scheme. Based on the experimental results, a simplified approach using the fiber element method is proposed to predict the moment–curvature relationship and ultimate moment capacity of the CFRP-CFST beams. The predictions are validated with the experimental results from this paper and other study. A stepwise analytic approach is developed for designing CFRP-CFST flexural members.

Finite Element Analysis of the Mechanical Properties of Axially Compressed Square High-Strength Concrete-Filled Steel Tube Stub Columns Based on a Constitutive Model for High-Strength Materials.

With the development of new concrete technology, high-strength concrete has been used worldwide. In particular, more economic benefits can be achieved by applying high-strength concrete-filled steel tube (HSCFST) columns in the concrete core walls of super high-rise buildings. A constitutive relation with high applicability for high-strength materials with different strength grades is proposed. Based on this constitutive model, a brick element model of 181 sets of axially compressed square HSCFST members is established and experimentally verified. The effects of the concrete strength, diameter-to-thickness ratio, and steel yield strength on the axial compressive capacities of these members were investigated based on finite element calculation results. The results showed that with an increase in the concrete strength, the ultimate bearing capacities of CS-CC, HS-HC, HS-CC, and CS-HC stub column members increased by 60%, 24%, 44%, and 21% at most, respectively. Additionally, as the steel yield strength increased, the ultimate bearing capacities of CS-CC, HS-HC, HS-CC, and CS-HC stub column members increased by 8.8%, 5.1%, 8.5%, and 5.2%, respectively, Hence, material strength has the greatest impact on CS-CC and HS-CC. The confinement effect of the square steel tube on the concrete weakens as the strength grade of steel or concrete increases. Notably, the confinement effect of steel tube on the concrete is strongest in CS-CC and weakest in the CS-HC. In addition, the confinement coefficients of square HSCFST stub columns with different combinations of concrete and steel strengths were analyzed. Based on the superposition principle in the ultimate state, a practical axial compressive capacity calculation formula for three types of square HSCFSTs is established. Compared with existing major design code formulas, the proposed formula is more accurate and concise and has a clear physical meaning.

Seismic behavior of square spiral-confined high-strength concrete-filled steel tube columns under high axial load ratio

Adding internal spiral reinforcement is an effective and practical approach to enhancing the ductility of square concrete-filled steel tube (CFST) columns with high-strength concrete (HSC). To investigate the seismic behavior of CFST and spiral-confined concrete-filled steel tube (SCCFST) columns with HSC under high axial load ratio, four CFST columns and four SCCFST columns with concrete strengths not less than 110 MPa were tested under combined axial and lateral loading. The test variables included the concrete compressive strength, the axial load ratio, and the lateral-loading protocol. The test results demonstrated that adding spiral reinforcement can offset the reduction of ductility caused by the increase of the axial load ratio. The ultimate drift ratios of the columns under an axial load ratio of 0.5 increased from approximately 2.0% to 3.5% after adding spiral reinforcement with a volumetric ratio of 3.13%. Increasing the concrete strength from 110 to 140 MPa essentially did not affect the deformation capacity of the columns. Due to the cyclic degradation effect, the deformation capacity of the specimen under cyclic loading was much smaller than that of the corresponding specimen under monotonic loading. The column axial shortening accumulated during cyclic lateral loading and increased exponentially when significant strength degradation started. The rotation at the column base contributed a significant portion to the total lateral displacement, indicating the necessity of considering the flexibility of the column base in structural analysis.

Use of the Gene-Expression Programming Equation and FEM for the High-Strength CFST Columns

The ultimate strength of composite columns is a significant factor for engineers and, therefore, finding a trustworthy and quick method to predict it with a good accuracy is very important. In the previous studies, the gene expression programming (GEP), as a new methodology, was trained and tested for a number of concrete-filled steel tube (CFST) samples and a GEP-based equation was proposed to estimate the ultimate bearing capacity of the CFST columns. In this study, however, the equation is considered to be validated for its results, and to ensure it is clearly capable of predicting the ultimate bearing capacity of the columns with high-strength concrete. Therefore, 32 samples with high-strength concrete were considered and they were modelled using the finite element method (FEM). The ultimate bearing capacity was obtained by FEM, and was compared with the results achieved from the GEP equation, and both were compared to the respective experimental results. It was evident from the results that the majority of values obtained from GEP were closer to the real experimental data than those obtained from FEM. This demonstrates the accuracy of the predictive equation obtained from GEP for these types of CFST column.

Behaviour and design of eccentrically loaded CFST columns with high strength materials and slender sections

High strength concrete-filled steel tube (CFST) columns have been adopted in many constructions requiring great load-carrying capacities, large floorplans, cost-effective and time-saving solutions such as modern skyscrapers or high-rise buildings, but their performance is still insufficiently understood due to the lack of experimental investigations and design specifications. The test program in this study consists of eight stub square CFST columns fabricated from high strength materials and slender sections which are beyond the permitted limits specified by many current design provisions. The failure patterns, load and moment resistances, load-mid-height deflection and load-axial strain responses were used to evaluate the overall performance of the test specimens. Furthermore, a numerical model for CFST columns, taking into account the actual factors such as the confinement on concrete cores, initial imperfections and residual stresses on steel tubular members, was developed and demonstrated a good agreement with the experimental results. A parametric study then adopted this verified model to explore the confinement effect on concrete cores and the influence of various material strengths, eccentricity ratios, section slendernesses and member slendernesses on the performance of composite columns. Finally, the load-moment interaction curves of specimens under different loading conditions obtained from experiment and numerical analysis were compared to the predictions of AISC 360-16, Eurocode 4 and AS/NZS 2327 for validating these specifications. In short, this study is expected to advance the understanding in the area of eccentrically loaded CFST columns with high strength materials and slender sections.

Seismic behavior of high-strength concrete-filled square steel tube columns reinforced with ultrahigh-strength reinforcing bars

This paper investigated the seismic behavior of high-strength concrete-filled square steel tube columns reinforced with ultrahigh-strength reinforcing bars. Three full-scale specimens with different amounts of ultrahigh-strength longitudinal reinforcing bars under a high axial load ratio and cyclic lateral load were tested. The test results demonstrate that the existence of ultrahigh-strength longitudinal reinforcing bars had a significant influence on the seismic behavior of high-strength concrete-filled steel tube columns. Compared with conventional concrete-filled steel tube (CFT) columns, the deformation capacity, bearing capacity and cumulative energy dissipation of these innovative CFT columns are greatly improved, and the cumulative damage and plastic deformation of the specimens decreased with the introduction of steel bars. A calculation model based on AISC360-16 using the superposition method was proposed and verified to evaluate the compressive-flexural strength of the specimen.

Finite Element Analysis and Theoretical Verification of Natural Vibration of Novel Simply-supported Beams

In order to investigate the dynamic characteristics of I-shaped novel simply-supported beams with the upper flange of rectangular high-strength concrete filled steel tube (RHCFST), the modal analysis of 16 simply-supported beams is conducted by ANSYS finite element software. The first order frequencies of the specimen beams are obtained in the Y direction. Based on the concentrated mass method, the frequencies of the specimens are theoretically calculated. By comparison the maximum error of the first-order frequency is 3.80%, which is in good agreement and can meet the precise requirements of practical engineering. The result shows that the first three order vibration modes of the specimen are consistent with the theoretical vibration modes. For the first-order vibration mode, the proportion of the effective mass to the total mass of the specimen is 79.2 % in the Y direction. Therefore, the first-order vibration mode in Y direction should be mainly taken into consideration for this type of beams during the structural design.

Axial compressive performance of CFRP confined rectangular CFST columns using high-strength materials with moderate slenderness

The bearing capacity of concrete-filled steel tube (CFST) columns can be significantly increased by using high-strength materials. However, the ductility of high-strength CFST columns is reduced due to the brittleness of high-strength concrete and the buckling of thin-walled high-strength steel tubes, especially for slender columns. By wrapping the carbon fiber reinforced polymer (CFRP) to increase the confinement on the steel tube, the ductility can be improved. This paper presents experimental and theoretical studies on the axial compressive performance of CFRP confined rectangular CFST columns using high-strength steel and concrete with moderate slenderness. The influence of the section aspect ratio, concrete strength, and CFRP confinement on the axial behavior was analyzed. The experimental results indicate that the local buckling of CFRP-confined specimens is delayed compared with the unconfined specimens. The horizontal CFRP confinement increases the yield load by 1.5–25.6%, and the peak load by 2.5–11.2%. The bearing capacity and ductility of specimens are improved under the confinement of CFRP. The calculation method for axial compression bearing capacity considering the stability of moderate slender specimens was proposed. The ratio of the experimental results to the calculated value is between 0.86 and 1.09, which verified the accuracy of the calculation method.

Behaviour and design of high strength CFST columns with slender sections

The behaviour of high strength concrete-filled steel tube (CFST) columns with slender sections under concentric loading is experimentally explored in this study. The test program includes a series of six CFST columns made of welded built-up square tube with very high strength materials and different width-to-thickness ratios. The performance of these specimens is evaluated under compression based on their failure mode, ultimate strength, strain development and axial load-shortening relationship. Moreover, a numerical model is developed considering the influence of initial local imperfections, residual stress of the steel tube and material models are proposed for composite columns. The finite element model (FEM) is verified using the experimental results and is shown to provide good predictions of the mechanical properties of CFST. Then the FEM is used to investigate the equation of AS/NZS 2327 code and propose a modified form factor for predicting the loading capacity of square CFST columns under axial compression. A database of 185 test samples is compiled with a wide range of concrete grade and section slenderness to evaluate the accuracy of the newly proposed form factor equation. Furthermore, the study also examines the optimized section of CFST columns in terms of performance and cost savings.

Seismic performance of RC columns with encased prefabricated high-strength CFST core

This paper proposed an innovative RC column with encased prefabricated high-strength concrete filled steel tube core, and four RC columns with encased prefabricated high-strength CFST core and a RC control-column were tested under lateral low cyclic loading. All specimens were evaluated by the cracks developments, failure patterns, hysteretic behavior, skeleton curves, strength and stiffness degradation, ductility and energy dissipation capacity. The effects of stirrup ratio and welding studs of prefabricated CFST core were investigated in details. The experiment results indicated that compared with the RC control-column, the performances of RC columns with encased prefabricated high-strength CFST core, including the hysteretic behavior, strength degradation, ductility and energy dissipation, were significantly improved. Higher stirrup ratio of the RC column with encased prefabricated high-strength CFST core leaded to higher ductility and more satisfactory energy dissipation capacity, stiffness degradation. Studs could effectively combine prefabricated high-strength CFST core and surrounding concrete, which significantly increase the integrity of RC column with encased prefabricated high-strength CFST core. Based on the test results, a numerical model was established to further analyze the cyclic behavior of the test specimens, and the numerical results agreed well with the test results, which showed the feasibility for the further parametric study. Finally, on the basis of the plastic stress theory, a calculation model for seismic bending moment capacity of RC column with encased prefabricated high-strength CFST core was established, and the results obtained form the formulas showed good agreement with the experiments.