High-strength Concrete-filled Steel Tube Research Articles

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.

Behavior of eccentrically loaded square spiral-confined high-strength concrete-filled steel tube columns

Concrete-filled steel tube (CFST) columns have been widely used in super tall buildings. Since these columns are subjected to very high axial loads, the use of high-strength concrete (HSC) is preferred in order to reduce their cross-sectional size. For applications in seismic regions, additional confinement to the HSC may be needed to ensure ductility. Among various approaches, spiral confinement is an effective and practical way for square CFST columns. Six spiral-confined concrete-filled steel tube (SCCFST) columns and three CFST columns with the concrete compressive strength of 111 MPa were tested under eccentric axial loading. The behavior of the columns under different levels of eccentricity ratio was investigated, and the use of high-strength spirals to further enhance confinement was also examined. Concrete crushing was found to be the main cause for the loss of the load-carrying capacity. Spiral reinforcement is not effective in increasing the load-carrying capacity; however, it can considerably improve ductility, which is improved with an increase of the yield strength of spiral reinforcement. Based on the test results, an equivalent uniaxial stress–strain model was developed for the HSC in CFST and SCCFST columns. A fiber beam-column element using the proposed stress–strain models was found to provide good predictions of the measured load–displacement responses. Such fiber element can be used for modeling of SCCCFST columns with HSC in nonlinear structural analysis of tall buildings.

Seismic performance assessment of a precast concrete-encased CFST composite wall with twin steel tube connections

A new type of Precast Concrete-encased high-strength Concrete-filled Steel tube (PCCS) composite shear wall with twin steel tube connections is proposed. Six two-storey PCCS wall specimens were prepared and tested under reversed quasi-static cyclic loading. The axial load ratio, the out-of-plane load eccentricity and the connection of the outer steel tubes are considered in this investigation of the assembly integrity and seismic performance of the PCCS walls. The experimental results show that the PCCS walls exhibit good assembly integrity and satisfactory seismic performance in terms of hysteresis behaviour, ductility, stiffness degradation and energy dissipation capacity. The use of bolts to radially fasten the inner and outer steel tubes can constrain the twin steel tubes. The out-of-plane load eccentricity significantly influences the cyclic behaviour. The specimens with large load eccentricity still have good deformation and ductility capabilities, which can be attributed to the ability of the embedded high-strength concrete-filled steel tube columns to improve the out-of-plane flexural capacity of a PCCS wall. Finally, the PCCS walls are compared with other typical precast concrete walls and cast-in-place concrete-encased concrete-filled steel tube composite walls in terms of ductility capacity, and a calculation model for shear bearing capacity is proposed and evaluated.

Compressive performance of steel fiber-reinforced rubberized concrete core detached from heated CFST

In this study, the compressive behavior of concrete cores detached from the high-strength concrete-filled steel tube (CFST) members reinforced with steel fibers and containing crumb tire rubber was investigated experimentally after exposure to elevated temperatures. The test variables included the diameter-to-thickness ratio of the steel tube, volume content of crumb rubber replacing natural sand, volume fraction of steel fibers, and temperature. After the exposure of the CFST specimens to the elevated temperatures and subsequently removing the steel tubes, the detached concrete core was subjected to the axial compression test, and the parameters of compressive strength, modulus of elasticity, strain at peak stress, and weight loss, stress-strain relationship together with the ultrasonic pulse velocity, visual appearance, and failure mode were evaluated. Using the experimental results, expressions were developed to predict the mechanical properties of the concrete core at elevated temperatures taking into account the steel tube wall thickness. The findings indicated that increasing the thickness of the steel tube had a greater damaging effect on the heated concrete core, while in the fiber-reinforced specimens, this damaging effect was lower. Moreover, the addition of crumb rubber to the concrete mix as the natural sand replacement degraded the mechanical properties of the heated and unheated concrete core.

Interfacial Bond Behavior of High Strength Concrete Filled Steel Tube after Exposure to Elevated Temperatures and Cooled by Fire Hydrant

For the engineering structure in case of fire, a fire hydrant is generally used for extinguishing the fire. This paper presents an experimental investigation on interfacial bond behavior of high-strength concrete-filled steel tube (HSCFST) after exposure to elevated temperatures and cooled by fire hydrant using the pull-out test of 22 specimens. According to the experimental study, the failure mechanism of HSCFST exposed to elevated temperatures and water-cooling (ETWC) was revealed, the influence of various parameters on the bond behavior was analyzed, and the calculation formula of the bond strength of HSCFST subjected to ETWC was put forward. The results show that the load-slip curves of the loading end and the free end of the specimen are basically similar, and can be divided into three types of typical curves. In the push out test, the strain on the outer surface of the steel tube is exponentially distributed with its distance from the loading end. After ETWC exposure, the bond strength of the specimen is less affected by the concrete strength, which is inversely proportional to the anchorage length, and it is basically stable after the constant temperature duration is longer than 60 min. With the increase of the maximum temperature, the ultimate bond strength increases first, then decreases and then increases, and the residual bond strength increases first and then decreases. Besides, the study indicate that cooling method has significant influence on the bond behavior, compared with natural cooling specimens, the ultimate bond strength, residual bond strength, and shear bond stiffness of water-cooling specimens are smaller, and the interfacial energy dissipation capacity is larger.

Creep of concrete-filled steel tube considering creep-recovery of the concrete core

Concrete creep results in the strain increment of concrete-filled steel tube. In this article, a new method was established for creep prediction of concrete-filled steel tube considering the creep-recovery effect of the concrete core with creep model and separate creep-recovery model, named two-function method. Five creep models including the recent proposed B4 model and two separate creep-recovery models were used in the method. First, the creep models were compared with 197 test curves of creep of sealed concrete. The results show that the B4 model offers the highest prediction accuracy for load duration time longer than 200 days. Then, the two-function method was used for creep prediction of normal strength concrete-filled steel tube and high-strength concrete-filled steel tube. For comparison, the creep prediction method for concrete-filled steel tube based on the principle of superposition was also adopted. The principle of superposition did not predict the creep of concrete-filled steel tube accurately. The calculation results based on the two-function method are closer to the test data, especially for normal strength concrete-filled steel tube. Finally, the uncertainty of creep in concrete-filled steel tube was analyzed considering the variability of the concrete strength. The results indicate that the variances calculated by the methods with the MC 2010 creep model are smaller than that of the B4 model. The creep strains of concrete-filled steel tube calculated by the two-function method are subject to normal distribution.

Model Experiment of Construction Process of Long-Span Concrete Arch Bridge Rigid Skeleton

High-speed railway passenger dedicated line have high demands for track smoothness, beipanjiang bridge as shanghai-kunming high-speed railway control project, mainspan 445m, constructed by using rigid skeleton, C80 high-strength concrete filled steel tube structure for the first time used in railway bridge; rigid skeleton in the process of construction has the traits: long span, long construction period, multi-step, mechanical behavior was extremely complex. In this paper, the design principle and similarity relation of rigid skeleton model are introduced, simplified the model construction process, and the finite element simulation analysis model is established, compared with measured values, it is shown that model test can well simulate the long span rigid skeleton arch bridge stress state in the construction process; the original construction steps have more safety reserves than simplified; the indicators of model test are within the scope of design, the results can provide technical support for the bridge construction.

Experimental evaluation of high-strength concrete-filled steel rectangular tube columns under axial compression

This paper presents the effect of the high-strength concrete on the behavior and strength of concrete-filled steel tube (CFST) columns. To evaluate the structural performance, load-axial shortening responses, failure modes and axial load capacity are performed for ten high-strength concrete-filled steel tube (HSCFT) columns. All of the columns are 750 mm in height, and have compressive strength of 66 MPa. Test parameters included the wall thicknesses of the rectangular tube (3.0 mm, 4.5 mm and 6.0 mm) and three different of rectangular cross-sections (150×50 mm, 150×75 mm and 150×100 mm). From the experimental results, it is indicate that the load-axial shortening response of the columns have a linear behavior up to approximately 80-90% of their axial load capacity. Then, the behavior of the HSCFT columns is nonlinear with a very large deformation before failure. It is also shown that the axial load capacity and ductility of the HSCFT columns are increased significantly compared to the RC columns (reference columns), depending mainly on the wall thickness and cross-section of the hollow steel tube. Finally, by comparing the axial load capacity of the test results with those obtained from the AISC/LRFD design equation, the comparison results show that calculation formula in AISC/LRFD specification can be applied to compute the axial load capacity of HSCFT columns under axial compression.

Axial Compression Behavior of the High-Strength Concrete Squat Wall with Distributed Steel Tubes

Three high-strength concrete walls with distributed steel tubes (HC-DST squat walls) and three high-strength concrete-filled steel tubes (CFSTs) were tested to study the mechanical behavior of this type of squat wall. The strains of the steel tubes and the failure mode and axial force distribution of the squat wall were analyzed. Then, the mechanism behavior and the axial capacity formula for this type of shear wall were obtained. The rotation angle of the principal strain of the steel tube was between 3° and 7°. Therefore, the minimum principal strain and the maximum principal strain of the steel tube were replaced by the longitudinal compressive strain and the circumferential strain of the steel tube, respectively. The axial force of each part of the squat wall was distributed according to the vertical compressive stiffness before the peak load. Finally, the parametric analysis of the HC-DST squat wall was performed using the finite element method. The analysis results indicate that the axial capacity of the HC-DST squat wall increases with the increase of the confined area ratio or steel area ratio and decreases with an increase in the confinement coefficient.

Mechanical behavior of a steel tube-confined high-strength concrete shear wall under combined tensile and shear loading

The steel tube-reinforced concrete (STRC) shear wall is a type of composite shear wall that consists of a reinforced concrete shear wall embedded into high-strength concrete-filled steel tube (CFST) columns in the boundary element and/or the center of a shear wall. The use of STRC walls has gained popularity in the construction of high-rise buildings because their performance is superior to conventional reinforced concrete (RC) walls. To study the influence of various parameters (i.e., the number of CFSTs embedded in the shear wall, the shear span ratio, the ratio of the vertical reinforcement, the strength of the concrete outside the embedded steel tube, and the initial tensile stress) on the mechanical properties of an STRC composite shear wall under combined tension and shear forces, tensile shear tests were conducted on 9 specimens whose shear span ratios were less than or equal to 1.0. The experimental results demonstrated that the STRC shear wall retained its high shear capacity and its good ductility under a high axial tensile force. Using statistical analyses, a formula to calculate the bearing capacity of an STRC shear wall was proposed. The calculated values are in good agreement with the experimental results.

High-Strength Rectangular CFT Members: Database, Modeling, and Design of Short Columns

AISC 360-16 (the current AISC Specification) does not endorse the use of high-strength materials (Fy≥525 MPa and fc′≥70 MPa) for concrete-filled steel tube (CFT) columns because of a lack of adequate research and comprehensive design equations. This paper makes a contribution toward addressing this gap and proposes effective stress-strain relationships and design equations for high-strength rectangular CFT members using a three-step method. The first step consists of compiling the experimental database of high-strength rectangular CFT column tests in the literature and evaluating the possibility of extending the current AISC 360-16 design equations to high-strength rectangular CFT short columns. The second step consists of developing and benchmarking detailed three-dimensional (3D) nonlinear finite-element models for predicting the behavior of high-strength CFT columns from the database. The benchmarked models are then used to perform comprehensive parametric studies to (1) address gaps in the database and (2) develop effective stress-strain relationships for modeling the steel tube and concrete infill of high-strength rectangular CFT members, while indirectly accounting for the effects of confinement, yielding, and local buckling. The third step consists of using these effective stress-strain relationships and the enhanced database to propose a new design approach (including equations) for high-strength rectangular CFT short columns. Finally, a reliability analysis is performed to establish a resistance (strength reduction) factor (ϕ) to be used with the proposed design equations.

Properties of High-Strength Concrete Filled Steel Tube Columns

Properties of High-Strength Concrete Filled Steel Tube Columns

Numerical study on temperature distribution of high-strength concrete-filled steel tubes subjected to a fire

High-strength concrete-filled steel tube (CFST) columns offer a number of benefits and are widely used in high-rise building construction. This paper presents a brand new thermal modeling approach in comparison with tremendous test data for the heat transfer analysis. A heat transfer model has been developed to predict the thermal response of high-strength CFST columns under standard fire conditions with consideration of a number of parameters: steel and furnace emissivity, thermal interface conductance and concrete strength. The verified numerical models discussed the variation of emissivity of steel surfaces and thermal interface conductance in fire. It can be concluded that the influence of emissivity and thermal interface conductance is considerable in the numerical analysis. It is also demonstrated that thermal behavior of high-strength CFST columns subjected to fire during heating and cooling stages, and providing the guidance on predicting thermal response of high-strength concrete-filled steel tube columns.

Experimental investigation of axially loaded steel fiber reinforced high strength concrete-filled steel tube columns

An experimental study on the compressive behavior of steel fiber reinforced concrete-filled steel tube columns is presented. Specimens were tested to investigate the effects of the concrete strength, the thickness of steel tube and the steel fiber volume fraction on the ultimate strength and the ductility. The experimental results indicate that the addition of steel fibers in concrete can significantly improve the ductility and the energy dissipation capacity of the concrete-filled steel tube columns and delay the local buckling of the steel tube, but has no obvious effect on the failure mode. It has also been found that the addition of steel fibers is a more effective method than using thicker steel tube in enhancing the ductility, and more advantageous in the case of higher strength concrete. An analytical model to estimate the load capacity is proposed for steel tube columns filled with both plain concrete and steel fiber reinforced concrete. The predicted results are in good agreement with the experimental ones obtained in this work and literatures.

INFLUENCE OF THE LOAD TRANFERENCE METHOD ON SHORT TUBE CONCRETE ELEMENTS FILLED WITH HIGH-STRENGTH CONCRETE

This article deals with advantages and disadvantages of use of concrete filled with steel tube structures with cores of high-strength concrete. Characteristics of used materials are shown, including the physical and mechanical properties of high-strength concrete compositions designed with using the local materials. Results of experiments of short centrally compressed structural elements of steel tubes filled with high-strength concrete and according to the load transference onto complex cross-section and concrete core are presented. It has been determined the capacity relatively the deformations and it has been found the fracture character according to the way of loading and concrete core strength. The method of transference of the load results in efficiency of tube concrete element from the high strong. Load transfer method impacts the efficient of high-strength concrete filled steel tubes structure element. Carrying capacity of test samples N1 increased by 8 – 20% when load is transferred to concrete compared to complex cross-section. Longitudinal deformation decreases during the load transfer onto concrete core compared to complex cross-section load transfer.

Experimental and theoretical studies of confined HSCFST columns under uni-axial compression

The development of modern concrete technology makes it much easier to produce high-strength concrete (HSC) or ultra-high-strength concrete (UHSC) with high workability. However, the application of this concrete is limited in practical construction of traditional reinforced concrete (RC) structures due to low-ductility performance. To further push up the limit of the design concrete strength, concrete-filled-steel-tube (CFST) columns have been recommended considering its superior strength and ductility performance. However, the beneficial composite action cannot be fully developed at early elastic stage as steel dilates more than concrete and thereby reducing the elastic strength and stiffness of the CFST columns. To resolve this problem, external confinement in the form of steel rings is proposed in this study to restrict the lateral dilation of concrete and steel. In this paper, a total of 29 high-strength CFST (HSCFST) columns of various dimensions cast with concrete strength of 75 to 120 MPa concrete and installed with external steel rings were tested under uni-axial compression. From the results, it can be concluded that the proposed ring installation can further improve both strength and ductility of HSCFST columns by restricting the column dilation. Lastly, an analytical model calculating the uni-axial strength of ring-confined HSCFST columns is proposed and verified based on the Von-Mises and Mohr-Coulomb failure criteria for steel tube and in-filled concrete, respectively.

Behaviour of high-strength concrete filled steel tubes under transverse impact loading

This paper presents a set of new test data for high strength concrete filled steel tubular (CFST) members subjected to transverse impact. A total of 12 circular CFST specimens and reference hollow steel tubes were tested in a drop hammer rig. The cube strength of the concrete used for the CFST members is up to 75MPa. A finite element analysis (FEA) model is established to predict the impact behaviour of high strength CFST members, and the accuracy of which is then verified by the presented test results. Full-range analyses of the behaviour of CFST members under impact loading are then carried out using the FEA model to produce the force state, internal force distribution, and flexural capacity. Finally, a simplified model is obtained based on a parametric analysis to calculate the flexural capacity of CFST members under impact load.

일정축력하에 고온을 받는 고강도 콘크리트 충전강관 기둥의 구조적 거동에 관한 연구

Fire-resistant (FR) test data for a square concrete-filled steel tube (CFT) columns consisting of high-strength steel (fy>650MPa) and high strength concrete (fck>100MPa) under axial loads are insufficient. The FR behavior of square high-strength CFT members was investigated experimentally for two specimens having with two axial load cases (5,000kN and 2,500kN). The results show that the FR performance of the high-strength CFT was rapidly decreased at earlier time (much earlier at high axial load) than expected due to high strength concrete spalling and cracks. In addition, a fiber element analysis (FEA) model was proposed and used to simulate the fiber behaviour of the columns. For steel and concrete, the mechanical and thermal properties recommended in EN 1994-1-2 are adopted. Test results were compared to those of numerical analyses considering a combination of temperature and axial compression. The numerical model can reasonably predict the time-axial deformation relationship.