Circular Concrete-filled Steel Tube Research Articles

Structural performance of composite steel rubberised concrete members under combined loading conditions

AbstractThis paper investigates the structural behaviour of rubberised concrete‐filled steel tubular (RuCFST) members under a wide range of axial‐bending loading conditions. The results of an experimental programme on circular concrete‐filled steel tubes (CFST), incorporating conventional and rubberised concrete (RuC) materials with various rubber content ratios, are presented. After describing the specimen details and testing arrangements, the influence of the RuC infill on the behaviour of test members in terms of moment‐axial interaction and ductility, is examined. The test results show that whilst the cross‐sectional capacity of RuCFST members is reduced with the increase in rubber content, significantly higher ductility is obtained compared with conventional counterparts. An enhancement of about 85% in ductility is obtained for members with 60% rubber content compared to those with conventional concrete. Finally, the test results are compared with current design guidelines, indicating that the latter tend to overestimate the capacity, particularly for relatively high rubber contents.

Comparison for Solid Concrete-filled Circular Steel Tubes Load-carrying Capacities in Frame Structures Basing on Chinese codes

Solid steel tubular concrete members have excellent axial compression capacity. In terms of the calculation method of their axial compression bearing capacity, the current ten domestic standards all provide specific provisions. On the basis of main structure provisions such as diameter-thickness ratio and confinement factor, the formula differences in axial-force compression capability of domestic and international standards are compared and analyzed. First, the difference value of bearing capacities reach up to 62.79% according to current standards when diameter-thickness ratio range from 4 to 30. Second, concrete-filled steel tube members with diameter-thickness ratio greater than 20 and diameter less than 200, the current specifications cannot calculate the axial compressive bearing capacity.

Compatibility Optimal Design of Axially Loaded Circular Concrete-Filled Steel Tube Stub Columns.

Numerous studies have been carried out on the axially loaded circular concrete-filled steel tube (CCFST) stub columns. However, to date, no clear evaluation criterion for the compatibility of its design parameters has been established. In the present study, the compatibility of the design parameters (concrete compressive strength , steel yield strength , diameter D and thickness of steel tube t) of axially loaded CCFST stub columns was quantitatively investigated in terms of the contribution of the composite actions to the axial bearing capacity of the columns. The composite ratio was proposed as an indicator to represent the effectiveness of the composite actions. A numerical framework of the determination of was established, making use of a series of existing widely recognized constitutive models of structural steel and concrete. Some modifications were carried out on these models to ensure the numerical stability of the presented analysis. Moreover, the rationality of the combined use of these models was verified. The analytical results show that excessive or very small ratio should be avoided in design. Meanwhile, the combined use of low-strength steel and high-strength concrete should be avoided. A table of optimal ratios corresponding to different material strength matches was provided for designers. Finally, an optimization of the design parameters using the proposed method and the existing design specification was performed.

Compressive behaviour and design of compact to slender octagonal concrete-filled steel tubular stub columns

This paper presents an experimental investigation into the compressive behaviour of compact to slender octagonal concrete-filled steel tubular (OctCFST) stub columns. In this study, a total of twenty OctCFST stub column specimens were tested. Five octagonal hollow sections with various cross-sectional slenderness were fabricated from 3 mm and 6 mm 460 steel plates. The measured yield strengths of the two batches of steel plates are 546.5 and 580.7 MPa, respectively. Three grades of concrete with measured cylinder strengths varying from 27.9 to 92.1 MPa were used to infill the hollow tubes. The test results, in terms of failure modes, axial load versus axial strain responses and cross-sectional capacities of the stub column tests are fully presented and discussed. The experimental results were then used to evaluate the applicability of current design methods in Eurocode EN 1994-1-1, American standard ANSI/AISC 360-16, Chinese code GB 50936-2014 and equivalent circle methods. It is shown that the circumcircle method and GB 50936-2014 approach produce excellent cross-sectional strength predictions for the OctCFSTs and the current cross-section slenderness limit of 135×235/fy for circular concrete-filled steel tubes specified in GB 50936-2014 can be safely extended to the design of OctCFSTs when the equivalent circumcircle method is used.

Performance of new CFST square column-to-foundation connections for cyclic loads

Although the structural behavior of circular Concrete Filled Steel Tube (CFST) columns-to-foundation connections has been studied substantially, there are limited studies on connections for square CFST columns, which are more in demand in high-rise buildings. The acceptable behavior of these columns for their use in high-rise buildings require sufficient stiffness for controlling the drift, safe and efficient transfer of imposed loads, and adequate ductility for withstanding earthquake loads. In the present study, authors aim to develop two such column-to-foundation connections for the square CFST columns. The performance of the two square CFST column-to-foundation connections was assessed experimentally as well as numerically under cyclic lateral loading. The comparison of the connections was made with one of the prevalent square CFST column-to-foundation connections and a conventional RC connection to study the inelastic deformation capacity of connections under cyclic loads. The test results illustrate the efficient and improved behavior of the suggested connections. Numerical models were also developed, and the experimentally obtained results were predicted and compared. The developed numerical models were capable of predicting the experimental response of connections to large drift levels with acceptable accuracy. The numerical results were reasonably close to the experimental response.

Analytical model for circular high strength concrete filled steel tubes under compression

It is well known that the effect of confinement in concrete filled steel tube (CFST) can enhance the axial compressive strength of the concrete core. The confining pressure is a key issue to determine the compressive strength of concrete while the development of confining pressure is related to the lateral responses of steel tube and concrete core. This paper firstly presents a review on the lateral behaviour of circular CFST and existing lateral-axial strain models for concrete. These models are assessed by the authors’ previous experimental observations. Assessment results reveal deficiencies of current lateral-axial strain models, especially on the CFST with high strength concrete. A modified lateral-axial strain model for both normal and high strength concrete core in CFST was then proposed to accurately capture the interaction behaviour of the steel tube and the concrete. An analytical model of CFST with uniformly confined concrete core (i.e. with circular section) was then proposed to capture the development of confining pressure in CFST. A reduction factor was considered when the high strength concrete was used. The proposed analytical model can well predict the load bearing capacity, load-shortening curve, axial strain-lateral strain relationship of CFST with normal and high strength concrete.

Experimental study of axial compression behavior of circular concrete-filled steel tubes after being loaded at an early age

This paper presents an experimental study on the effects of very early age loading of concrete-filled steel tubular (CFST) short columns on their long-term axial compression behaviors. The specimens include CFSTs wrapped in FRP (fiber reinforced polymer) cloths and bare CFSTs. When concrete is at an age of 1d, 2d or 3d, the circular CFST column specimen is loaded with axial compression, unloaded after reaching the peak value, and then put into the curing room to continue curing. At the age of 28d, the axial compression bearing capacity is tested and the load–displacement curve is obtained. In the tests, five CFST columns are set as the control group, which are directly cured until 28d. The effects of initial loading age, steel tube wall thickness and constraint level are investigated. The results show that very early age loading can improve the long-term load bearing performance of CFST columns. For the configurations applied in the present study, the ultimate bearing capacity are increased by 1.01% to 24.59%, the yield loads by 2.33% to 52.53%, and the combined elastic moduli are also increased.

Hysteresis Performance of Through Bolted Connections in Steel Beam to Circular CFST Column

Concrete Filled Steel Tubes (CFSTs) with through bolt connections are demonstrated to be satisfactory for seismic performance. Due to better confinement effect, circular CFST columns possess higher strength to weight ratio compared to square CFST. The only limiting factor in application of circular CFST in construction is the difficulty in joining with the beams. This paper presents an experimental study on hysteretic behavior of interior joint between circular CFST column and steel beam using extended end plates with through bolt. The study focuses on the comparison of connection using flat extended end plate with straight bolts and curved extended end plate with split bolt assembly for circular CFST columns. The experimental results were analysed in terms of strength, stiffness, ductility, failure mode and energy dissipation for the evaluation of cyclic performance. The results indicate that the seismic resistance agrees with seismic recommendations of American Institute of Steel Construction (AISC) and the split bolt assembly increases the performance remarkably. For comprehension of mechanism and prediction of shear capacity of the panel zone for curved endplates, a mathematical model was generated. This includes the influence of axial load, prestressing induced by bolt tightening, anchorage and the concrete strut action. The prediction accuracy was compared with the test result and it showed sound correlation.

Revisiting the composite action in axially loaded circular CFST columns through direct measurement of load components

The composite action in circular concrete-filled steel tube (CFST) columns under axial compression is one of the most fundamental problems in the research area of steel-concrete composite structures. However, it cannot be easily investigated through experiments as the load components of the steel tube and concrete infill cannot be measured in a conventional axial loading test. In this study, an innovative testing method was devised to directly measure the load components of circular CFST columns under axial compression. Six innovative specimens with varying concrete strength and diameter-to-thickness (D/t) ratio and their conventional counterparts were tested. Although some difference existed between the load-deformation responses of the innovative and corresponding conventional specimens, it was essentially insignificant. The development of hoop stresses in the steel tube tends to reduce the corresponding axial stresses, but this effect can be offset by the strain hardening of the steel tube. Strain hardening of the steel tube should be considered when evaluating the confinement effect at the ultimate load, especially for columns with low concrete strength and small D/t ratio. The average values of the coefficients related to the axial and hoop stresses of the steel tube at the ultimate load, computed using available tests results, are 0.92 and −0.14, respectively. The proposed axial load capacity formula with these two values can reasonably predict the ultimate loads of circular CFST columns under axial compression.

Effects of variability in experimental database on machine-learning-based prediction of ultimate load of circular concrete-filled steel tubes

This study investigates the performance and robustness of regression machine-learning models in the presence of variability in the experimental database. The main objective of this work is to predict the ultimate load of circular concrete-filled steel tubes. The simulations were designed by combining size of the learning dataset, random realizations and prediction models. The variability (i.e. probability density function of each variable) is propagated to the output response through the regression machine-learning models. Results show that such variability must be considered when training and testing regression machine-learning models. The performance and robustness of the prediction models are presented and discussed. Based on the most robust and efficient model, a prediction equation is proposed for practical use. After conducting a comparison investigation, the performance of the proposed equation is found superior to one of current models. Finally, the proposed equation is implemented in Excel and appended to this paper.

A displacement/damage controlled seismic design method for MRFs with concrete-filled steel tubular columns and composite beams

A displacement/damage controlled (DDC) seismic design method for composite (steel/concrete) frames, consisting of circular concrete-filled steel tube (CFT) columns and composite beams (steel beams connected with concrete floor slabs) is developed in this study. The proposed seismic design method controls displacement and damage in a direct way for all seismic performance levels including the one near collapse. Through empirical expressions this method can estimate the inter-storey drift ratio (IDR) of a designed structure and evaluate the damage index (DI) of critical members for a given seismic intensity. A reduced number of design iterations is achieved while the computationally demanding non-linear time-history analysis can be avoided. The necessary empirical expressions of the design method are derived by means of statistical and sensitivity analysis of a large response databank consists of IDR and DI that cover all the way from elastic behavior to final global dynamic instability. This response databank is created by performing extensive parametric incremental dynamic analyses of many composite framed structures of the kind considered here under many seismic motions and different soil types. Design examples reveal that the DDC design method successfully estimates the targeted IDR for the desired seismic performance level as well as controls the DI in critical beam-to-column joints in order to avoid a soft-storey failure mechanism or partial loss of structure. Compared to all steel framed structures, the composite frames considered here exhibit a better seismic performance with beams and columns exhibiting a lower DI. The low-damage performance of composite frames is mainly emphasized as the number of storeys increases, while both the IDR and DI tend to fall within lower performance levels than those of the corresponding all steel frames for the same seismic intensity.

Behaviour of concrete-filled steel tubes with concrete imperfection under axial tension

In concrete-filled steel tubes (CFSTs), a gap between the steel tube and the core concrete can exist due to inappropriate construction, which will result in a change in the load-carrying capacity of the member. To assess the change in tensile strength, the tensile behaviour of circular CFST members with small gaps was investigated. To this end, a finite-element model was established and validated. Two types of gaps were considered in the analysis – a circular-segment gap and a circumferential gap. The full-range mechanical behaviour of CFST tensile members with a gap was evaluated. CFST members with and without a gap were compared in terms of their failure pattern, load–deformation response, composite action and ultimate strength. A parametric study was also performed and simplified formulae for calculating the tensile strength of CFST members with gaps were developed. It was found that the tensile strength of a CFST member with a gap decreases due to reduced composite action. The tensile strength of CFST members with gaps can be assessed reasonably well using the proposed method.

Ultimate axial capacity prediction of CCFST columns using hybrid intelligence models - a new approach

This study aims to propose a new intelligence technique of predicting the ultimate capacity of axially loaded circular concrete-filled steel tube (CCFST) columns. A hybrid system based on one of the evolution algorithm – Genetic Algorithm (GA), fused with a well-known data-driven model of multivariate adaptive regression splines (MARS), namely G-MARS, was proposed and applied. To construct the MARS model, a database of 504 experimental cases was collected from the available literature. The GA was utilized to determine an optimal set of MARS's hyperparameters, to improve the prediction accuracy. The compiled database covered five input variables, including the diameter of the circular cross section-section (D), the wall thickness of the steel tube (t), the length of the column (L), the compressive strength of the concrete (fc), and the yield strength of the steel tube (fy). A new explicit formulation was derived from MARS in further analysis, and its estimation accuracy was validated against a benchmark model, G-ANN, an artificial neural network (ANN) optimized using the same metaheuristic algorithm. The simulation results in terms of error range and statistical indices indicated that the derived formula had a superior capability in predicting the ultimate capacity of CCFST columns, relative to the G-ANN model and the other existing empirical methods.

Seismic Performance of Recycled Concrete Filled Circular Steel Tube Columns

This study was conducted to experimentally investigate the behavior of recycled concrete-filled circular steel tube (RCFST) columns subjected to cyclic loading. Ten specimens were prepared and tested. Four parameters were used to characterize seismic behavior: the replacement percentage of recycled coarse aggregate, slenderness ratio, axial compression level, and steel ratio. A novel calculation method for the bearing capacity for RCFST columns is established. The failure processes and modes of RCFST columns are found to be similar to normal concrete-filled steel tube columns. Varying the replacement percentage of recycled coarse aggregate has little effect on the hysteresis curves of the RCFST columns. The RCFST columns also show seismic performance similar to that of concrete-filled steel tubes. The displacement ductility of all specimens is larger than 3.0 and the equivalent viscous damping coefficients corresponding to the ultimate load range from 0.305 to 0.460.

Behavior of CFRP-confined reactive powder concrete-filled steel tubes under axial compression

Reactive powder concrete-filled steel tubes (RPCFSTs) have become an important research target in recent years. In engineering applications, RPCFSTs can provide superior vertical components for high-rise and tower buildings, thereby enabling developers to provide more floor space. However, this type of composite structure is prone to inelastic outward local buckling. The use of carbon fiber reinforced polymer (CFRP) wrapping to suppress such local buckling has shown great potential in limited test results. This paper presents experimental results concerning the axial compression of CFRP-confined reactive powder concrete-filled circular steel tubes (CF-RPCFSTs). We included 18 specimens in our experimental investigation, varying the number of CFRP layers, steel tube thickness, and RPC strength. According to our test results, CF-RPCFSTs exhibit compression shear failure and drum-shaped failure. The CFRP wrap can effectively enhance bearing capacity and postpone local buckling of the steel tube. In addition, three-layer CFRP-confined RPC-filled thin-wall steel tubes are suitable for engineering. We also propose a model to calculate the bearing capacity of CF-RPCFSTs. Compared to the existing model of CFRP-confined concrete-filled steel tubes, the results obtained using the proposed model are in good agreement with our experimental results.

An Exposition of Speculative and Numerical Analysis of CFST Columns

Owing to the advantages like intense compressive strength, large ductility and enormous energy incorporation, Concrete filled steel tube column system is gaining momentum when set side by side to regular steel or reinforced concrete system. Local buckling is one such major advantage that CFST gives experimental results from various articles has proven that there is an increase in the properties of CFST over normal concrete systems. This paper aims to give brief understanding on the experiments works conducted and numerical results obtained from various articles are discussed. Experimental works exposed to view that circular concrete filled steel tubes were more appropriate than square concrete filled steel tubes. In this paper, nonlinear finite element analysis for concrete filled steel tube is effectuated through varied parameters like concrete infill grade, as short columns with diameter ratio (L/D) not exceeding 4.5 with devoid of slenderness have also been discussed. The past studies divulged that the increment of D/T ratio dwindles the load carrying capacity of CFST and moreover, the high concrete infill grade buttress the load carrying capacity. From various literature reviews it was also concluded that the confinement efficacy of concrete enlarges the resistance to axial loading of compound columns with concrete filled circular sections. Furthermorefor the CFST columns the buckling failure can be evaded, as well as the load carrying capacity can be strengthened by hauling down the slenderness ratio. Therefore ample scope exists in carrying of numerical analysis using ANSYS to find the potential of CFST columns.

A numerical investigation on the through rib stiffener beam to concrete-filled steel tube column connections subjected to cyclic loading

This research presents a numerical investigation on the behaviour of through rib stiffener steel beam connected to circular concrete-filled steel tube (CFT) column subjected to the cyclic loading. A nonlinear Finite Element (FE) model was established using ABAQUS/Standard program. The accuracy of the FE result was examined through comparing it with the previous experimental test results. The effects of various parameters on the failure mode and ultimate moment capacity of the connections were investigated on twenty-four FE models. The parameters were stiffener’ height, length, and thickness including varied thicknesses and diameters of the hollow circular steel tube. The FE results indicated that the larger height of the through rib stiffener improved buckling behaviour of the steel tube column and the moment capacity by 8%. Meanwhile, the longer and thicker through rib stiffener effectively shifted the beam plastic hinge’s location away from the column, which resulted in 4% improvement in ultimate moment capacity of the connection.

Structural performance of nano-silica based blended CFST stub circular column

The objective of this study is to find the optimum percentage replacement of cement with the nano-silica in a concrete mix to improve the structural performance Concrete Filled Steel Tube (CFST) stub circular column under the axial compressive load. The CFST stub columns with circular cross-sectional area of conventional concrete mix (without nS) and nano-silica blended concrete mix (with nS-CFST) had been experimentally and numerically studied. The amorphous silica is a major component of the cementitious materials, and it reacts with the calcium hydroxide, which is comprised of the calcium silicate hydration. The addition of this nano-silica affects the different properties of the concrete and increases its performance. On the other hand, CFST is known for the composite behavior generated between the hollow tube of steel and a core constructed of concrete, which improves the collected performance of the member. In present study, the concrete mix of M25 had been modified with 4%, 6%, 8% and 10% of nano silica. The structural behavior of stub columns (with / without nS) are assessed under axial compressive load. Also, the CFST stub columns were then modelled into ABAQUS for finite element analysis. The nano-silica blended concrete in CFST stub columns showed an improved the structural performance than the conventional cement concrete stub circular CFST column.

Effect of dimensional parameters on the seismic performance of circular CFST columns

Due to the expansion of cities and increase in population, the need for construction of high rise buildings becomes more essential in the society. As earthquakes are of the greatest damaging natural hazards to the buildings, the design and construction of structures which are capable of resisting the adverse effects of earthquakes become necessary. Concrete Filled Steel Tubes (CFSTs) are the steel tubular sections in-filled with concrete and with a composite action. CFSTs are becoming a popular solution for structures located in seismic areas, due to their high strength, excellent ductility and large energy absorption capacity. This study was mainly concentrated to find out the effect of dimensional parameters on the seismic performance of CFSTs. Width - thickness ratio (D/t) and slenderness ratio (L/D) were taken as two parameters and the study was carried out using circular CFST buildings. It was observed that, there will be a particular limit for D/t and L/D ratio upto which the thickness and diameter of the CFSTs can be decreased. At that point the building will show a better seismic performance.

Post-Fire Exposure Behavior of Circular Concrete-filled Steel Tube Column under Axial Loading

Concrete-filled steel tube (CFST) columns are a composite member which mainly consists of steel tube infilled with concrete that are increasingly being used as load-carrying members these days. In construction industry, CFST columns are being preferred for the development of tall buildings and long-span bridges. This paper presents an experimental investigation on concrete-filled steel tube (CFST) columns which were post heated and were subjected to axial loading. The thickness of the casing steel was 4 mm and 5 mm and diameter was 100 mm, 125 mm and 150 mm and were infilled with concrete of grade M30 which were used in the present study. This study also represents the behavior of CFST columns for two cooling regimes (annealing and quenching) after exposure to elevated temperatures of 600 °C and 800 °C. The results obtained from experimental analysis were compared to each other in terms of load-deformation pattern, ultimate load capacity, residual strength index, secant stiffness and ductility index. The test results showed that as compared with water quenching, annealing is slightly better for post fire cooling of CFST columns. Also, the results obtained by the experimental investigation were compared with each other on the basis of two cooling regimes.