Circular Concrete-filled Steel Tube Research Articles

Modeling of circular concrete-filled steel tubes subjected to cyclic lateral loading

In this paper, a simple and efficient yet fairly accurate analytical model for the cyclic behavior and strength capacity of circular concrete-filled steel tubes (CFT) under axial load and cyclically varying flexural loading is developed. Firstly, an accurate nonlinear finite element model is created using the ATENA software. The validity of this model is established by comparing its analyses results with those of experimental data published in the pertinent literature. Then, using this finite element model, an extensive parametric study is conducted to create a fairly broad databank of hysteretic behavior of circular CFTs, involving numerous circular CFT columns with different diameter to thickness ratios, steel tube yield stress and concrete strength. On the basis of this databank, empirical expressions are developed to evaluate the phenomenological parameters of the well-known Ramberg-Osgood hysteretic model. Additionally, empirical analytical relations providing a direct and efficient representation of the ultimate strength of circular CFT columns are constructed and validated. Comparisons between analytical and experimental results demonstrate that the proposed analytical model can describe efficiently and reliably the behavior of circular CFT columns under cyclic lateral loading.

A theoretical axial stress-strain model for circular concrete-filled-steel-tube columns

Concrete-filled-steel-tube (CFST) columns are widely adopted in many structures nowadays attributed to the superior behaviour developed by the composite action. However, the composite action cannot be fully developed because of different dilation properties of steel tube and concrete in the elastic stage. Moreover, due to the inelastic outward buckling of steel tube, CFST columns may suffer serious degradation. To overcome these problems, external confinement, such as rings, ties, spirals and FRP wraps have been studied recently and proven experimentally to have potential in improving the uni-axial behaviour of CFST columns. In this paper, an experimental database containing 422 uni-axial compression test results of unconfined and externally confined CFST columns has been assembled. In addition, a theoretical model has been proposed for predicting the uni-axial behaviour of circular CFST columns. This model consists of mainly three components: (1) Constitutive model of confined concrete modified from Attard and Setunge’s actively confined concrete model. (2) Constitutive model of steel tube under complex stress-state using Prandtl-Reuss theory. (3) The interaction among external confinement, steel tube and core concrete based on new a hoop strain equation. The validity of the proposed model has been verified by comparing the predicted results with the experimental database.

Flexural behavior of circular concrete filled steel tubes (CFST) under sustained load and chloride corrosion

This paper studies the flexural behavior of circular concrete filled steel tubes (CFST) under sustained load and chloride corrosion. 7 CFST specimens were tested under a four-point bending load. It was found that corrosion causes noticeable deterioration to the flexural strength, while the ductility of CFST keeps well. A finite element analysis (FEA) model was developed to study the full-range behavior of CFST under corrosion. A parametric study was conducted to find the main parameters that influence the residual flexural strength, based on which a simplified model was proposed to calculate the residual flexural strength of circular CFST under long-term load and corrosion.

Design of CFST column-to-foundation/cap beam connections for moderate and high seismic regions

Seismic design of new bridges requires strength, stiffness and stable inelastic cyclic deformation capacity. In addition, rapid construction and rapid return to service after an earthquake are desired. Recent research demonstrates that circular concrete filled steel tubes (CFSTs) satisfy these diverse requirements. CFSTs are composite elements consisting of steel tubes with concrete infill. CFST columns offer larger strength and stiffness than conventional reinforced concrete bridge piers. Under seismic loading, CFSTs undergo reversed cyclic loading, and robust connections capable of sustaining cyclic lateral load demands, while minimizing damage and degradation, are required. This research project investigates foundation and cap-beam connections for these bridge applications. Several connections have been investigated, but the embedded ring CFST connection is the focus of this work. This connection develops the full strength, stiffness and ductility of the CFST bridge pier. The connection permits accelerated bridge construction and it sustains minimal damage during large inelastic deformations during earthquake loading. Results from the experiments were used to develop practical design expressions, which are presented here, and a design example is included to facilitate the implementation of the research results into practice.

EXPERIMENTAL STUDY ON CIRCULAR AND SQUARE CONCRETE FILLED STEEL TUBE COLUMNS SUBJECTED TO AXIAL COMPRESSION LOADS

D. R. Panchal1, V. P. Sheta2 1Assistant Professor, Department of Applied Mechanics, FTE-MSU, Vadodara, Gujarat, India panchaldr@gmail.com 2M.E. (Structural Engineering) Student, Department of Applied Mechanics, FTE-MSU, Vadodara, Gujarat, India vpsheta63@gmail.com Abstract This paper investigates the behaviour of concentrically axially loaded circular and square high strength steel tube (Fe310 grade) columns filled with different grades (M20, M30 & M40 etc.) of concrete. The effects of grade of concrete and composite action between the steel tube and the concrete core on axial load capacity are studied & graphs of Axial load v/s vertical deflection (axial shortening curves) are plotted experimentally. Some important performance indices entitled as Ductility Index (DI), Strength factor (SI) and Concrete Contribution Ratio(CCR) are also evaluated and compared between the circular and the square shaped Concrete Filled Steel Tube (CFST) columns.By referring many research works, we can know firmly that confinement effect is restricted to short column only. So, we choose 300mm length for all the specimens of 88.9mm dia and 80×80mm size, to keep the columns stub only for getting benefit of confinement effect especially in circular sections.From the results, it has been noted that square columns have higher axial capacity when compared with circular columns of same resisting area under compression. Hence, size reduction for square column is possible. While ductility index for circular columns are quite better than square ones. Ultimate failure patterns are obtained by applying axial compression loads concentrically up to ultimate stage on circular and square CFST stub columns. as expected, the results shows a Local buckling i.e. crushing of concrete type of failure at supports. Ultimate axial load capacity of Concrete filled steel tubular columns are evaluated and compared experimentally as well as by calculations also, as per the guidelines suggested by EC4(Eurocode part-4, British Standards Institutions) code, which is the best suitable code for design of CFST columns amongst all other international codes for composite structure design. From the study it is concluded that, loads obtained by EC4 fits nearer to perfection and depicts the behaviour well enough by 15-20% error with experimental ultimate loads. The results obtained by experimental and EC4 code are validated well with the previous scholar researchers too

Experimental behaviour of circular concrete filled steel tube columns and design specifications

This paper presents 18 tests conducted on short, medium and long circular Concrete Filled Steel Tube (CFST) columns. To explore the impact of column parameters and confinement effect three L/D ratios, two D/t ratios, two steel qualities and three concrete classes were employed. Some specimens have properties within application limits of EC4 and AISC 360-10 whereas others have properties beyond the application limits. Since new, large and efficient structures require adoption of high strength materials, it is compulsory to push the limits of design specifications. It is shown that 56MPa and 66MPa concretes provide very smooth and ductile load-shortening curves which imply high deformation capacity of such concrete classes. Brittle nature of 107MPa concrete is shown by very sharp transitions from pre-peak to post-peak region and sudden discharge of loading in load-shortening curves. Additionally, 239 experimental data were collected from literature to assess EC4 and AISC 360-10 predictions within application limits and beyond application limits. Instead of focusing on a narrow region of configurations, this paper examines the performance of prediction methods on short, medium and long CFST columns. EC4 predictions indicate much better agreement with the test results. However AISC 360-10 predictions are conservative for all combinations of parameters. The application limits of EC4 can be widened to cover solutions of columns with broader properties. Confinement effect should be handled elaborately in AISC 360-10 formulations. L/D and relative slenderness are key parameters and have direct impact on column behaviour. However D/t does not have direct impact on column behaviour.

Axial and lateral stress–strain model for circular concrete-filled steel tubes with external steel confinement

In a concrete-filled steel tube, delamination often occurs at the steel tube–concrete core interface due to the larger Poisson’s ratio of the steel tube. For resolving this problem, it has been advocated to install external restraints in the form of steel rings or spirals so as to restrain the lateral expansion of the steel tube. This also provides additional confinement to the concrete core. To study the effectiveness of such external steel confinement, a theoretical model for evaluating the confining stress and axial load in a concrete-filled steel tube with external steel confinement up to the post-peak stage is developed. The theoretical model is first verified by analyzing a total of 98 specimens tested by previous researchers and comparing the measured and predicted lateral strain–axial strain curves and axial load–strain curves of the specimens. It is then used to perform a parametric study to evaluate the required equivalent thickness/diameter ratios of the external steel confinement for eliminating the delamination effect and for achieving Level I ductility.

Behaviour of concrete filled-steel tubes under axial load

In order to study the mechanical mechanism of concrete-filled steel tubes (CFSTs) under axial compression, a theoretical model for analysing the behaviour of axially loaded CFST columns is presented in this paper. The model is derived from the theory of elasto-plasticity, non-linear elastic mechanics and materials properties. Based on a related numerical program, analytical results were compared with experimental data. The complete load–deformation relationship of specimens, steel tube stress, performance of the confined core concrete and the interaction between the outer tube and the core concrete were investigated. The analytical results indicated that this model is able to capture the mechanical behaviour of concrete-filled circular steel tubes. It was found that the axial and circumferential stresses of the steel tubes behaved non-linearly during the loading history. It was concluded that the behaviour of the confined core concrete was significantly influenced by the confining pressure. The constraining factor had an important effect on the improvement of the core concrete strength. The analytical results also indicated that the axial stress–strain relationship of concrete under constant confinement was not appropriate for the core concrete in CFST specimens. Finally, a comparison between the confining pressure and the confined concrete response is presented.

Investigation of through beam connection to concrete filled circular steel tube (CFCST) column

Nonlinear finite element models of through beam connection to concrete filled circular steel tube (CFCST) column with three types of connection failures (column, beam and joint shear failures) under monotonic loading were proposed by ABAQUS programme. The connection detail composed of a steel I-beam which is completely passed through the circular steel tube column and welded to it by fillet or full penetration weld and the tube is then filled with concrete. The connection parameters investigated included different ratios of column-to-beam flexural strength, fillet or full penetration weld to connect the beam to the tube, using rebar inside of the column and effect of beam web and concrete core inside the joint. Buckling analysis, concrete damage, weld fractured and tube tearing were defined in the model. Good agreement was achieved between the model and existing test results in terms of the beam tip force–storey drift relationship, joint distortion and joint shear strain. As a result, the models could correctly predict the linear, nonlinear and post-failure behaviours of the connection. In addition, other parameters of the connection were investigated for the specimens. Finally, the effect of column axial load level for the connection behaviour in the three types of failures was investigated as a parametric study.

Bond behavior in concrete-filled steel tubes

In this paper, the bond behavior between the steel tube and the concrete in concrete-filled steel tubes (CFST) is investigated. A series of push-out tests on circular and square CFST specimens were conducted, and the main parameters considered in the test program were: (a) cross-sectional dimension (120–600mm); (b) steel type (carbon and stainless steels); (c) concrete type (normal, recycled aggregate and expansive concretes); (d) concrete age (31–1176days); and (e) interface type (normal interface, interface with shear studs and interface with an internal ring). The experimental results indicate that stainless steel CFST columns have lower bond strength compared with carbon steel counterparts, and the bond strength decreases remarkably with increasing cross-sectional dimension and concrete age. To enhance the bond strength, welding internal ring(s) onto the inner surface of the steel tube is the most effective method, followed by the methods of welding shear studs and using expansive concrete.

Finite element analysis of circular concrete filled tube connections

A parametric study based on finite element technique is performed to investigate the performance of different connection configurations between circular concrete filled steel tube (CCFT) columns and gusset plates subjected to axial compression loadings. The study focuses on the effect of the pipe and gusset plate dimensions on the connection behavior. The modeling assumptions and techniques used to perform the analysis are detailed. The models are verified using experimental test data performed earlier by the authors. A notable effect was observed on the behavior of the connections due to its detailing changes with respect to failure mode, yield and ultimate capacity, stress distribution and initial and final stiffness.

Failure mechanism of full-size concrete filled steel circle and square tubes under uniaxial compression

In this paper, the failure mechanisms of full-size concrete filled steel tubes (CFST) under uniaxial compression were investigated with nonlinear finite element method. Existing experimental results were employed to verify the validity of the finite element models of CFST specimens. Then, the numerical analysis was further conducted to study the mechanical behaviors of full-size CFST columns with circular and square cross sections under uniaxial compression. The simulation results indicate that the distribution of the contact pressure between circular steel tube and core concrete is much more uniform than that between square steel tube and concrete, resulting in much higher confinement and more efficient interaction between steel tube and core concrete in circular CFST columns, as well as ultimate load capacity and ultimate displacement. Extensive parametric analysis was also conducted to examine the effect of various parameters on the uniaxial compression behaviors of circular and square CFST columns.

Structural behavior of beam-to-column connections of circular CFST columns by using mixed diaphragms

Concrete Filled Steel Tube (CFST) is a structure made of a steel tube filled with concrete, in which the rigidity of the member can be increased as the steel tube confines the concrete and the concrete reduces the local buckling of the steel tube. The object of this study is a mixed diaphragm which uses a penetrated diaphragm and an exterior diaphragm in combination at beam-tocolumn connections of circular CFST columns to improve the concrete filling property. Verifying that the resistance force of mixed diaphragm is same as or superior to that of current type, the study has been conducted on the optimal dimension of mixed diaphragm of which the validity and superiority at connections of circular CFST structure have been confirmed. A size of the circular CFST column of which the yield moment is bigger than that of the beam has been selected. A type of circular CFST which exceeds the yield and full plastic moments of the beam and satisfies the condition of maintaining the initial rigidity and the maximum resistance force has been selected through finite element analysis, and a regression analysis has been conducted to obtain an approximate expression used to calculate the optimal diaphragm. The reliability and validity of the obtained approximate expression have been confirmed through a statistical theory based analysis and an approximate plane analysis. This approximate expression can be used to calculate the optimal size of the mixed diaphragm required at the beam-to-column connection of a circular CFST column.

The correction coefficient of size effect on the axial compression strength of circular concrete-filled steel tubular short columns

In this paper, 413 experimental results from circular concrete-filled steel tube short columns, which were subjected to formulae under the design specifications of CECS 28:201219, AIJ (2008)20, ANSI /AISC 360-0521 and EC4 (2004)22 are presented. Generally, when the specimen diameter increases, the ratio of experimental values to calculated values decreases; this illustrates the influence of existent size effect on the compression strength of concrete-filled steel tube columns. The correction coefficients of size effect obtained from the data fitting were subjected to the formulae. Subsequently, the experimental values to calculated values ratio did not change with size of the specimen. Having considered 1·0 as the critical value of the confinement index, the data were divided into two groups. When the correction coefficients of the two groups using the same formula were compared, it was seen that the smaller the confinement index was, the larger the size effect would be. When the results obtained from different formulae were compared, the size effect on the formulae using the confinement index was found to be smaller than that on the formulae with a superposition form of capacity.

Statistical-based evaluation of design codes for circular concrete-filled steel tube columns

This study addresses the load capacity prediction of circular concrete-filled steel tube (CFST) columns under axial compression using current design codes. Design methods given in the Chinese code CECS 28:2012 (2012), American code AISC 360-10 (2010) and EC4 (2004) are presented and described briefly. A wide range of experimental data of 353 CFST columns is used to evaluate the applicability of CECS 28:2012 in calculating the strength of circular CFST columns. AISC 360-10 and EC4 (2004) are also compared with the test results. The comparisons indicate that all three codes give conservative predictions for both short and long CFST columns. The effects of concrete strength, steel strength and diameter-to-thickness ratio on the accuracy of prediction according to CECS 28:2012 are discussed, which indicate a possibility of extending the limitations on the material strengths and diameter-to-thickness ratio to higher values. A revised equation for slenderness reduction factor in CECS 28:2012 is given.

Noncompact and slender circular CFT members: Experimental database, analysis, and design

The AISC 360-10 Specification categorizes circular concrete filled steel tube (CFT) members as compact, noncompact or slender depending on the slenderness ratio (diameter-to-thickness D/t ratio) of the steel tube walls. International design codes typically focus on the design of compact circular CFT members with relatively small slenderness (D/t) ratios, and the behavior and design of noncompact and slender circular CFT members is not addressed directly. This paper presents the basis of the current AISC Specification (AISC 360-10) for the design of noncompact and slender circular CFT members under axial compression, flexure, and combined axial and flexural loading. An experimental database of tests conducted on noncompact and slender circular CFT members is compiled, and used to establish the conservatism of the design equations in the AISC Specification. Detailed 3D finite element method (FEM) models are developed for circular CFT members, and benchmarked using the database of experimental results. The benchmarked models are used to conduct parametric studies to address gaps in the experimental database, and further verify the conservatism of the design equations. The AISC 360-10 Specification is recommended for the design of noncompact and slender circular CFT members. Additionally, the numerical models developed and benchmarked for circular CFT members are recommended for future investigations.

Behaviour of CFRP externally-reinforced circular CFST members under combined tension and bending

This paper investigates the performance of circular concrete-filled steel tubes (CFST) externally reinforced by carbon fibre reinforced polymer (CFRP) sheets under the combined actions of tension and bending. The strengthening effect of CFRP layers was examined through tensile tests of 19 CFST specimens. The main experimental parameters include fibre orientation, load eccentricity, number of CFRP layers and anchorage system of CFRP sheets. A finite element analysis (FEA) model was developed to conduct mechanism analysis and parametric analysis of the composite members investigated. In general, CFRP with fibres oriented in the longitudinal direction is very efficient in increasing the strength of the CFST member. On the other hand, a wrapped CFST member strengthened at ±45° angles to the longitudinal axis has the largest deformation capacity. Finally, a simplified strength model was proposed to predict the load-carrying capacity of CFRP-reinforced CFST members under combined tension and bending.

Bond behaviors of shape steel embedded in recycled aggregate concrete and recycled aggregate concrete filled in steel tubes

Thirty one push-out tests were carried out in order to investigate the bond behavior between shape steel, steel tube (named steels) and recycled aggregate concrete (RAC), including 11 steel reinforced recycled aggregate concrete (SRRAC) columns, 10 recycled aggregate concrete-filled circular steel tube (RACFCST) columns and 10 recycled aggregate concrete-filled square steel tube (RACFSST) columns. Eleven recycled coarse aggregate (RCA) replacement ratios (i.e., 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%) were considered for SRRAC specimens, while five RCA replacement ratios (i.e., 0%, 25%, 50%, 75% and 100%), concrete type and length-diameter ratio for recycled aggregate concretefilled steel tube (RACFST) specimens were designed in this paper. Based on the test results, the influences of all variable parameters on the bond strength between steels and RAC were investigated. It was found that the load-slip curves at the loading end appeared the initial slip earlier than the curves at the free end. In addition, eight practical bond strength models were applied to make checking computations for all the specimens. The theoretical analytical model for interfacial bond shear transmission length in each type of steel-RAC composite columns was established through the mechanical derivation, which can be used to design and evaluate the performance of anchorage zones in steel-RAC composite structures.

Flexural Behaviour of Stainless Steel-Concrete-Carbon Steel Circular Concrete-Filled Double Skin Steel Tubes

In order to improve the traditional concrete filled steel tube's corrosion problems,when it uses as holding column of offshore platforms and as support bar of long-span bridges,this paper mainly studies stainless steel-concrete-carbon steel circular concrete-filled double skin steel tubes's flexural behaviour. Six specimen's tests were carried out.The main parameters varied in the test were the hollow ratio (χ) was 0.45 or 0.69,and shear span ratio (λ) was 1 or 3.The experiment results show how the hollow ratio and shear span ratio effect specimens' ultimate flexural capacity.Also,the experiment results provide experimental basis for structure design.

Behavior of gusset plate-T0-CCFT connections with different configurations

Concrete-filled steel tube (CFT) composite columns, either circular (CCFT) or rectangular (RCFT), have many economical and aesthetic advantages but the behavior of their connections are complicated. This study aims to investigate, through an experimental program, the performance and behavior of different connections configurations between circular concrete filled steel tube columns (CCFT) and gusset plates subjected to shear and axial compression loadings. The study included seventeen connection subassemblies consisting of a fixed length steel tube and gusset plate connected to the tube end with different details tested under half cyclic loading. A notable effect was observed on the behavior of the connections due to its detailing changes with respect to capacity, failure mode, ductility, and stress distribution.