Circular Concrete-filled Steel Tube Research Articles

Flexural strength analysis of non-post-tensioned and post-tensioned concrete-filled circular steel tubes

Concrete-filled circular steel tubes (CFT) have recently gained significant attention for their enhanced strength and ductility over the conventional steel and reinforced concrete construction. The concrete compressive strength is significantly increased by the lateral confinement provided by the steel tube and local buckling of the steel tube is restrained by the concrete infill. This paper investigates the further enhancement of these composite actions due to post-tensioning the concrete cores inside circular steel tubes. The flexural behavior of a post-tensioned CFT as well as a non-post-tensioned CFT was studied experimentally and analytically. A numerical algorithm for predicting the moment capacities of circular CFT, with or without the post-tensioning effects, has been validated against the test results by the authors as well as those published in the literature.

Study on push-out test and bond stress-slip relationship of circular concrete filled steel tube

According to the results of 9 circular concrete filled steel tube (CFT) push-out tests, a new theoretical model for average bond stress versus free end slip curve is proposed. The relationship between verage bond stress and free end slip is obtained considering some varying influential parameters such as slenderness ratio and diameter-to-thickness ratio. Based on measured steel tube strain and relative slip at different longitudinal positions, the distribution of bond stress and relative slip along the length of steel tube is obtained. An equation for predicting the varying bond-slip relationship along longitudinal length and a position function reflecting the variation are proposed. The presented method can be used in the application of finite element method to analyze the behavior of CFT structures.

Nonlinear finite element analysis of circular concrete-filled steel tube structures

The structural behaviors of circular concrete filled steel tube (CFT) structures are investigated by nonlinear finite element method. An efficient three-dimensional (3D) degenerated beam element is adopted. Based on those previous studies, a modified stress-strain relationship for confined concrete which introduces the influence of eccentricity on confining stress is presented. Updated Lagrange formulation is used to consider the geometrical nonlinearity induced by large deformation effect. The nonlinear behaviors of CFT structures are investigated, and the accuracy of the proposed constitutive model for confined concrete is mainly concerned. The results demonstrate that the confining effect in CFT elements subjected to combining action of axial force and bending moment is far sophisticated than that in axial loaded columns, and an appropriate evaluation about this effect may be important for nonlinear numerical simulation of CFT structures.

Axial strength of circular concrete-filled steel tube columns — DOE approach

This paper presents the effect of changes in diameter of the steel tube ( D ), wall thickness of the steel tube ( t ), strength of in-fill concrete ( f c u ), and length of the tube ( L ) on ultimate axial load ( P u e ) and axial shortening at the ultimate point ( δ u e ) of circular Concrete Filled steel Tubes (CFT). Taguchi’s approach with an L9 orthogonal array is used to reduce the number of experiments. With the help of initial experiments, linear regression models are developed to predict the axial load and the axial shortening at the ultimate point. A total of 243 circular CFT samples are tested to verify the accuracy of these models at three factors with three levels. The experimental results are analyzed using Analysis Of Variance to investigate the most influencing factor on strength and axial shortening of CFT samples. Comparisons are made with predicted column strengths using the existing design codes, AISC-LRFD-2005 and EC4-1994.

Suggested empirical models for the axial capacity of circular CFT stub columns

In this paper, a total of 250 experimental tests of axially loaded circular concrete-filled steel tube (CFT) stub columns, published in the literature was summarized. The applicability of the current design codes such as ACI, Australian Standards, AISC, AIJ, Eurocode 4, DL/T and some available empirical models proposed by various researchers for calculating the axial capacity of circular CFT stub columns was examined using these experimental data. Based on the investigations, four new empirical models for predicting the axial capacity of circular CFT stub columns are proposed. The comparisons between the experimental results and the predictions of these models show that the proposed empirical models provide a direct, compact, and efficient representation of the ultimate strength of circular CFT stub columns made with not only normal strength but also high strength steel tubes and concrete. Finally, the limiting values of the maximum effective length, the compressive strength of concrete, the yield strength of steel tubes and the diameter-to-thickness for circular CFT stub columns with respect to the present empirical models are suggested. It is expected that engineers can easily use the present empirical models to estimate the axial capacities of circular CFT stub columns for engineering designs.

A unified formulation for hollow and solid concrete-filled steel tube columns under axial compression

On the basis of the theory of elasticity, a unified formulation is proposed in this paper to predict the composite compressive strength of circular concrete-filled steel tube (CCFST) columns. The formula is obtained from the analytic solution of an elastic composite cylinder under axial compression. The formula is further calibrated by introducing a number of correlation coefficients that are validated by test results. Furthermore, the stability factor of CCFST long columns under axial compression is derived on the basis of the Perry–Robertson formula. The newly derived formulas of stability bearing capacity are applicable for both hollow and solid CCFST columns, which is again validated through comparisons with experimental results. One of the important features of the new formulas is that they provide a unified formulation for both hollow and solid CCFST columns that relates the compressive strength or the stability capacity of a CCFST column and a series of design parameters.

Analytical Studies of Concrete-Filled Circular Steel Tubes under Axial Compression

This paper describes a numerical program to analyze the behavior of the concrete-filled steel tubular (CFT) stub columns, and predict various modes of lateral interactions between steel tube and filled-in-concrete under axial compression. The behavior of the CFT columns is controlled by both the strength and the confinement effect of steel tube and filled-in-concrete in the columns. Various lateral interactions between steel tube and filled-in-concrete in CFT columns are classified into eight different cases by the contact between steel tube and filled-in concrete at different stress stages. The paper examines the analytical models with various experimental results in literature. The comparisons showed that the numerical program is reasonable to capture mechanical behavior of concrete-filled circular steel tubes.

Prediction of pre- and post-peak behavior of concrete-filled circular steel tube columns under cyclic loads using fiber element method

The study of cyclic load–deformation relationships of columns in the post-peak region is essential to prevent the collapse of buildings under earthquake excitation. This paper provides an efficient method to predict the pre- and post-peak hysteretic behavior of concrete-filled circular steel tube columns under the combination of constant axial load and cyclic lateral load. A simplified nonlinear fiber element method is used to investigate uni-axial stress and strain relationship of materials in terms of the composite action between the steel tube and the concrete. The accuracy of the proposed computational technique is verified by comparing the results from this technique and experimental results.

Experiment Study on Cyclic Behavior of Concrete Filled Steel Tube (CFST) Column-Reinforced Concrete (RC) Beam Connections

The test results of six connections under cyclic loading are presented in the paper. Each test specimen was properly designed to model the interior joint of a moment resisting frame, and was identically comprised of three parts that including the circular concrete filled steel tube columns, the reinforced concrete beams, and the short fabricated connection stubs. Energy dissipation was designed to occur in the beams during a severe earthquake. Steel bars which were embedded into concrete core and welded to the connection stubs, were used to transfer the force distributed by the reinforcing bars of concrete beam to the concrete core. The results indicated that the embedded steel bars were very efficient in eliminating the stress concentration on the tube wall and there was no visible deformation occurred on the tube wall until the collapse of the specimen. Furthermore, the connection of each specimen had enough capacity and thus the plastic hinge appeared in the beams. As results, the ductility of this new type structure directly depended on the RC beams.

Strength deformation behaviour of circular concrete filled steel tubes subjected to pure bending

The strength deformation behaviour of circular steel tubes filled with different grades of concrete under flexure is presented. The effects of steel tube thickness, the cross sectional area of concrete, strength of in-filled concrete and the confinement of concrete on moment capacity and curvature of Concrete Filled steel Tubes (CFTs) are examined. Measured flexural strengths are compared with the values predicted by EC4-1994 and LRFD-AISC-1999 code provisions. A total of ninety nine specimens, all one metre long, were tested with concrete fills of 20, 30 and 40 N/mm 2 characteristic strength and with D / t ratio 22.3 to 50.8. Based on the experimental results, an interaction model to predict moment and curvature of the CFT sample is developed.

A Simple Formula for Predicting the Compressive Strength of Circular CFT Stub Columns

In this paper, the predictions of ultimate strength of circular concrete-filled steel tube (CFT) stub columns using current design codes such as AIJ-2001, AISC-2005, DL/T-1999, and Eurocode 4-2004, are examined and compared with published experimental tests of CFT short columns made with normal- and high-strength steel and concrete. Based on the investigations, a simple formula for predicting the ultimate strength of CFT stub columns is proposed. The concrete confinement, which depends mainly on the ratio of the external diameter of the steel tube to the plate thickness, the yield stress of the steel tube, and the unconfined compressive strength of the filled concrete, was reasonably achieved in the proposed formula. From comparisons with the published experimental data for CFT short columns of normal and high strength steel and concrete, the present formula is shown to give a good representation of the ultimate strength of circular CFT columns.

Seismic behavior of exterior connections with steel beams bolted to CFT columns

In order to improve the constructability and meanwhile ensure satisfactory seismic behavior, an innovative type of connection for concrete filled circular steel tube (CFT) column-to-steel beam composite structures was conceived and studied. The proposed connection details are characterized by an extended endplate that is welded to a steel beam in factory and then bolted to a CFT column using high-strength steel rods in the field. An experimental investigation on seismic behavior of the proposed bolted endplate connection and evaluation of the effect of concrete floor slabs and reduced beam sections was conducted by testing three full scale joint models. The experimental results indicated that the presence of floor slabs contributed to the strength of joints significantly and reduced beam sections were effective in moving the buckling zone away from the welds. Analytical models for the bolted endplate connections were also constructed using OPENSEES1.7.3 to simulate the experimental results.

Low-Cycle Fatigue Damage of Circular Concrete-Filled-Tube Columns

Under seismic actions, the induced damage in critical regions of a concrete-filled steel tube (CFT) column could be attributed to cumulative damage of the steel tube caused by repeated cyclic loading in the post-yield strain range. An experimental study was undertaken to develop an analytical model of cumulative damage for circular CFT columns subjected to cyclic loading. The low-cycle fatigue behavior of CFT columns was investigated and a relationship between fatigue life and normalized displacement amplitude was established for typical controlling parameters. A total of 12 large-scale model column specimens, representing two types of circular CFT columns with different steel tube diameter-to-thickness ratios were tested under quasistatic loading with either constant drift amplitude or standard cyclic loading histories with increasing peak drift displacement. Test results indicate that the low cycle fatigue behavior of CFT columns depends on the thickness of the steel tube. Fatigue life expressions useful for application in damage-based seismic design are developed and used to predict the damage index for additional CFT columns based on experimental data reported in the literature.

Analytical behavior of circular concrete-filled thin-walled steel tubes subjected to bending

This paper presents a finite element analysis (FEA) modeling to study the flexural performance of circular concrete-filled thin-walled steel tubular (CFST) beam. A set of test data was used to verify the FEA modeling; generally, good agreement was achieved. The FEA modeling was then used to investigate the stress and strain distributions across the composite section in the whole loading procedure. The composite action between the steel tube and its concrete core was analyzed. A strut–tie model was proposed for the load transfer mechanism of the circular composite member subjected to pure bending.

Flexural Behavior of Nonposttensioned and Posttensioned Concrete-Filled Circular Steel Tubes

Concrete-filled steel tubes (CFTs) have recently been used for columns and bridge piers, especially in seismic regions, for their enhanced strength and ductility over the conventional steel and concrete construction. This enhancement is due to the composite actions of the concrete infill and the steel tube: The concrete compressive strength is significantly increased by the lateral confinement provided by the steel; and local buckling of the steel tube is delayed by the concrete infill. Similar enhancement has also been observed in flexural strength of CFT. A novel concept of posttensioning the concrete core inside circular steel tubes to further enhance the synergistic interactions has been developed, and numerical analyses have shown significant increase in their flexural strength over nonprestressed CFT. A numerical algorithm has been developed for predicting the moment capacities of circular CFT. The increased concrete strength and ductility due to confinement were taken into account. The analytical predictions compared well with flexural test data. The validated algorithm was further developed to include the posttensioning effects. Numerical results suggest that moment capacity of a CFT can be significantly increased by prestressing the concrete core with only moderate posttensioning forces.

Fire resistance of axially loaded concrete filled steel tube columns

The behaviour of axially loaded square and circular concrete-filled steel tube (CFST) columns when exposed to elevated temperatures is investigated in this paper. The fire resistance of this kind of composite tubes is calculated. Comparison of the square and circular columns in the fire resistance shows that, for columns with the same steel and concrete cross-section areas, the circular column has slightly better fire resistance than the square column.

Recent Findings from Phase V of the United States–Japan Cooperative Earthquake Research Program

Recent Findings from Phase V of the United States–Japan Cooperative Earthquake Research Program

Axial capacity of circular concrete-filled tube columns

The behaviour of circular concrete-filled steel tubes (CFT) with various concrete strengths under axial load is presented. The effects of steel tube thickness, the bond strength between the concrete and the steel tube, and the confinement of concrete are examined. Measured column strengths are compared with the values predicted by Eurocode 4, Australian Standards and American Codes. 15 specimens were tested with 30, 60 and 100 N/mm 2 concrete strength, with a D/ t ratio from 22.9 to 30.5. All the columns were 114 mm in diameter and 300 mm in length. The effect due to concrete shrinkage is critical for high-strength concrete and negligible for normal strength concrete. All three codes predicted lower values than that measured during the experiments. Eurocode 4 gives the best estimation for both CFT with normal and high-strength concrete.

Seismic performance of steel beams to concrete-filled steel tubular column connections

Abstract A nonlinear force-deformation model to simulate shear transfer behavior in the panel zone of CFT (Concrete-Filled Steel Tube) beam-column connections is proposed. In this model, influence of axial load on the shear transfer behavior is accounted for. To validate the proposed theory, five circular CFT beam-column connections were constructed and tested. Test results showed that all specimens failed by the welding fracture while entering nonlinear stage. It is found that the higher the axial load was applied, the better the ductility of connections was obtained. Comparison of analytical and experimental results shows that the proposed prediction for panel shear falls in a reasonable range for higher axial load tests, but tends to be conservative for lower axial load tests.

Concrete-filled circular steel tubes subjected to pure bending

Current design codes and standards provide little information on the flextural behaviour of circular concrete filled tubes (CFT) as there have been few experimental studies. There are significant differences in d/ t-limits recommended in various codes for CFT under bending. This paper presents an experimental investigation of the flexural behaviour of circular CFT subjected to large deformation pure bending where d/ t=12 to 110. The paper compares the behaviour of empty and void-filled, cold-formed circular hollow sections under pure plastic bending. It was found that for the range of d/ t≤40, void filling prevented local buckling for very large rotations, whereas multiple plastic ripples formed in the inelastic range for specimens with 74< d/ t≤110. In general, void filling of the steel tube enhances strength, ductility and energy absorption especially for thinner sections. Based on the measured material properties, the plastic d/ t-limit was found to be 112. A simplified formula is provided to determine the ultimate flexural capacity of CFT. The existing design rules for the ultimate moment capacity of CFT may be extended conservatively to a new slenderness range of 100< λ s<188.