Behavior Of Concrete-filled Steel Tubes Research Articles

Application of Ultrasonic Testing in Concrete Filled Steel Tubular Arch Bridge

Concrete Filled Steel Tube(CFST) is widely used in civil engineering structures because of its superior mechanical performance. Yet the mechanical behavior of CFST is highly depended on the construction quality of the filled concrete. Hence it is very important for the inspection of the construction quality of the filled concrete in CFST structures. In this paper, the ultrasonic testing technique was used to detect the defect of the filled concrete of a CFST arch bridge. During the inspection, the ultrasonic transducer was moved along the circumference of the cross-section of the arch, and the defect of the concrete was comprehensively judged by detecting the change of sonic time, sonic amplitude and sonic frequency. Based on the analysis of the ultrasonic transmission path, the influences of different defects on the sonic time, sonic amplitude and sonic frequency were discussed. The detecting results were verified by core-drilling method. The verification showed that different kinds of defects defected by ultrasonic testing was in good accordance with the drilling samples, which demonstrates the adaptability of the ultrasonic detection technique in the construction quality inspection of CFST structures.

Monotonic and cyclic behavior of concrete-filled steel-tube beam-columns considering local buckling effect

A mathematical model is developed to evaluate the monotonic and cyclic behavior of concrete-filled steel tube (CFST) beam-columns with rectangular cross section. The model includes the reduction in the steel compressive strength due the local buckling effect. The degradations in unloading and reloading stiffness of steel tube due to local buckling are also included. The model is based on fiber element method in which uniaxial stress–strain material laws are used for cross section components. The results obtained from the mathematical model were compared with experimental results for columns under monotonic as well as cyclic loads. It is observed that the proposed model predicts well the columns and beams nonlinear behavior compared with the experimental results.

Behaviour of concrete-filled steel tubes under static and variable repeated loading

This paper presents the results of static loading (SL) and variable repeated loading (VRL) tests on concrete filled steel tubular (CFST) columns. Assessment of the columns was based on its length, concrete strength and load eccentricity. The column behaviour (with and without filling) from the tests was studied. The ultimate strength of the columns subjected to VRL reduced by up to 16% after undergoing a number of load cycles. The incremental collapse (IC) limit was found to lie between 70% and 88% of the static collapse load for CFST columns. The deformations at IC limit were significant and could affect practical designs. The theoretical strengths of the stub and long columns tested are determined on the basis of building code 318 of the American Concrete Institute, and compared with the test results. The squash load equation of the code was found to underestimate the nominal strength of short composite columns.

Further study on the flexural behaviour of concrete-filled steel tubes

This paper is a further study on the flexural behaviour of concrete-filled steel tubes based on the former work presented by Han [Han LH. Flexural behaviour of concrete-filled steel tubes. Journal of Constructional Steel Research 2004;60(2):313–37]. A total of 36 composite beam specimens filled with self-consolidating concrete (SCC) were tested. The main parameters varied in the tests are: (1) sectional types (circular and square); (2) steel yielding strength (from 235 to 282 MPa); (3) the ratio of tube diameter (or width) to wall thickness, D / t (from 47 to 105), and (4) the ratio of shear span to depth (from 1.25 to 6). Comparisons are made with predicted beam capacities using the existing methods, such as AIJ-1997 [Architectural Institute of Japan (AIJ). Recommendations for design and construction of concrete filled steel tubular structures. 1997], AISC-LRFD-1999 [AISC. Load and resistance factor design specification for structural steel buildings. Chicago: American Institute of Steel Construction, Inc.; 1999], BS5400-1979 [British Standard Institute: BS5400, Part 5, Concrete and composite bridges. 1979], EC4-1994 [Eurocode 4. Design of composite steel and concrete structures, Part 1.1: General rules and rules for buildings (together with United Kingdom National Application Document). DD ENV 1994-1-1:1994. London W1A2BS: British Standards Institution; 1994] and the method proposed by Han [Han LH. Flexural behaviour of concrete-filled steel tubes. Journal of Constructional Steel Research 2004;60(2):313–37]. Applied calculation formulae of moment versus curvature curves and the flexural stiffness of concrete-filled steel tubular (CFST) beams are presented, based on the mechanics model of Han [Han LH. Flexural behaviour of concrete-filled steel tubes. Journal of Constructional Steel Research 2004;60(2):313–37]. Comparisons are made with predicted beam flexural stiffness using different methods, such as AIJ-1997, AISC-LRFD-1999, BS5400-1979, EC4-1994 and the method proposed in this paper. Comparisons are also made between the simplified model and the mechanics model, and generally good agreement is achieved.

CONCRETE-FILLED TUBULAR COLUMNS: PART 1 — CROSS-SECTION ANALYSIS

A fiber-based approach is adopted to study the cross-sectional behavior of concrete-filled steel tubes subjected to monotonic and cyclic loading. Constitutive behavior of the cross-section is formulated by explicitly dividing the section into fibers, and uniaxial stress–strain rules for steel and concrete are defined. Interaction curves, Moment–Thrust–Curvature (M–P–φ) relations and Moment–Thrust–Strain (M–P–ε) relations are developed for circular and box-shaped in-filled sections based on fiber analysis. A fiber model is also developed to describe the nonlinear response of concrete-filled steel tubes subjected to cyclic loading. M–P–φ relations are developed for arbitrary loading histories using this simple yet reliable approach. Results are compared with the numerical and experimental data available in published literature, and the accuracy of the proposed model is thus ascertained.

Flexural behaviour of concrete-filled steel tubes

This paper develops a mechanics model that can predict the behaviour of concrete-filled hollow structural section (HSS) beams. A form of unified theory, where a confinement factor ( ξ) was introduced [Steel Compos. Struct.—Int. J. (2001) 1(1) 51] to describe the composite action between the steel tube and filled concrete, is used in the analysis. A series of concrete-filled square and rectangular tube beam tests were carried out. The main parameters varied in the tests were the depth-to-width ratio ( β) from 1 to 2, and tube depth to wall thickness ratio from 20 to 50. The load vs. lateral deflection relationship was established for concrete-filled HSS beams both experimentally and theoretically. The predicted curves of load vs. mid-span deflection are in good agreement with the presented test results. Formulas which should be suitable for incorporation into building codes are developed for calculating the moment capacity of concrete-filled HSS beams. Comparisons are made with predicted beam capacities and flexural stiffness using the existing codes, such as AIJ-1997, BS5400-1979, EC4-1994, and LRFD-AISC-1999.

Creep modeling for concrete-filled steel tubes

Using the rate of flow method and the double power law function for basic creep of concrete, an algorithm is developed for the time-dependent behavior of concrete-filled steel tube (CFT), with or without the interface bond. The model adheres to geometric compatibility and static equilibrium, and considers the effects of sealed concrete, multi-axial state of stresses, creep Poisson’s ratio, stress redistribution, variable creep stress history, and creep failure of the column. The model is verified against previous creep tests for bonded and unbonded specimens. A study is then carried out on the practical design parameters that may affect creep of CFT columns under service loads, or lead to their creep rupture at high levels of sustained load. The study indicates that creep of CFT columns should be considered in the design, however, with creep coefficients much lower than those prescribed in the current ACI. Creep of CFT is shown to be a function of concrete mix, column geometry, and interface bond. Therefore, a single ultimate creep coefficient cannot be used for all concrete mixes, column geometries, and construction types. Bonded tubes curtail creep of concrete much more than the equivalent unbonded ones, mainly because of the stress relaxation phenomenon, which is more pronounced for smaller diameter-to-thickness ratios. For diameter-to-thickness ratios of 40 or less, bonded tubes are more durable in creep rupture than the equivalent unbonded ones. Creep rupture life of 75 years is quite feasible in bonded CFT, with diameter-to-thickness ratio of 40 or less, for sustained loads as high as 65% of the static capacity of the column.

Slenderness limits for filled circular steel tubes

This paper presents a theoretical study of the local and post-local buckling of thin-walled circular steel tubes that contain a rigid infill, with the motivation being an investigation of the cross-sectional behaviour of concrete-filled steel tubes used as columns in composite construction. An analysis is undertaken to determine a closed form solution for the elastic buckling of a circular tube with a rigid infill, and a model of the post-local buckling response is proposed. This generic approach is calibrated against test data, and a cross-section slenderness limit is proposed that delineates between a fully effective cross-section and a slender cross-section. A simple prescriptive equation is proposed for the buckling strength of the steel cross-section that is consistent with many design codes, and illustrates that the presence of an infill may enhance the cross-sectional strength, not only by the added strength of the infill itself, but by delaying the buckling of the steel tube.