Concrete-filled Double Tube Research Articles

Analytical solution for predicting the interaction stress of axially loaded concrete-filled double-tube columns

Concrete-filled double-tube (CFDT) members are promising steel-concrete composite structural forms. Existing studies mainly focus on members’ load-deformation performance. The interaction stress between components, which is a fundamental cause for change in load-deformation performance, receives much less attention. This study presents an analytical solution for interaction stress between components of axially loaded CFDT columns. The stress–strain relationships and force equilibrium equations of the inner concrete, inner steel tube, sandwiched concrete, and outer steel tube were analysed using elasticity method first. Subsequently, the deformation compatibility equations were established concerning the interactions between the components. Based on the above analysis, an analytical solution was derived. Equivalent lateral deformation coefficients were used to substitute Poisson’s ratio of concrete to account for the nonlinear behaviour of materials during loading. The proposed model shows reasonable agreement with experimental results. Finite element analysis further shows the accuracy of the interaction stress model till an axial compressive strain of 0.01 of the composite column. The derived interaction stress model was successfully applied in predicting the load-bearing capacities of CFDT columns through validation of 39 specimens from five independent studies. The average ratio of the predicted-to-experimental bearing capacities was 1.054, with a coefficient of variation of 0.077. The accuracy of models in Eurocode 4, ACI 318-11, and GB 50936 were also evaluated. Thereby the proposed model provides insights into the interaction analysis between components and mechanism-based load-bearing capacity prediction of CFDT columns.

Axial compression performance analysis and bearing capacity calculation on square concrete-filled double-tube short columns

The concrete-filled double-tube (CFDT) columns have been proposed in recent years. The existence of inner steel tube can not only increase the confinement on core concrete, but also increase its fire resistance and anticorrosion performance. This makes it have a great application prospect in the field of ocean structure engineering. In this paper, the axial compression performance of square CFDT short columns is comprehensively analyzed. The specimens studied consist of an external steel tube (square hollow section) and an inner steel tube (circular hollow section), in which concrete is completely filled. The fine finite element (FE) method is used to simulate the performance of this column. Two constitutive models of confined concrete for square CFDT core concrete are proposed. In order to ensure the correctness of the model, the verification analysis of the model is carried out based on the previous experimental results, and the whole process of axial compression performance of CFDT columns is analyzed in detail. The concrete strength (fc'), width-thickness ratio (B/to), diameter-thickness ratio (d/ti), diameter-width ratio (d/B) and yield strength (fy) of square CFDT columns are compared and analyzed. The performance of CFDT columns with the same steel content is also analyzed, it is of great significance to improve the application of CFDT columns in marine environment or high temperature environment. Finally, new formulas for calculating the ultimate bearing capacity of CFDT short columns under axial compression are proposed. The results show that the new formulas can predict the strength of CFDT short columns well.

Numerical investigation for the behaviour of stiffened circular concrete filled double tube columns

Concrete filled double tube (CFDT) columns are considered effective to be used in heavy loaded structures, such as bridges, subway stations, and skyscrapers owing to the numerous advantages of concrete filled steel tube (CFST) columns. This paper aimed to numerically study the performance of CFDT columns under concentric loads and present innovative strengthening methods for CFDT columns, which yield more load bearing capacity compared to the existing ones. The proposed stiffened CFDT column will meet the requirements for use in heavy loaded structures. Circular steel hollow sections have been used for the inner and outer tubes. Stiffeners with different profiles (rectangle, T and bar stiffeners) have been welded to the outer and inner tube to check their contribution in increasing the load capacity and enhancing the performance of the circular CFDT columns. The behaviour of CFDT columns is investigated numerically by using a verified finite element (FE) model from the ABAQUS program. Verification of the FE model is achieved by comparing the obtained results with previous test results. Several parameters have been investigated using the FE model. The result revealed that using stiffeners in CFDT Column has a significant effect in increasing its load capacity, which can be increased to more than 15% of its original capacity by increasing number of stiffener and decreasing its slenderness. Based on the parametric study design guide line is presented. A new simplified design model is proposed for the prediction of the axial load capacity of stiffened circular CFDT columns.

Compressive strength and stiffness of concrete-filled double-tube columns

Concrete-filled double-tube (CFDT) columns have the potential to be widely used in high-rise buildings due to their very high load-carrying capacity, excellent ductility and good fire resistance. This new type of composite construction has attracted increasing research attention in the last decade, but there is a need to develop practical design recommendations for this construction. This paper aims to study the ultimate strength, the ultimate strain corresponding to the ultimate strength (ductility), and the stiffness of CFDT stub columns under axial compression. An existing finite element model is directly used to study square CFDT stub columns and this model is further modified to simulate circular CFDT stub columns under axial compression. Numerical data are generated to cover a wide range of parameters for CFDT stub columns. Through regression analysis, simple equations are proposed to calculate the ultimate strength, the compressive stiffness and the ultimate strain of CFDT stub columns. These equations can assist engineers to conduct structural analysis and design of CFDT columns.

Axial compressive behaviour of concrete-filled double-tube stub columns with stiffeners

Concrete-filled double-tube (CFDT) columns, which are a new type of composite columns, have high fire resistance and the potential to be widely used in high-rise buildings. It is expected that the amount of steel used in a regular CFDT column will be relatively high due to the use of double tubes. To reduce the steel consumption, a thin-walled steel tube with longitudinal stiffeners may be adopted for the outer tube in a CFDT column. This paper studies the behaviour of concrete-filled stiffened double-tube (CFSDT) stub columns under axial compression. Tests on 12 CFSDT stub columns and two reference columns were carried out accordingly, and the test results confirm that the stiffened columns have high strength and good deformation capacity. A finite element model was developed to analyse the interaction between the steel tubes and concrete. Based on further parametric studies, a superposition model was proposed to predict the axial compressive strength of CFSDT stub columns.

EXPERIMENTAL STUDY ON STRUCTURAL BEHAVIOR OF FIRE PROTECTED DOUBLE CFT COLUMN SUBJECTED TO FIRE LOAD

New concrete filled double-tube (CFDT) sections consist of an inner and outer tube with fire protection mortar (FPM) filling the cavity between them and the inner tube also filled with concrete or not. An investigation into the fire performance of CFDT during the standard fire test is reported. Six full size FPM filled CFDT columns were designed for the fire tests. Detail failure modes of overall specimens and each component in the columns as well as temperature, deformation and fire endurance were presented. It showed that the fire resistance in the CFDT columns is significantly higher than that in concrete filled steel tubular (CFT) columns. Investigation into the fire performance of the columns reveals possible solutions to improve the fire resistance of CFT members.

Experimental study on the seismic behavior of high strength concrete filled double-tube columns

To study the seismic behavior of high strength concrete filled double-tube (CFDT) columns, each consisting of an external square steel tube and an internal circular steel tube, quasi-static tests on eight CFDT column specimens were conducted. The test variables included the width-to-thickness ratio (β1) and the area ratio (β2) of the square steel tube, the wall thickness of the circular steel tube, and the axial force (or the axial force ratio) applied to the CFDT columns. The test results indicate that for CFDT columns with a square steel tube with β1 of 50.1 and 24.5, local buckling of the specimen was found at a drift ratio of 1/150 and 1/50, respectively. The lateral force-displacement hysteretic loops of all specimens were plump and stable. Reducing the width-to-thickness ratio of the square steel tube, increasing its area ratio, or increasing the wall thickness of the internal circular steel tube, led to an increased flexural strength and deformation capacity of the specimens. Increasing the design value of the axial force ratio from 0.8 to 1.0 may increase the flexural strength of the specimens, while it may also decrease the ultimate deformation capacity of the specimen with β1 of 50.1.