Local Buckling Restraining Behavior Of Concrete-Filled Steel Tubular Columns Under Seismic Loads

Extensive investigations have verified that frame systems consist of concrete-filled steel tubular (CFST) columns have more benefits than ordinary reinforced concrete and steel systems. The CFST column increases earthquake resistant capabilities due to the concrete filling inside the steel tubes and are ideal for buildings subjected to large compressive stress. The use of CFST columns is drawing attention due to their strength and quake-proof advantages. Local buckling of the steel tube is delayed by the restraint of the concrete, and the strength of concrete is increased by the confining effect of the steel tube. This thesis deals with the local buckling restraining behavior of thin-walled CFST columns under seismic loads by conducting a bidirectional cyclic loading numerical analysis. The CFST columns are modeled and analyzed, by the commercial computer program ABAQUS, to calculate the responses of the CFST columns under bidirectional cyclic load. The obtained results from analysis indicate that the buckling deformation should be slowed for the reduction in compressive force on buckled part due to shifting of compressive force from steel tube to the in-filled concrete. In addition, under a cyclic load applied after the occurrence of local buckling, the opening and closing of major horizontal cracks and dilation occur in the in-filled concrete. As a result, a predominant tensile axial force will act repeatedly on the buckled part of the outer steel tube. This tensile force restrains or restores the local buckling deformations by stretching them. The magnitude of the tensile force could be enhanced by installing diaphragms on the steel tube at the upper surface of the in-filled concrete. The ratio between the residual sway displacement δr and the maximum response sway displacement δm, defined as δr/δm for partially CFST columns, is smaller than that for hollow columns because of the enhanced strength and ductility of CFST columns. An extensive study will be carried out to derive seismic design equations for Concrete Filled Steel Tubular Columns

Confining and hoop stresses in ring-confined thin-walled concrete-filled steel tube columns

Concrete-filled steel tube (CFST) columns are now widely adopted in many structures due to the superior behaviour provided by composite action. However, this composite action is limited because of the different dilation properties of the constitutive materials at the early elastic stage. Furthermore, due to inelastic outward buckling of the steel tube, CFST columns may suffer serious degradation of the steel confinement. To overcome these problems, external confinement in the form of rings has been recently studied, and test results have shown that such provision can improve the strength, elastic stiffness, ductility and interface bonding of CFST columns. In the work reported in this paper, as a continuation of a previous study of ring-confined CFST columns, ten concrete-filled thin-walled steel tubes were fabricated and tested under uniaxial compression. The main parameters examined were the concrete cylinder strength, the steel tube thickness and the ring spacing. The test results show that external rings are highly effective in improving the uniaxial behaviour of CFST columns. Based on the experimental hoop–axial strain relationships and Prandtl–Reuss theory for the steel tube, the true structural behaviour of unconfined and ring-confined CFST columns, such as the steel–concrete interface bonding condition, the behaviour of the steel tube and core concrete as well as the confining mechanisms, were explored

Axial behaviour of concrete filled steel tube stub columns: a review

[EN] Concrete-filled steel tubular (CFST) columns are widely used in construction of high-rise buildings and peers of bridges to increase the lateral stiffness of the buildings, the axial load capacity, ductility, toughness, and resistance of corrosion of the columns. The CFST columns have much superior characteristics compared with traditionally reinforced concrete columns. The position of the concrete and steel tube in the cross-section of the CFST column is the most appropriate solution in terms of the strength and ductility. The steel tube, which is placed outside of the cross-section of the column, withstand the bending moment effectively. The concrete that is placed into the steel tube delay the local buckling of the steel tube and increase the axial load capacity of the column due to continually lateral confining. This paper presents a review on experimental results of the axial behavior of CFST columns performed by various researchers.Güler, S.; Korkut, F.; Yaltay, N.; Yavuz, D. (2018). Axial behaviour of concrete filled steel tube stub columns: a review. En Proceedings of the 12th International Conference on Advances in Steel-Concrete Composite Structures. ASCCS 2018. Editorial Universitat Politècnica de València. 359-364. https://doi.org/10.4995/ASCCS2018.2018.7602OCS35936

Improved behaviour of concrete-filled-stee-tube columns with external confinement

In high-strength concrete columns, because of the heavy demand of confining steel to restore the column ductility, it is more efficient to provide the confinement in the form of steel tube to form concrete-filled-steel-tube (CFST) column. Comparing with the transverse steel, CFST columns provide a stronger and more uniform confining pressure to the concrete core, and reduce the steel congestion problem for better concrete placing quality. However, a major shortcoming of CFST columns is the imperfect steel-concrete interface bonding at the elastic stage as steel dilates more than concrete in compression. This adversely affects the confinement of the steel tube and decrease the elastic modulus. To resolve the problem, it is proposed in this study to use external steel confinement in the forms of rings and ties to restrict the dilation of steel tube. For verification, a series of uni-axial compression test was performed on some CFST columns to study the effectiveness of external confinement. From the results, it was found that: (1) Both rings and ties improved the stiffness of the CFST columns; (2) The rings improve significantly the axial strength of the CFST columns while the ties did not improve the axial strength; (3) All externally confined CFST columns can reach a strain of at least 20% before failure occurs.postprintThe Australian Earthquake Engineering Society (AEES) 2011 Conference, Barossa Valley, South Australia, 18-20 November 2011. In Proceedings of the AEES Conference, 2011, paper 2

Investigation of the effect of impact load on concrete-filled steel tube columns under fire

Concrete-filled steel tube (CFST) columns are increasingly used in the construction of high-rise buildings which require high strength and large working space especially at lower stories. As compared to reinforced concrete columns, existence of the exterior steel tube not only bears a portion of axial load but also most importantly provides confinement to the infill concrete.with the confinement provided by the steel tube, axial strength of the infill concrete can be largely enhanced.this paper presents the investigation effect of impact load on concrete-filled steel tube columns under fire by numerical simulations using ABAQUS software.the results indicate that the CFST sections with larger confinement factor ξ=1.23 behaved in a very ductile manner under lateral impact. And the sections with smaller confinement factor ξ=0.44  generally behaved in a brittle mechanism

Plastic dilation rate characteristic of concrete confined with steel tube

The use of external confining devices to confine concrete has become widely used. One of the purposes is to gain additional concrete strength and ductility. Although there are many types of external confining devices, in this paper, the attention is limited to the use ofthe steel tube as anexternal confining device. One of the main objectives ofthis research is to study the plastic dilation rate behavior of concrete-filled-steel-tube (CFST) columns. The experimental data for the plastic dilation rate is extracted, and compared with the authors concrete plasticity model. In the authors’ previous research, the calibration of the plastic dilation rate model was based on confined concrete tested under both active and passive confinement using FRP wraps. Since the behavior of the steel tube and the FRP materials are different, the author’s plastic dilation rate model needs to be re-evaluated for CFST columns. Comparisons of the extracted experimental plastic dilation rates with the model prediction for CFST specimens with normal strength concrete show good agreement and requires no adjustment in the formulation. However, for a specimen with 80 MPa concrete, the proposed formulation showsslightly lowerplastic dilation rates.More experimental data for CFST using high strength concretes is required for further investigation. For the sake of completeness, the overall response of two CFST specimensisalso evaluated using anin-house three-dimensional non-linear finite element analysis (3D-NLFEA) using the author’s proposed plasticity formulation for confined concrete

Concrete-encased CFST structures: behaviour and application

[EN] Concrete-encased CFST (concrete-filled steel tube) is a kind of composite structure comprised of a CFST component and a reinforced concrete (RC) component. The concrete encased CFST possesses superior ductility and higher stiffness. They are gaining popularity in high-rise buildings, large-span structures, bridges, subway stations and workshops. This paper initially reviews the recent research on concrete-encased CFST structures. The major research findings on bond performance, static performance, dynamic performance and fire resistance are presented. This paper also outlines some construction considerations, such as the utilization of materials, the fabrication of the steel tube, and the methods of casting the inner and outer concrete. Finally, some typical practical projects utilizing concrete-encased CFST members are presented and reviewed.The research reported in this paper is part of the Project 51678341 supported by the National Natural Science Foundation of China (NSFC). The financial support is highly appreciated.Han, L.; Ma, D.; Zhou, K. (2018). Concrete-encased CFST structures: behaviour and application. En Proceedings of the 12th International Conference on Advances in Steel-Concrete Composite Structures. ASCCS 2018. Editorial Universitat Politècnica de València. 1-10. https://doi.org/10.4995/ASCCS2018.2018.7109OCS11

Fire resistance of concrete-filled steel tube columns with preload:Part II: Numerical and analytical investigation

Cast-in-situ concrete-filled steel tube (CFST) structures are inevitably subjected to preload that are developed in the steel tube during the construction process. These preloads may have detrimental effects on the overall performance of a CFST component, such as a CFST column, especially when the column is subjected to elevated temperature. However, existing design methods of CFST exclude the impacts of preload in fire resistance design. In this paper, a three-dimensional finite element model for predicting fire resistance of CFST with preload is developed and validated by experimental tests. The model is then used to predict fire resistance time of CFST columns with different slenderness, load and preload ratios. The results show that preload of the steel tube have little influence on the fire resistance of short CFST columns, while the influence of preload on the fire resistance can be significant when the slenderness ratio is greater. Further increase of the slenderness ratio exceeding a certain range, however, reduces the effect of preload. It can be generally concluded that fire resistance of slender CFST columns decreases with increase of preload ratios and the effect of preload on fire resistance of CFST columns is more prominent when the load ratio is greater. In addition, formulas for calculating fire resistance of cast-in-situ concrete-filled steel tubes (CFST) with preload are proposed. This paper is a companion paper of Yu et al. (submitted for publication)

Analytical behaviour of concrete-encased CFST box stub columns under axial compression

[EN] Concrete-encased CFST (concrete-filled steel tube) members have been widely used in high-rise buildings and bridge structures. In this paper, the axial performance of a typical concrete-encased CFST box member with inner CFST and outer reinforced concrete (RC) is investigated. A finite element analysis (FEA) model is established to analyze the compressive behavior of the composite member. The material nonlinearity and the interaction between concrete and steel tube are considered. A good agreement is achieved between the measured and predicted results in terms of the failure mode and the load-deformation relation. The verified FEA model is then used to conduct the full range analysis on the load versus deformation relations. The loading distributions of different components inclouding concrete, steel tube and longitudinal bar during four stages are discussed. Typical failure modes, internal force distribution, stress development and the contact stress between concrete and steel tube are also presented. The parametric study on the compressive behavior is conducted to investigate the effects of various parameters, e.g. the strength of concrete and steel, longitudinal bar ratio and stirrup space on the sectional capacity and the ductility of the concrete-encased CSFT box member.Chen, J.; Han, L.; Wang, F.; Mu, T. (2018). Analytical behaviour of concrete-encased CFST box stub columns under axial compression. En Proceedings of the 12th International Conference on Advances in Steel-Concrete Composite Structures. ASCCS 2018. Editorial Universitat Politècnica de València. 401-408. https://doi.org/10.4995/ASCCS2018.2018.6966OCS40140

Static behavior of T-shaped concrete-filled steel tubular columns subjected to concentric and eccentric compressive loads

In this paper, a batch of T-shaped concrete-filled steel tubular (CFST) columns, T-shaped steel tube confined concrete (STCC) columns and T-shaped reinforced concrete (RC) counterparts were tested subjected to concentric compressive load or eccentric compressive load. The battlement-shaped bar and tensile bar were developed as stiffeners to delay local buckling in tube wall. The stiffening mechanism, the failure modes of concrete and steel tubes, the strength and ductility of specimens were investigated in detail during the experimental research. A numerical modeling program was developed and verified against the experimental data. The numerical program, which incorporates the effect of the stiffeners on postponing local buckling of the tube and the confinement of the tube on concrete core, was used to carry out comprehensive parametric analysis of influencing factors on the structural behavior of T-shaped CFST columns