Concrete-filled Steel Tubular Joints Research Articles

Fatigue assessment of CFST joints using the effective notch stress approach

Compared to tubular joints formed by hollow steel sections, concrete-filled steel tubular (CFST) joints exhibit more complicated failure modes and mechanism under fatigue load due to the nonlinear material confinement and the associated stress redistribution. For a reliable fatigue design of such composite tubular joints, these complexities need to be fully addressed and reasonably quantified, which necessitates an accurate approach for assessing their fatigue performance. The effective notch stress approach is widely recognized in fatigue design for its applicability and efficiency in evaluating the fatigue strength since it can rationally account for the effects of geometric configurations and welding defects on the fatigue performance. However, it has not been widely adopted in the assessment of CFST joints due to the high costs involved in modelling and computing. In this paper, a refined nonlinear finite element model is established, which facilitates the acquisition of reliable notch stress values. Subsequently, a statistical analysis is conducted on the available fatigue data of CFST joints, employing the computed notch stress to yield an S–N curve. A comparative evaluation on the results obtained from the nominal stress approach, the hot-spot stress approach and the effective notch stress approach testifies the superior accuracy of the last in assessing the fatigue strength of CFST joints.

Fire and post-fire performance of blind-bolted concrete-filled steel tubular joints with restrained steel beams: Experiment and simulation

Blind-bolted connection is a type of new developed prefabricated beam-column connection, which can be used to connect the concrete filled steel tubular (CFST) columns and the steel beams easily. The static and dynamic performance of CFST column to steel beam joints with blind-bolted connections have been well explored, but limited research has been conducted to investigate the fire and post-fire performance of this type of composite joint. In this paper, a series of reversed T-shaped joint fire tests were conducted to investigate the fire and post-fire performance of blind-bolted CFST joints with axially restrained steel beams. The influence of restrained effect from adjacent members in the entire frame structure on the response of blind-bolted joints in and after fire could be reasonably considered by the reversed T-shaped joint fire test. The joint temperature, failure modes, beam and column displacements and temperature internal force development during the entire loading and fire stage were recorded and discussed. The test results indicated that the blind-bolted connections showed very good working performance in and after fire, and no bolt shank fracture or bolt pull-out failure was observed. Finite element analysis (FEA) models were established to simulate the fire and post-fire performance of blind-bolted CFST T-shaped joints with axially restrained steel beams, and the joint moment versus relative rotation angle relationship curves of the blind-bolted joints during the entire fire and loading process were extracted and discussed based on the FEA models. The results indicated that the joint negative moment generated by the axially restrained beam boundary in the heating and cooling phases could go up to around 1.4 times the joint negative moment at ambient temperature.

Post-fire seismic performance of concrete-filled steel tubular column to steel beam joints with blind-bolted connections: Experiment and simulation

During the life-cycle of a building, the structure might be subjected to multiple hazards such as fire, earthquake and impact. Compared with the traditional structure design methods based on a single type of disaster load, the structure subjected to multiple and coupled disaster loads would be more dangerous and worth to be investigated. In this paper, the post-fire seismic behavior of concrete-filled steel tubular (CFST) column to steel beam joints with blind-bolted connections subjected to coupled fire and cyclic loads were investigated. A total of ten post-fire hysteresis joint tests were designed and conducted, and the testing parameters included heating time, post-fire column load ratio and thickness of fire protection layer. The test results indicated that the joint failure mode changed from column failure to connection failure when the post-fire column load ratio ranged from 0.3 to 0.6. Generally, the blind bolts in the panel zone of this joint specimen showed acceptable working performance in fire, but gap between the stee tube and beam end plate might be formed during the post-fire cyclic loading process. Finite element analysis (FEA) models were established to simulate and discuss the post-fire seismic performance of this type of blind-bolted CFST joints under the coupled fire and loading process. Finally, a simplified post-fire hysteretic relationship model for this type of joint was proposed.

Experimental investigation on the seismic performance of concrete-filled steel tubular joints in diagrid structures

Concrete-filled steel tubular (CFST) joints are widely used as a common joint form in diagrid structure systems. However, previous studies on CFST diagrid joints have mainly focused on the compressive bearing capacity, and they lack relevant experimental studies on the seismic performance of diagrid structure joints. In this paper, five CFST diagrid joints are tested under cycle loading to study the failure mode and hysteretic characteristics. In addition, the influence of the intersection angle, structural forms and the wall thickness of the steel tube on the seismic performance of the joints, such as the bearing capacity, ductility, and energy dissipation, are analysed. Three CFST joints are mainly damaged by shear cracking in the centre of the joints, and the other two specimens are damaged by buckling of the inclined column. Test results showed that the increase in the intersection angle reduces the bearing capacity of the specimen and changes the distribution of the main energy dissipation; the increase in the wall thickness of the steel tube can effectively improve the bearing capacity and energy dissipation capacity of the specimen. The joint reinforcing ring can effectively improve the ductility, energy consumption and other seismic performance of the joint. In the CFST diagrid joints, the weak parts along the horizontal intersecting line of the joint and the intersection between the joint and inclined column need to be strengthened in the actual structure.

Parametric study and equations of the maximum SCF for concrete filled steel tubular T-joints under in-plane and out-of-plane bending

Due to being increasingly used in infrastructure, studying stress concentration in concrete-filled steel tubular joints is necessary. In this study, the variation of the maximum Stress Concentration Factor (SCFmax) with non-dimensional geometric parameters in steel circular hollow section brace welded to concrete-filled circular hollow section chord (CHS-to-CFCHS) T-joints under in-plane and out-of-plane bending moment has been investigated. A database of the maximum SCFs in CHS-to-CFCHS T-joints under in-plane and out-of-plane bending moment is developed based on three-dimensional (3D) finite element (FE) models. The 3D FE models developed using ABAQUS software, have been verified using experimental results. Graphs showing variation of the maximum SCF, in CHS-to-CFCHS T-joints, with non-dimensional geometric parameters have been produced and compared with those for non-filled empty T-joints. Parametric equations for predicting the maximum SCFs in CHS-to-CFCHS T-joints under in-plane and out-of-plane bending moment have been developed in a multiple nonlinear regression analysis. There is a good agreement between the maximum SCFs predicted by the parametric equations and those determined from the experiments.

Parametric study and equation of the maximum SCF for concrete filled steel tubular T-joints under axial tension

Concrete-filled steel tubular joints are increasingly being used for built infrastructure such as bridges and towers. In this study, the variation of the maximum Stress Concentration Factor (SCFmax) with non-dimensional geometric parameters in concrete-filled steel tubular (CFST) T-joints under axial tension has been investigated. A database of the maximum SCFs in CFST T-joints under axial tension is developed based on three-dimensional (3D) finite element (FE) models. The 3D FE models developed using ABAQUS software have been verified using experimental results. Graphs showing variation of the maximum SCF, in CFST T-joints, with non-dimensional geometric parameters have been produced and compared with those for non-filled empty T-joints. A parametric equation for predicting the maximum SCFs in CFST T-joints, a useful parameter for design, has been developed in a multiple nonlinear regression analysis. There is a good agreement between the maximum SCFs predicted by the parametric equation and those determined from the experiments.

Experimental study on joint resistance and failure modes of concrete filled steel tubular (CFST) truss girders

Concrete Filled Steel Tubular (CFST) structures are increasingly used not only for columns in tall buildings but also in the arch trusses of many arch bridges and in the truss girders of buildings and bridge decks. Therefore, the chords of Circular Hollow Section (CHS) truss arches and girders are increasingly filled with concrete, effectively making them CFST structures. In addition to the strength and stiffness of the CFST members, the failure mode of the CFST joint connecting them to the tubular member is also affected by the concrete filling. In this study, truss girders with different web arrangements were tested, and their behaviour investigated. The girders were not slender because they were designed to attain the peak limiting state for joint failure rather than chord failure due to bending moments. Moreover, two other types of girders were tested: one without concrete-filled chords (CHS girder) and another with only the upper chord filled with concrete; thus allowing an investigation of how a concrete-filled chord affects joint failure mode. The geometry of the CHS girder joints was such that only chord face failure and punching shear failure could occur. The former required an inward deformation that was prevented by the concrete filling in a CFST girder with similar geometry. Finally, the study considers extending the Eurocode 3 and the AWS D1.1 code formulae, originally proposed for CHS joints, to calculate the resistance of CFST joints.

08.40: Numerical investigation of mechanical properties of multi‐planar concrete‐filled steel tubular joints

ABSTRACTWith the development of large span and heavy load modern engineering, concrete‐filled steel tubular structures appear constantly. The joints of these structures are mostly multi‐planar joints. In order to solve the problem of lacking of the investigation of multi‐planar concrete‐filled steel tubular (CFST) joints, the mechanical behaviour of CFST KK joints were studied through refined finite element simulation. Accurate finite element models of CFST KK joints were established by considering the material nonlinearity, the geometry nonlinearity, and the contact nonlinearity between the tube and the concrete. The spatial effect of multi‐planar joints was be obtained by comparing with planar joints. The FEA results showed that, the multi‐planar carry‐over effects had the maximum effect for the 60° CFST joints, the strength of multi‐planar joints decreased compared with the planar joints, and the stiffness of them did not vary substantially. While for the 90° CFST joints, both the strength and stiffness of multi‐planar joints did not vary substantially with the planar joints. This research can provide scientific basis for the design of multi‐planar CFST joints, and effectively promote the application of CFST structures.

Temperature analysis of extended end plate joints to square CFST columns in fire

Purpose This paper aims to obtain fire resistance of semi-rigid joints for concrete-filled steel tubular (CFST) composite frames and temperature filed distribution of composite joints in fire. Design/methodology/approach The temperature filed model of semi-rigid joints to CFST columns with slabs was made by using ABAQUS finite element (FE) software, in considering temperature heating-up stage of fire modelling. The effects of composite slab, fire type and construction location were discussed, and the model was verified by the test results. The temperature distribution of composite joint under three-side or four-side fire condition was studied by the sequentially coupled thermal analysis method. The temperature versus time curves and temperature distribution of various construction and location were analyzed. Findings The paper provides FE analysis and numerical simulation on temperature field of semi-rigid joints for CFST composite frames in fire. The effects of composite slab, fire type and construction location were discussed, and the model was verified by the test results. It suggests that the temperature distribution of composite joint in three- or four-side fire condition showed a different development trend. Research limitations/implications Because of the chosen FE analysis approach, the research results may lack generalizability. Therefore, researchers are encouraged to test the proposed propositions further. Practical implications The research results will become the scientific foundation of mechanical behavior and design method of semi-rigid CFST composite frames in fire. Originality/value This paper fulfils an identified need to study the temperature field distribution of the semi-rigid joints to CFST columns and investigate the mechanical behavior of the semi-rigid CFST joints in fire.

Fatigue behaviour of concrete-filled steel tubular joints - a review

High stress concentration at weld vicinity in welded steel tubular joints has led researchers to try finding an effective method to reduce these stresses. Since the beginning of the last decade, concrete filling of the steel tubes has been effectively used to reduce stresses at the tubular joints. This paper summarises research on stress concentration factors and fatigue behaviour conducted on concrete-filled steel tubular T-, Y-, K-, KT-, N-, and X-joints. Based on available experimental results, nominal stress range and hot spot stress range design S-N curves have been proposed for concrete-filled steel tubular joints. These curves are compared with that of the tubular joints without concrete filling. Research gaps have also been highlighted and it has been found that the current research is insufficient to reasonably predict fatigue behaviour of concrete filled tubular joints.

Fatigue behaviour of concrete-filled steel tubular joints - a review

Fatigue behaviour of concrete-filled steel tubular joints - a review

DESIGN S-N CURVES FOR T-, K-, AND X-CONCRETE-FILLED STEEL TUBULAR JOINTS

High stress concentration at the weld vicinity in welded steel tubular joints results in fatigue failure. Researchers have tried finding an effective method to reduce these stresses. Although concrete filling of the steel tubes has been used to reduce stresses at the joints in the recent two decades, the fatigue performance of concrete filled tubular joints is not yet fully understood. This paper summarizes research on the fatigue strength of concrete-filled steel tubular T-, K-, and X-joints. Based on available experimental results, the nominal stress and hot spot stress design S-N curves for concrete-filled steel tubular joints have been derived. The least squares method is used in the derivation of the design S-N curves according to the American Society for Testing and Materials (ASTM) specifications.

Post-fire behaviour of concrete-filled steel tubular column to axially and rotationally restrained steel beam joint

This paper presents a numerical investigation on the post-fire behaviour of concrete filled steel tubular (CFST) column to restrained steel beam joint. An entire loading and fire phase, including ambient loading, heating with constant loads, cooling with constant loads and post-fire loading, was employed in the numerical analysis, and a finite element analysis (FEA) model was built to simulate the behaviour of CFST column to axially and rotationally restrained steel beam joints with external diaphragm connections under the entire loading and fire phase. For validation, the proposed modelling method was used to predict the test results of CFST columns and joints in fire and post-fire. The comparison demonstrates that the accuracy of the proposed FEA model is acceptable. Afterwards, the FEA model was used to analyse the mechanics characteristics of CFST column to restrained steel beam joints in the entire loading and fire phase. Based on the numerical analysis, the joint moment versus relative rotation angle relationship in the entire loading and fire phase was addressed, and the residual joint strength index and stiffness index were defined to evaluate the post-fire performance of joints. Finally, simplified calculating formulas were proposed to calculate the two indexes, which provide a simply and feasible method to evaluate the post-fire performance of external diaphragm joints in the CFST column - steel beam framed structure.

Mechanical Behavior Experimental Study on Lattice Wind Turbine Tower KT-Type Joints of Concrete-Filled Steel Tubular

The mechanical behavior of joints on lattice wind turbine tower of concrete-filled steel tubular(CFST) is very complex, the safety and reliability of joints is very important for the whole structure. To better understand the mechanical properties of the joints, we design a static loading test of 3 KT-type CFST joints. According to the test, we make analysis about the failure progress, equivalent stress of joint intersection and the relation of load-defomation. The results showed that due to the presence of concrete, the failure mode of CFST joints is web members failed. The failure characteristic of overlap joints is the instability of compression web member while the gap joints are the weld tear of tensile web member. With the increase of the overlap ratio, the distribution of the equivalent stress is more uniform. The high stress area is in the gap and the tower wall which between the crown point and saddle point in tensile side.

Experimental Research on Seismic Behavior of Exterior Frame Joints with T-Shaped CFST Column and Steel Beam

In order to understand the mechanical performance and related tectonic design requirements of the concrete-filled steel tubular joints with special-shaped cross-section , three T-shaped concrete filled steel tubular column joints with exterior diaphragms were manufactured and experimented under cyclic loading. By changing the axial compression ratio of the columns and increasing the width and overhanging length of the stiffening ring, the hysteretic performance and working mechanism of such joints under cyclic loading were studied. The results show that by increasing the width and overhanging length of the stiffening ring, the hysteretic performance, ductility and energy dissipation capacity of joints were obviously improved.