Circular Concrete-filled Steel Tubular Research Articles

Seismic Response Investigation and Analyses of End Plate Moment-Resisting CFST Frames Under Pseudo-Dynamic Loads

This paper reported an innovative semi-rigid composite frame comprised of concrete-filled steel tubular (CFST) columns and H-shaped steel beams assembled using flush or extended end plates with blind fasteners. Two specimens of two-story single-bay end plate moment-resisting square or circular CFST frames were investigated by a series of pseudo-dynamic tests. To analyze dynamic response of this type of structure under the earthquake action, time history response and story drift were investigated. The influence of the column section type and the end plate type on seismic performance of semi-rigid moment-resisting CFST frames under different earthquake loading levels were also discussed according to hysteretic behavior, rigidity degradation and energy dissipation capacity. The experimental and analytical results indicated that at the same section width and steel ratio of the column, the rigidity of end plate moment-resisting square CFST frame was greater than that of circular CFST frame, while the story drift of square CFST frame was smaller than that of circular CFST frame. The type of end plate moment-resisting CFST frames with blind fasteners, which can provide good seismic behavior and energy dissipation ability, can be well used in a seismic region.

Numerical Simulation Parametric Studies on Concrete-Filled Steel Tubular Columns

The finite element(FE) software is used to simulate concrete-filled steel tubular(CFST) columns. Based on the existing experimental data, the model is verified. The parameters studied include axial load ratio and shear span ratio, and the difference of seismic performance between circular CFST column and square CFST column under the condition of equal steel ratio is compared. The results show that with the decrease of axial load ratio, the hysteretic curve is plumper and the seismic performance is more stable. With the decrease of shear span ratio, the horizontal bearing capacity of the specimen increased significantly, but the cumulative damage effect was more significant. The simulation results also show that when the steel ratio is identical for circular and square CFST columns, the circular CFST column has superior seismic performance.

Post-heating response of concrete-filled steel tubular columns under sustained loads

Concrete-filled steel tubular (CFST) columns can generally be expected to better resist elevated temperatures as compared to unfilled steel hollow sections, whose evaluation after a fire is limited by the resulting deformation. A better understanding of the behaviour of CFST columns after a fire, which is dominated by the maximum temperature achieved by the concrete infill and plasticity of the steel, is required to properly estimate their residual strength and deformation and to adopt a reasonable strategy with minimum post-fire repair. In this paper, a fibre beam model for the simulation of the post-heating response of concrete-filled steel tubular (CFST) columns is presented. First, the model is validated against experimental results and subsequently it is employed to analyse the post-heating response of circular CFST columns under sustained loads. In reality, during a fire, the columns support load even during the cooling phase of a fire, so it is important to consider this loading condition when predicting the post-fire behaviour. The analysis presented in this paper comprises three stages: heating, cooling and post-fire (under sustained load) conditions. The model considers realistic features typical of the fire response of CFST columns, such as the existence of a gap conductance at the steel-concrete interface and the sliding and separation of the steel tube and the concrete. Based on the model, the response of CFST columns after heating is investigated via parametric analysis.

Concentrated-plasticity modelling of circular concrete-filled steel tubular members under flexure

The research reported herein aims at the proposal of an accurate and efficient simplified numerical modelling approach for circular concrete-filled steel tubular (CFST) members under flexural loading. Experimental tests were carried out to characterize the bending behaviour of CFST members under monotonic and cyclic loading. The observed behaviour was characterized by strength and stiffness deterioration effects, as a result of the development of local buckling of the steel tube and cracking of the concrete core. Numerical simulations of these tests were conducted by resorting to existing modelling approaches, namely through Distributed Plasticity (DP) and Concentrated Plasticity (CP) models. It was found that existing modelling approaches failed to accurately capture the levels of strength deterioration and pinching effects observed in the tests. Thus, a novel CP-based simplified model, designated by matCFSTdet, was implemented in OpenSees. The hysteretic response of the CP model is based on a novel rotational spring model. An advanced calibration framework was introduced with targets to calibrate the accuracy of the model. The validation analyses indicate that the model is able to capture well the deterioration in both strength and stiffness of CFST members under cyclic flexural loading. Furthermore, the elastic stiffness, ultimate strength and the pinching effects of the hysteretic loops were also well simulated. The proposed CP model, coupled with the advanced calibration framework, thus results in a more realistic simulation of the cyclic flexural response of circular CFST members.

Axial compressive performance of circular CFST columns partially wrapped by carbon FRP

To explore the axial compression property and failure mechanism of concrete-filled steel tubular (CFST) columns partially wrapped by carbon-fibre-reinforced polymer (FRP), a series of tests using carbon-FRP-strengthened circular CFST stubs and slender columns imposed with axial loads were conducted. The effects of various parameters including steel yield stress, number of carbon-FRP layer, slenderness ratio of the composite column, and spacing of carbon-FRP strip on the axial load bearing capacities of the tested specimens were studied. Moreover, the axial compression behaviour of the circular CFST columns with carbon-FRP composites was evaluated in terms of axial compression force (N)–longitudinal shortening displacement (δ) curves, axial stiffness, strain response, strength enhancement indexes, and ductility indexes. Subsequently, a nonlinear finite element (FE) analysis modelling concerning the surface contact action of carbon-FRP-strengthened circular CFST columns was established and validated experimentally. The experimental and analytical results indicate that strengthening the circular CFST stub columns using carbon-FRP wraps could improve their axial load bearing capacities and prevent outward local bulges for the thin-walled steel tubes. Only a slight effect was observed when laterally confined carbon-FRP strips were used for the type of slender composite columns. Finally, several simplified empirical formulas for predicting the axial load bearing capacity of the circular CFST column partially wrapped by carbon-FRP are proposed.

Circumferential gap and partial debonding effects on buckling loads and modes of slender CFST circular columns

This paper presents a new mathematical model for analytical investigation of global buckling behavior of slender concrete-filled steel tubular (CFST) columns with circumferential gaps and partial debonding between the concrete core and the steel tube. The analytical buckling load of circular and slender CFST columns with circumferential gaps and partial debonding is derived for the first time. The critical buckling load decreases as the magnitude and length of the circumferential gap increases. Nevertheless, it is shown that if the length of the circumferential gap is smaller than the length of the CFST column, this effect is less than 4%. On the other hand, for a fully delaminated CFST column, this effect can be up to approximately 40%. Similarly, the first buckling shape modes proved to be notably affected by the circumferential gap only if its length is greater than 75% of the CFST column length. The results can be used as a benchmark solution for the buckling problem of slender circular CFST columns with circumferential gaps and partial debonding between the materials.

Experimental and Numerical Studies on Concrete Filled Circular Steel Tubular (CFCST) Members Under Impact Loads

This paper presents experimental and numerical studies on the behavior of concrete filled circular steel tubular (CFCST) members under lateral impact loading. Fourteen CFCST member specimens were designed, fabricated and tested using a Drop Hammer Impact Test Machine in a simple double-supported boundary conditions. The main parameters include the outer diameter of CFCST member, the thickness of steel tube and the height of drop hammer. The failure modes, the impact force, the impact force time history curves and the strain time history curves were investigated to evaluate the behavior of CFCST member under an impact load. A finite element analysis (FEA) model of CFCST member under lateral impact loading was established using ABAQUS software, and the accuracy of the proposed model was verified by a comparison with the test results. The response of the specimen under impact loading simulated by FEA model, matches well with the test results.

Ultra-high strength circular short CFST columns: Axisymmetric analysis, behaviour and design

A nonlinear axisymmetric simulation of axially loaded ultra-high strength circular short Concrete-Filled Steel Tubular (CFST) columns is reported in this paper. An efficient material law of Ultra-High Strength Concrete (UHSC) is employed in this axisymmetric simulation to consider the confining effects which increase the ultimate strain, stress and axial ductility. The peak loads and load-strain responses of short CFST columns predicted by the axisymmetric model are verified with the independent test data and those predicted by the three-dimensional (3D) model. The structural characteristics of circular short CFST columns having different material and geometric parameters are subsequently examined by the axisymmetric analysis. The numerical results indicate that the column size can approximately be reduced by 50% when UHSC is used in the composite construction instead of Normal Strength Concrete (NSC). Comparisons of the current test data are made with the current structural codes including AS/NZS 5100.6, Eurocode 4, AISC and GB 50936. The comparisons demonstrate that the Australian/New Zealand and European design codes provide consistent predictions for the ultimate axial strengths of ultra-high strength circular short CFST columns. On the other hand, the North American design code is conservative, whereas the Chinese code is unconservative.

Seismic tests and numerical investigation of blind-bolted moment CFST frames infilled with thin-walled SPSWs

This paper aims to investigate the seismic response of blind-bolted moment concrete-filled steel tubular (CFST) frames infilled with thin-walled steel plate shear walls (SPSWs). Tests of two-storey blind-bolted circular or square CFST frames with thin-walled SPSWs subjected to horizontal low-cyclic loading were performed. Macroscopic finite element (FE) models of the specimens were primarily established by fibre beam-column elements, zero-length springs, and truss elements. The numerical simulation results were verified by the experimental data. A parametric analysis was conducted to analyse the influence of the axial load level, material strength, and height-thickness ratio of the steel plate etc. on the hysteresis behaviour and energy dissipation of blind-bolted moment CFST frames with thin-walled SPSWs. In addition, the mechanics of the semi-rigid frame with SPSWs were compared with the pinned and rigid frames with SPSWs using mechanical analysis model and FE model. The experimental and analytical results show that the blind-bolted moment CFST frames with SPSWs connected to the beams only exhibit preferable hysteretic response, ductility and energy dissipation. The peak strength, initial stiffness and energy dissipation of blind-bolted moment CFST frames infilled with thin-walled SPSWs can be significantly affected by the axial load level as well as the steel plates' yield strength, height-thickness ratio, aspect ratio, and connection type.

Mechanism and application of concrete-filled steel tubular support in deep and high stress roadway

The concrete-filled steel tubular (CFST) support has been proposed as a new passive support form for controlling the stability of surrounding rock in deep roadway. To explore the mechanism of CFST support in deep roadway, the mechanical performances of U-steel support and CFST support were compared by theoretical calculation and numerical simulation with the engineering background of 11031 ventilation roadway in Pingmei coal mine. Then, the section shape of CFST was optimized and the special-shaped CFST support was put forward. The special-shaped CFST support was applied to 11031 ventilation roadway of U36-steel support failed to control. The results indicate that: The ultimate bearing capacity of CFST support is much greater than that of U-steel support. The CFST support can provide great supporting force to the roof-floor and two sides of roadway. It can be applied to the roadway with large horizontal stress. The section bending modulus and ultimate bearing capacity of the special-shaped CFST support are greater than those of the circular CFST support. The displacements of 11031 ventilation roadway supported by the special-shaped CFST support was not great. So, the special-shaped CFST support can effectively control the surrounding rock in 11303 ventilation roadway.

Cyclic Experimental Behavior of CFST Column to Steel Beam Frames with Blind Bolted Connections

This paper aims to investigate the seismic behavior of blind bolted end plate composite frames between square or circular concrete filled steel tubular (CFST) columns and steel beams. Two composite frames were tested under a constant axial load on the CFST columns and a lateral cyclic load on the frame. Each specimen composed of CFST columns and steel beams was selected to represent a plan frame in an assembly building. Failure pattern of the type of frames was analyzed to comprehend the structural response. The seismic resistance ability of the blind bolted CFST frames was also estimated in terms of hysteretic curves, ductility and energy dissipation etc. The effect of column section type and end plate type on the type of semi-rigid CFST frames was studied. The test results showed that at the same steel ratio of the column section, the bearing capacity and energy dissipation of square CFST frame were higher than those of circular CFST frame at ultimate state. Strain response of main members in each CFST frame was also estimated. The internal force analysis of the test CFST frames with semi-rigid connections was discussed and evaluated the effect of the bending moment distribution. It was concluded that the novel typed CFST frames exhibited excellent seismic performance and structural internal force redistribution. The experimental studies will be useful for design and application of the blind bolted CFST frames in fabricated steel structure building.

Fire resistance of circular concrete-filled steel tubular (CFST) column protected by intumescent coating

For structural components with specific requirement of fire resistance rating for safety reasons, additional fire protection is usually necessary. Intumescent fire coating (IFC) is one of the well-used materials to protect steel structures, and it has advantages such as light weight, simple construction, aesthetic appearance, easy maintenance, etc. In this study, a group of eccentrically loaded concrete-filled steel tubular (CFST) columns with and without IFC protection was tested in standard fire circumstance up to 180 min. Three types of intumescent materials were adopted and different dry film thicknesses of the fire coating were designed. Temperature field of each CFST column and the load bearing behaviour were measured and analysed. Finite element modelling of the columns was also established for heat transfer analysis and fire resisting capacity analysis. The modelling was validated by the test data. This study expands the related experimental database with large scale column specimens. The investigation results provide further evidence that CFST column protected by intumescent fire coating could achieve excellent fire resistance. A technical process to determine intumescent fire coating for CFST columns coupling with experimental verification and FE modelling analysis is presented.

Experimental investigation on seismic performance of endplate composite joints to CFST columns

The aim of this research is to carry out pseudo-static tests and investigate the seismic behaviour of the endplate composite joints of concrete-filled steel tubular (CFST) columns using blind bolts. Four full-scale blind-bolted endplate composite joints of a reinforced concrete (RC) slab to circular or square CFST columns were subjected to lateral cyclic displacements. Thereafter, the data obtained were used to evaluate the seismic performance of the typical composite joints from many aspects, such as hysteretic response, strength and rigidity degradation, ductility and energy dissipation capacity. In addition, failure modes were summarized based on the test phenomena; the influence of the column section and endplate types on the seismic performance of the joints was discussed in detail. The findings of the experiment demonstrate that the blind-bolted endplate composite joints possess appropriate ductility and adequate energy dissipation and, through reasonable design, they can satisfy the anti-seismic requirements in most seismic zones.

Empirical Approach for Determining Axial Strength of Circular Concrete Filled Steel Tubular Columns

The concrete filled steel tubular (CFST) columns are highly regarded in recent years as an interesting option in the construction field by designers and structural engineers, due to their exquisite structural performance, with enhanced load bearing capacity and energy absorption capacity. This study presents a new approach to simulate the capacity of circular CFST columns under axial loading condition, using a large database of experimental results by applying artificial neural network (ANN). A well trained network is established and is used to simulate the axial capacity of CFST columns. The validation and testing of the ANN is carried out. The current study is focused on proposing a simplified equation that can predict the ultimate strength of the axially loaded columns with high level of accuracy. The predicted results are compared with five existing analytical models which estimate the strength of the CFST column. The ANN-based equation has good prediction with experimental data, when compared with the analytical models.

Behavior of polygonal concrete-filled steel tubular stub columns under axial loading

The objective of this paper is to investigate the mechanical performances of polygonal concrete-filled circular steel tubular (CFT) stub columns under axial loading through combined experimental and numerical study. A total of 32 specimens were designed to investigate the effect of the concrete strength and steel ratio on the compressive behavior of polygonal CFT stub columns. The ultimate bearing capacity, ductility and confinement effect were analyzed based on the experimental results and the failure modes were discussed in detail. Besides, ABAQUS was adopted to establish the three dimensional FE model. The composite action between the core concrete and steel tube was further discussed and clarified. It was found that the behavior of CFT stub column changes with the change of the cross-section, and the change is continuous. Finally, based on both experimental and numerical results, a unified formula was developed to estimate the ultimate bearing capacity of polygonal CFT stub columns according to the superposition principle with rational simplification. The predicted results showed satisfactory agreement with both experimental and FE results.

Behavior of circular concrete-filled steel tubular columns under pure torsion

Concrete-filled steel tubular (CFT) columns are commonly used in engineering structures and always subjected to torsion in practice. This paper is thus devoted to investigate the mechanical behavior of circular CFT columns under pure torsion.3D finite element models based on reasonable material constitutive relation were established for analyzing the load-strain (T‒γ) curves of circular CFT columns under pure torsion. The numerical simulation indicated that local bulking of the steel tube in CFT columns was prevented and the shear strength and ductility of the core concrete were significantly improved due to the confinement effect between the steel tube and the core concrete. Based on the results, formulas to predict the torsional ultimate bearing capacity of circular CFT columns were proposed with satisfactory correspondence with experimental results. Besides, formulas of composite shear stiffness and the overall process of the T‒γ relation of circular CFT columns under pure torsion were proposed.

Circular concrete filled steel tubular (CFST) columns under cyclic load and acid rain attack: Test simulation

Concrete filled steel tubular (CFST) structure attracts increasing engineering applications in earthquake prone regions due to its high section modulus, high strength, and good seismic performance. However, the seismic resistance of CFST columns may be affected by the environmental corrosions, such as acid rain attack. This paper makes an attempt to investigate the performance of CSFT columns with circular sections under both a cyclic load and an acid rain attack. First, the tensile mechanical properties of steel plates with various corrosion rates were tested. Second, a total of 12 columns with different corrosion rates were tested subjected to a reversed cyclic load. It was found that the corrosion leads to not only a loss in wall thickness but also an evident decrease in yield strength, elastic modulus, and tensile strain capacity of the steel coupons, and also to a significant deterioration in the load carrying capacity, ductility, and energy dissipation of the CFST columns. The larger the axial force ratio, the severer deterioration of deformation capacity of the columns.

Composite frame of circular CFST column to steel-concrete composite beam under lateral cyclic loading

In this paper we investigate the performance of composite frames composed of circular concrete-filled steel tubular (CFST) columns connected to steel-concrete composite beams subjected to a constant axial load and a cyclic lateral load. Seven single-story and single-bay in-plane frames were fabricated and tested. The effects of the slenderness ratio (λ), the axial compression ratio (n), and the beam-to-column linear stiffness ratio (k) on the seismic performance of the composite frame were studied. The experimental results, including damage development and stiffness degradation, load-deformation responses, energy dissipation capacity and ductility are discussed. It was found that these composite frames exhibited satisfactory seismic performance. Furthermore, a finite element (FE) model was developed and validated by comparisons with the experimental results, considering both material and geometrical nonlinearity for confined concrete and steel. The results obtained from the FE modeling were in good agreement with the experimental results in terms of failure modes, load-displacement hysteretic curves, and skeleton curves.

Pseudo-dynamic tests of assembly blind bolted composite frames to CFST columns

This paper presents the dynamic response and failure mechanism for the novel assembly of blind-bolted concrete-filled steel tubular (CFST) composite frames with steel-bar truss deck (SBTD) concrete slabs under seismic action. With this type of composite frame, the SBTD concrete slabs are attached to H-shaped steel beams using shear connectors, and the steel-concrete composite beams are connected to CFST columns using flush or extended end plates with blind bolts. A series of pseudo-dynamic tests (PDTs) were conducted on two two-story one-bay composite frames with square or circular section CFST columns. The time-history response and inter-story drift of each test specimen were investigated to analyse the dynamic response of the composite structures under a series of ground motion records. Meanwhile, the seismic behaviour of the composite structures was also evaluated in terms of their failure modes, hysteretic behaviour, ductility, and strain response. The test results indicate that the structural response under a simulated seismic loading is consistent with the desired performance under different earthquake hazard levels. The proposed blind-bolted square or circular CFST composite frame, which exhibits excellent seismic behaviour and ductility, can be used as a reliable and effective assembly composite structure in seismic regions.

Behaviours of concrete-filled cold-formed elliptical hollow section beam-columns with varying aspect ratios

Experimental and numerical studies were conducted to investigate the behaviours of the concrete-filled cold-formed elliptical hollow section beam-columns. A total of 11 specimens were tested to evaluate the failure modes, load-deformation histories and strains development in the steel tube. Complementary finite element (FE) models were developed and validated against experimental results. Validated FE methodology was then used to study the influence of key parameters, including aspect ratio, slenderness ratio, load eccentricity ratio, yield strength of steel, compressive strength of concrete and steel tube to concrete area ratio, on the load carrying capacity. As a result, the design method for elliptical concrete-filled steel tubular (CFST) columns in Chinese code - GB50936-2014 and the design method for circular CFST columns in EC4 were assessed to confirm their applicability for cold-formed elliptical CFST columns with aspect ratio ranging from 1.0 to 2.5.