Concrete-filled Thin-walled Steel Tubular Columns Research Articles

Shear behavior of thin-walled concrete-filled steel tubular columns

Shear failure is a brittle type of failures, which should be avoided in any structure as it could cause property loss and human casualties. The shear behavior of thin-walled concrete-filled steel tubular (CFST) columns was hence investigated in this study, involving the tests of 24 thin-walled CFST columns and considering the following parameters: shear span-to-depth ratios = 0.15, 0.3, 0.5, 1, 1.5, and 2; confinement factor = 0.63, 1.27, 1.44, and 1.70; and axial load ratios = 0, 0.2, 0.4, and 0.6. The test results revealed the insights of the initial shear stiffness, failure modes, ultimate bearing capacity, load-deflection curves, load-strain curves, and ductility of the specimens. Based on the failure modes, a method is proposed to evaluate the load resistance, and explain the relationship between the shear span-to-depth ratio and the peak bearing capacity. A compression strut mechanism considering the confinement provided by the steel tube flange is also proposed. The effect of axial load on the shear strength of the concrete core is negligible when the axial load ratio doesn’t exceed 0.4. The predicted results from using the proposed simple model agree generally well with the experimental results.

Experimental investigation on residual capacity of steel-reinforced concrete-filled thin-walled steel tubular columns subjected to combined loading and temperature

Steel-reinforced concrete-filled thin-walled steel tube (SRCFST) is gaining popularity in construction, primarily due to its superior structural performance. However, there is still a scarcity of studies investigating the residual capacity of SRCFST columns subjected to a full-range fire. A comprehensive research was conducted in this, involving both experimental testing and numerical simulation, to investigate the residual capacity of SRCFST columns. Six SRCFST columns were tested under combined loading and temperature to achieve the temperature distribution, failure modes, residual strength, and structural response within the cross-section. The developed finite element (FE) model was used to validate the test results. Three different paths, including after exposure to fire and without initial load, the full-range fire, and the actual path in the current test, were considered toevaluate the influence of different time-load-temperature paths on the mechanical properties of SRCFST columns. Results reveal that the region comprised of profiled steel and surrounded concrete forms a composite constrained region, with little loss of strength and stiffness occurring in the core region. The initial load level and heating time ratio were found to have a significant negative influence on the residual load-carrying capacity and ductility of SRCFST columns. In contrast, circular SRCFST columns are more resistant to the effects of fire than square SRCFST columns. Finally, the existing design methods were used and extended to evaluate the residual load-carrying capacity of SRCFST columns subjected to a complete temperature-load-time process.

Strength Behavior and Ultimate Capacity Prediction of Self-Compacting Concrete-Filled Thin-Walled Medium-Length Steel Tubular Columns under Eccentric Compression

The development of self-compacting concrete-filled thin-walled steel tubular columns is a potential strategy to ease the challenge of conserving resources in society, which are largely consumed by the quickly developing civil industry. However, the application of these columns in the civil industry is rare due to insufficient research, especially research concerning the strength behaviors of the columns under eccentric compression. Therefore, the eccentric compressive behaviors of medium-length tubular columns made up of self-compacting concrete and thin-walled steel with circular sections were experimentally studied in the present paper. The feasibility of predicting the columns’ ultimate capacities using existing design codes was explored, and then comparisons between the predictions and experimental values were carried out. The results showed that the eccentric compression columns had a failure morphology, buckling together with a lateral deflection while they were moved from the bottom to middle positions as the wall thickness increased. Moreover, the ratios of the predicted ultimate capacity of the eccentric compressive columns to the experimental values were within the range of 0.35 to 0.94. This indicates that the predicted ultimate capacity is conservative and safe. The codes AISC-LRFD and JCJ 01-89 achieved the most conservative and the most precise predictive results, respectively. Additionally, the decrease ratio of the predicted ultimate capacity of the eccentric compressive columns to the experimental values was more evident than that of axial compressive columns. This paper can serve as guidance for the design and application of these columns, as well as foster a sustainable and resilient civil industry.

Shear strength of stiffened square thin-walled concrete-filled steel tubular columns

In comparison to extensive researches on compressive or flexural behavior of stiffened square concrete-filled steel tubular (CFST) columns, few were conducted on shear behavior. The cyclic-shear behavior between stiffened and unstiffened square thin-walled CFST columns was different, and the cyclic-shear behavior of CFST columns stiffened by the diagonal ribs, which were welded on the adjacent sides of a square tube, was significantly improved, as demonstrated by recent experimental results. This work uses detailed finite element (FE) analysis and parametric analysis to explore the shear mechanism and shear contribution of each component in the stiffened CFST column. The refined FE model was developed using ABAQUS and verified by the test results of both stiffened and unstiffened CFST columns subjected to cyclic and monotonic shear loading. Based on the analysis of test results, a method based on the degree of the cross-sectional plasticity was proposed to distinguish shear failure from bending failure, as typical bending failure phenomena (concrete crushing and tube buckling) were also observed at the ends of column specimens with shear failure. Analytical results showed that the steel tube and diagonal ribs resisted the shear through shear stress, and the concrete resisted the shear through the developed compression strut confined by the horizontal stress of steel. A shear model considering the interaction between concrete and steel was then established. The mechanics-equations and regression-equations were proposed and compared with the shear test results reported in the existing literature and FE analysis. Both types of equations were found to well predict the shear strength of both stiffened and unstiffened square CFST columns.

Cyclic-shear behavior of square thin-walled concrete-filled steel tubular columns with diagonal ribs

There is no doubt that stiffeners for square concrete-filled steel tubular (CFST) columns can improve the composite action in terms of delaying local buckling of steel tubes and enhancing the confinement on infilled concrete. Among various proposed stiffening forms, diagonal binding ribs, made of perforated thin-walled steel plates and welded at all the four pairs of adjacent sides of a square tube, are found particularly effective. Recent research has experimentally evaluated the axial compressive and cyclic-bending behavior of square thin-walled CFST columns stiffened by diagonal ribs. However, further investigation is necessary to better understand their cyclic-shear behavior. In this work, four diagonal ribs stiffened square thin-walled CFST columns and one unstiffened counterpart were designed and tested under combined cyclic-shear load and constant axial compression. Main parameters included the shear span to depth ratio, axial load ratio, width-to-thickness ratio of the steel tube, and thickness of diagonal ribs. Test results showed that the diagonal ribs improved the confinement from the steel tube to the infilled concrete. The stiffened specimens exhibited satisfactory cyclic-shear behavior with much higher load capacity, better ductility and larger energy dissipation than the unstiffened counterpart. Numerical modeling of the shear specimens was then conducted, indicating that the concrete and steel part contributed to the shear strength by 41.3%-57.4% and 42.6%-58.7%, respectively. Finally, the test results were compared with current design provisions, the average ratio of the predicted to tested shear strength were 0.60 and 0.66 for the calculation methods in the AISC specification and Eurocode, respectively.

Nonlinear post-fire simulation of concentrically loaded rectangular thin-walled concrete-filled steel tubular short columns accounting for progressive local buckling

Abstract The repair of fire-damaged thin-walled rectangular concrete-filled steel tubular (CFST) columns in engineering structures after fire exposure requires the assessment of their residual strength and stiffness. Existing numerical models have not accounted for the effects of local buckling on the post-fire behavior of CFST columns with rectangular thin-walled sections. This paper describes a nonlinear post-fire simulation technique underlying the theory of fiber analysis for determining the residual strengths and post-fire responses of concentrically loaded short thin-walled rectangular CFST columns accounting for progressive local buckling. The post-fire stress-strain laws for concrete in rectangular CFST columns are proposed based on available test data and implemented in the theoretical model. An innovative numerical scheme for modeling the progressive local and post-local buckling of CFST thin-walled columns is discussed. The nonlinear post-fire simulation model is verified by experimental data and then used to investigate the significance of local buckling, material strengths and width-to-thickness ratio on the post-fire responses of CFST stub columns. The proposed post-fire computer model is shown to be capable of predicting well the residual stiffness and strength of concentrically loaded thin-walled CFST columns after fire exposure. A design formula is proposed that estimates well the post-fire residual strengths of CFST columns. Computational results presented provide a better understanding of the post-fire behavior of CFST columns fabricated by thin-walled sections incorporating local and post-local buckling.

Experimental study on seismic behavior of high-strength circular concrete-filled thin-walled steel tubular columns

This paper presents the results of an experimental study on the seismic behavior of high-strength circular concrete-filled thin-walled steel tubular (HCFTST) columns with ultra-large diameter-to-thickness (D/t) ratios exceeding the limitations of the current construction standards. Sixteen HCFTST columns with different combinations of D/t ratio, concrete cylinder compressive strength (fc), and axial compression ratio (n) were tested under constant axial compression combined with cyclic lateral loading. The ultimate failure state was achieved when the steel tubes ruptured severely and core concrete crushing occurred. The results from hysteretic curves indicated that the HCFTST columns with ultra-large D/t ratios displayed flexural failure and shear failure modes. Subsequently, the skeleton curve, ductility, energy dissipation capacity, and stiffness degradation were discussed in detail. Moreover, the effects of D/t ratio, concrete cylinder compressive strength and axial compression ratio on performance were investigated so that this work could serve as a basic reference to future studies, and a strength model was proposed to predict the moment-resisting capacity. The experimental investigation indicated that (i) using high-strength Q690 steel could significantly contribute to a larger elastoplastic deformation capacity and delay the onset of post-peak behavior, even though a lower ductility capacity was provided; (ii) the proposed strength model could satisfactorily predict the moment-resisting capacity; (iii) the out-of-code HCFTST columns with reasonable design could demonstrate favorable seismic behavior and could be accepted as aseismatic components in earthquake-prone regions.

Cyclic testing of Q690 circular high-strength concrete-filled thin-walled steel tubular columns

Under seismic action, the severe damage in critical regions of structures could be ascribed to the cumulative damage caused by cyclic loading. This article describes an investigation of the hysteresis behaviour of Q690 circular high-strength concrete-filled thin-walled steel tubular columns with out-of-code diameter-to-thickness ratios. A total of eight specimens were tested under constant axial compression and cyclic lateral loading. The study results of phase I testing consisting of a benchmark test were summarized to examine the seismic behaviour under standard loading, and those of the phase II testing that considered different fatigue loading modes and different concrete strengths were summarized to investigate the low-cycle fatigue behaviour. The load–displacement hysteretic curves, energy dissipation, strength and stiffness degradation were discussed in detail. A simplified method was proposed to predict the low-cycle fatigue life, which can be applied in the damage-based seismic design of circular concrete-filled steel tubular structures.

Local buckling of steel plates in concrete-filled steel tubular columns at elevated temperatures

Local buckling remarkably reduces the strength of steel plates in rectangular thin-walled concrete-filled steel tubular (CFST) columns at ambient temperature. This effect is more remarkable at elevated temperature. However, there have been very limited experimental and numerical investigations on the local and post-local buckling behavior of steel plates in CFST columns at elevated temperatures. This paper presents numerical studies on the local and post-local buckling behavior of thin steel plates under stress gradients in rectangular CFST columns at elevated temperatures. For this purpose, finite element models are developed, accounting for geometric and material nonlinearities at elevated temperatures. The initial geometric imperfections and residual stresses presented in steel plates are considered. Based on the finite element results, new formulas are proposed for determining the initial local buckling stress and post-local buckling strength of clamped steel plates under in-plane stress gradients at elevated temperatures. Moreover, new effective width formulas are developed for clamped steel plates at elevated temperatures. The proposed formulas are compared with existing ones with a good agreement. The effective width formulas developed are used in the calculations of the ultimate axial loads of rectangular CFST short columns exposed to fire and the results obtained are compared well with the finite element solutions provided by other researchers. The initial local buckling and effective width formulas can be implemented in numerical techniques to account for local buckling effects on the responses of rectangular thin-walled CFST columns at elevated temperatures.

Experimental and analytical studies on CFRP strengthened circular thin-walled CFST stub columns under eccentric compression

Carbon fiber reinforced polymer (CFRP) strengthened concrete-filled thin-walled steel tubular (CFST) column can improve its load carrying capacity and resolve local bugles or corrosion of thin-walled steel tube. However, little attention to CFRP strengthened CFST columns has been paid. This paper reported an experimental and numerical analysis on eccentric compressive behavior of circular CFST stub columns partially-wrapped by CFRP strips. A series of circular composite columns, including nine CFRP strengthened CFST stub columns and one bare CFST stub column, were tested subjected to eccentric compression. Moreover, a nonlinear finite element (FE) modeling in considering contact interactions of the composite columns was developed and verified by the test results in terms of eccentric load (N) - longitudinal shortening (δ) curves and failure patterns. Then, the influence of extensive parameters on the eccentric compressive behavior of CFRP strengthened circular CFST columns was also evaluated. Meanwhile, a simplified empirical method on the eccentrically-loaded stub composite columns was proposed on the basis of unified theory. The experimental and analytical data indicated that the eccentric compressive strength of CFRP strengthened circular CFST stub column was obviously influenced by load eccentricity, CFRP confinement factor, steel strength, core concrete strength and CFRP strength. The proposed simplified empirical formulas may provide a considerable approach for designing this type of composite structures in engineering practice.

Seismic performance assessment of blind bolted steel-concrete composite joints based on pseudo-dynamic testing

This paper aims to investigate seismic behavior and dynamic responses of semi-rigid composite joints composed of steel-concrete composite beams and concrete-filled thin-walled steel tubular (CFTST) columns. A range of pseudo-dynamic tests (PDTs) was performed on two full-scale blind bolted end-plate joint specimens, including one flush and one extended end-plate typed joint. The input ground motion adopted for the PDTs was Ispara seismic acceleration record and scaled to represent various seismic hazard levels. Using Ispara earthquake record with different peak ground accelerations (PGAs), a lot of useful data about hysteresis behavior, stiffness degradation and energy dissipation capacity was obtained. The data was also estimated in detail to characterize the seismic performance of this kind of composite joints. Dynamic responses of beam-to-column joint including displacement and acceleration responses, and dynamic magnification factor were discussed. The experimental results indicated that the structural responses of the test joints under seismic actions coincided with the desired performances analyzed for varying damage levels. The study affirms that the blind bolted end-plate composite joints to CFTST columns possess good energy dissipation seismic performance, so they can provide sufficient ductility and adequate strength to satisfy the seismic design requirement.

Experimental and numerical analysis of blind bolted moment joints to CFTST columns

Experiments and numerical analysis were conducted to investigate the mechanical behavior of the blind bolted end plate joints to concrete-filled thin-walled steel tubular (CFTST) columns. Four blind bolted end plate joints to CFTST columns subjected to the monotonic loading were tested. Then, finite element analysis (FEA) modeling of the tested specimens was developed, and the results obtained from FEA modeling were verified against those from the test results. Parametric studies were conducted to investigate the influence of end plate thickness, axial load level, steel ratio and slenderness ratio etc. on moment-rotation curves of composite joints. Some methods of effective provisions were proposed and discussed in order to enhance the connection behavior. The test and numerical analysis results indicated that the typed joints to CFTST columns behaved in a semi-rigid and partial strength manner. The proposed innovative blind bolted joint using reliable and effective solutions could be applied in the mid- and low-rise buildings.

Seismic behavior of blind bolted end plate composite joints to CFTST columns

The seismic behavior and failure mechanism of a novel type of composite joints are presented in this paper. These composite joints are composed of composite steel-concrete beams and square concrete-filled thin-walled steel tubular (CFTST) columns. The steel beams are connected to the CFTST columns using end plates with blind bolts, while the cast-in-situ RC slabs are attached to steel beams with shear connectors. Some quasi-static tests were performed on flush and extended end plate joints and horizontal cyclic displacements at the column ends were experienced during the test. A detailed account of the results and observations from the cyclic tests are investigated, and the main behavioral aspects are discussed. The seismic performance of the blind bolted composite joint to CFTST column is estimated in terms of the hysteretic behavior, strength and stiffness degradation, ductility, and energy dissipation capacity, etc. The results of this study show that the novel composite joints exhibit adequate ductility and energy dissipation, whilst they behave in a semi-rigid manner according to the EC3 specification.

Structural performance of blind bolted end plate joints to concrete-filled thin-walled steel tubular columns

This paper presents the results of an experimental study on the structural behavior of blind bolted end plate connections between steel beams and concrete-filled thin-walled steel tubular (CFTST) columns under monotonic loading. The square tube in each CFTST column connection was fabricated by seam welding four pieces of lipped angle with nominal wall thickness 1.5mm or 3mm together. In order to enhance the stiffness of the connection, extensions are provided to the blind bolts to link the connection back into the concrete with the tube. The objective of this study is to develop a novel bolted moment connection which is easy to use in the thin-walled structures. The structural performance of the blind bolted connection is evaluated in terms of the failure modes, moment–rotation relationship, connection rigidity and strain response of critical components. Some anchorage strengthening methods were proposed to enhance the strength and stiffness of the connection. The experimental results indicate that the proposed CFTST column connections behave in a semi-rigid and partial strength manner according to the EC3 specification, whilst its rotation capacity satisfies the ductility design requirements for earthquake-resistance in most aseismic region. Adopting reasonable connection measurement, the proposed blind bolted connection to the CFTST columns was a reliable and effective solution for low-layer and multilayer frame structures.

Performance of CFTST column to steel beam joints with blind bolts under cyclic loading

The objective of this research is to investigate the seismic performance of the composite joint consisting of square concrete filled thin-walled steel tubular (CFTST) column and steel beam with end plate and blind bolts. The cold-formed square tube in each CFTST column connection was fabricated by seam welding together four pieces of lipped angle with nominal wall thickness 1.5mm or 3mm. Four exterior joint specimens were tested under axially compressive load on the top of the columns and cyclic loads on the beam tip. The experimental parameters in the study were the thickness of the steel tube and the type of end plate. The seismic response of the blind bolted moment joints to CFTST columns was analyzed and evaluated in terms of the hysteretic behavior, failure modes, stiffness and strength degradation, ductility, and energy dissipation capacities of the joints. To improve the tension behavior of the blind bolted moment connections to the thin tube wall, the anchorage action of reinforcing rebar welded to the bolts with concrete-filled steel tubes was also investigated to consider the effect of cyclic loading. The experimental and analytical results indicated that when the end plate thickness is not less than 3mm, the flush or extended end plate joints to CFTST columns exhibited large hysteretic loops and excellent seismic performance, such as ductility and energy dissipation capacity. The proposed innovative blind bolted joint was verified as a reliable and effective solution applied in mid- and low-rise buildings through properly design and detailing.

Experimental Study of Blind Bolted Joints to Concrete-Filled Thin-Walled Steel Tubular Columns under Cyclic Loading

This paper discusses results of experiments on blind bolted end plate joints to concrete-filled thin-walled steel tubular (CFTST) columns. Four exterior joints to CFTST columns subjected to cyclic loadings. A feature of this novel joint is the use of the blind bolts and extensions to these bolts into the concrete-filled square steel tubular column. Failure modes, moment-rotation hysteretic curves and energy consumption of the connections were analyzed. Further, the connection rigidity and ductility were also elevated by present specifications. The test results showed that the end plate type and the steel tube thickness affect the seismic behaviour of the typed blind bolted end plate joints. The proposed joint has reasonable strength, stiffness and ductility by taking reasonable end plate type, steel tube thickness and blind bolt anchorage; its ultimate connection rotation satisfies the ductility design requirements, and could be reliably and safety used in low-layer or multi-layer composite frames.

Behaviour and design calculations on very slender thin-walled CFST columns

This paper studies the behavior of very slender thin-walled concrete filled steel tubular (CFST) columns under axial compression. A finite element analysis (FEA) was developed to carry out the behavior of compressive columns. Initial out-of-straightness imperfection was discussed. Generally good agreement was obtained between the predicted and measured results. The FEA model was then used to perform mechanism analysis on very slender circular CFST columns. Parametric studies were carried out and the ultimate strengths from tested results and design codes were compared and discussed. The reliability analysis method was used to calibrate the existing design formulas given in DBJ/T13-51-2010, ANSI/AISC 360-05 and Eurocode 4.

Performance-based analysis of concrete-filled steel tubular beam–columns, Part II: Verification and applications

The theory and algorithms of a performance-based analysis (PBA) technique for the nonlinear analysis and performance-based design of thin-walled concrete-filled steel tubular (CFST) beam–columns with local buckling effects were presented in a companion paper. Initial local buckling and effective strength/width formulas for steel plates are incorporated in the PBA program to account for local buckling effects. Performance indices are used in the PBA program to quantify the section, axial ductility and curvature ductility performance of thin-walled CFST beam–columns. This paper presents the verification and applications of the PBA program developed. The axial load–strain curves, ultimate axial loads and moment–curvature curves for thin-walled CFST columns predicted by the PBA program are verified by experimental data. The PBA program is then utilized to investigate the effects of local buckling, depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility performance of thin-walled CFST beam–columns under axial load and biaxial bending. The PBA technique developed is shown to be efficient and accurate and can be used directly in the performance-based design of thin-walled CFST beam–columns and implemented in advanced analysis programs for composite columns and frames.

Experimental study of hysteretic behaviour for concrete-filled square thin-walled steel tubular columns

In this paper, the hysteretic behaviour of concrete-filled thin-walled steel tubular (CFTST) columns was investigated experimentally. The parameters in the study included the axial load level and steel tube section type. Nine CFTST columns, including six columns with a longitudinal stiffener on each inner face of the steel tube and three columns with a longitudinal stiffener on the two opposite inner faces of the steel tube, were tested under a constant axial load in addition to a cyclic lateral load. The effect of axial load level on the hysteretic behaviour (stiffness, ductility and energy dissipation) was studied. Experimental results indicated that the CFTST columns under an axial load level below 0.5 exhibited plump hysteretic loops with a slight pinching effect, better ductility and energy dissipation capacity. The displacement ductility decreases significantly with an increase in the axial load level. Columns with two steel tube sections had almost the same load capacity, whilst the ductility and energy dissipation capacity of columns with a longitudinal stiffener on each inner face of the steel tube was better than that of columns with two opposite stiffeners.