Concrete-filled Steel Tubular Beams Research Articles

Experimental investigations on the partially concrete-filled steel tubular slender beams

This paper focuses on the flexural behaviour of partially concrete-filled steel tubular (PCFST) beams having slender cross-sections (D/t > 150). PCFST beam is developed by optimising the tension zone concrete in a concrete-filled steel tubular (CFST) beam. Twelve specimens were investigated under a four-point load to understand the flexural behaviour and load transfer mechanism of PCFST slender beams. Parameters considered for the tests are concrete compressive strength, shear-span-to-depth ratio, and concrete wall thickness. The failure mode, load-deflection, strain distribution and moment-rotation data of the test specimens are reported and compared. In general, PCFST beams exhibit the same flexure behaviour as CFST beams in terms of flexural resistance and stiffness, and the weight reduction is around 40 %. However, composite action is lost in PCFST without tension zone concrete, and the beam behaves like an empty steel tube. Therefore, a minimum concrete wall thickness (tc,min) in the tension zone is required in PCFST and an equation for tc,min is proposed based on the test results. Concrete compressive strength is found to have lesser significance in the PCFST beam, whereas a decrease in the shear span-to-depth ratio from 2 to 1 can alter the mode of failure from flexure to shear. A numerical model is developed using ABAQUS to support the test results. This study finds the PCFST beam an optimised replacement for CFST beams in flexure-predominant structural systems. Additionally, a flexure capacity equation is proposed using the plastic stress distribution method, and made good predictions with the experimental and finite element results.

Seismic behavior of an improved drilled flange connection for I-beam to CFST column

The use of cold-formed steel tubes in the concrete-filled steel tubular (CFST) columns makes the design of the moment connections between beams and CFST columns much more challenging. This is because setting the internal diaphragms in the CFST column to transfer the bending moment of the beam end becomes impossible, and thus new joints are needed. A novel joint was proposed in a previous study by the authors, where two end plates were used for the force transmission between CFST columns and beam flanges. Moreover, to satisfy the “weak member strong joint” requirement, the I-beams were reduced by drilling a series of circular holes in their flanges. However, most of the tested specimens failed by the unexpected rupture of the complete joint penetration (CJP) groove welds between the beam flanges and end plates. In this paper, three specimens are redesigned and tested, where modifications are made to improve their behaviors under the seismic load. In this test, the local buckling of beam flanges at the drilled sections was observed in all specimens, and two specimens failed due to the fracture at the drilled sections, while CJP weld fracture and significant bending of beam flanges both occurred at the failure of the rest specimen. The hysteresis curves of all specimens are fusiform, indicating their good energy dissipations (the maximum equivalent viscous damping coefficients between 0.35 and 0.40). The failure rotation angles are about 2.44–3.01, and the ductility coefficients are between 2.26 and 2.66. According to EC3, all specimens can be classified into the rigid and full-strength joints. Subsequently, the parametric analyses using the validated finite element model are carried out, where the effects of main parameters are investigated on the seismic behavior of the presented joint. This study brings insights into the moment connections between the steel beam and CFST column with the outer cold-formed section.

Flexural behaviour of optimised steel tubular beams partially filled with concrete

A partially concrete-filled steel tubular beam (PCFST) is proposed in this study to optimise the design and construction of the concrete-filled steel tubular beams (CFST). Concrete filling prevents the local buckling of the slender tubes. However, it is not needed in the beam’s tension zone. Therefore, in PCFST, concrete is filled in the compression zone while the tension zone is kept empty. In this paper, the flexural behaviour of the PCFST beam is studied experimentally and numerically. The ultimate moment resistance and flexural stiffness of PCFST are the same as CFST beams, while the weight reduction in PCFST is up to 60%. A finite element (FE) model is developed using ABAQUS software to investigate the effect of geometric and material properties of the PCFST beam on flexural behaviour. The FE model is validated by the present test results and the literature reports on CFST beams. Next, a parametric study is conducted to study the effect of concrete wall thickness in the web region, length-to-depth ratio, steel yield strength, concrete compressive strength, and beam shear span-to-depth ratio. The results show that the concrete filling is useful for slender steel tubular cross-sections rather than compact ones. A higher grade of concrete increases the beam ductility, while high-strength steel reduces the ductility. The results also show that the PCFST beam is more suitable for beam lengths where flexure is predominant than shear. Further, a flexural capacity equation is proposed for PCFST beams, and it agrees well with the test and numerical results. The reliability of the proposed equation is checked as per the recommendation in EN:1994–2004 and AISC 360–2016 code provisions.

Flexural Strength Evaluation of Multi-Cell Composite L-Shaped Concrete-Filled Steel Tubular Beams

Concrete-filled steel tubular (CFST) members have been widely used in industrial structures and high-rise residential buildings. The multi-cell composite L-shaped concrete-filled steel tubular (ML-CFST) cross-section, as an innovative, special-shaped structural arrangement, may solve the issue of normal CFST members protruding from walls and result in more usable interior space. Currently, no design rules are available for the application of ML-CFST members. One of the primary objectives of the present study is to develop recommendations in line with the unified theory to evaluate the bending moment resistance of ML-CFST beams. According to the unified theory, the bending moment resistance of an ML-CFST beam is related to the compressive strength (fsc) and the flexural strength index (γm) of a composite section, in which the accuracy of γm and fsc are affected by a confinement effect factor (ξ). Nevertheless, the original expression of ξ is not suitable for ML-CFST sections, since the appreciable effect of the irregular shape on confinement is neglected. Considering the cross-sectional geometry and boundary conditions of the cells, an equivalent shape factor to modify the confinement effect was proposed in this study through dividing the infill concrete into highly confined areas and less confined areas. An adequate formula to calculate the fsc and an approximate expression of γm for the ML-CFST sections was then developed. Furthermore, four-point bending tests on eight specimens were carried out to investigate the flexural performance of the ML-CFST beams. Lastly, the proposed formulas were assessed against experimental and numerical results. The comparisons show that the proposed unified theory-based approach produced accurate and generally conservative results for the ML-CFST beams studied.

Lightweight concrete filled steel tubular beam under bending-shear

Lightweight concrete filled steel tubular (L-CFT) member is the new type of concrete filled steel tubular consisting of hollow steel tube and lightweight concrete. This study aims to investigate bending-shear behavior of the new type of lightweight concrete filled steel tubular, L-CFT, member, which is lighter than normal concrete filled steel tubular, N-CFT, member. The two test parameters, large diameter-to-thickness and shear span ratios are selected. The influence of diameter-to-thickness ratio on L-CFT deep beam under bending-shear through asymmetric four-point loading is discussed. Moreover, methods to predict two types of bending-shear capacities of L-CFT deep beam are provided. The elasto-plastic biaxial stress behavior of the steel tube under plane stress condition is also mentioned. The obtained experimental strengths agreed with proposed strength based upon RC deep beam theory.

Compressive behavior of concrete-filled steel tubular columns with internal high-strength steel spiral confinement

Concrete-filled steel tubular (CFST) columns have been extensively studied and widely used in practice. Existing research has shown that non-circular CFST columns is much less ductile than their circular counterparts, particularly when thin/high strength steel (HSS) tubes and high-strength concrete are used. To address this problem, a new form of CFST columns has recently been proposed by the first author. The new column consists of a steel tube filled with concrete that is confined with HSS spiral reinforcement typically with a yield stress exceeding 1000 MPa. These columns, referred to as confined concrete-filled steel tubular (CCFST) columns, also maintain the ease for connection to CFST or steel beams. This paper presents the results of a series of concentric axial compression tests on such columns of square cross-section to demonstrate their advantages. The experimental program included 13 CCFST columns, four CFST columns without internal spiral confinement, two hollow steel tube (HST) columns, and 11 circular HSS spiral-confined concrete columns. Three different compressive strengths and three HSS spiral pitches were examined in the experimental program. The CFST columns, HST columns, and HSS spiral-confined concrete columns were all tested under axial compression to gain a good understanding of the confinement mechanism in a CCFST column. The test results show that the new columns possess much greater ductility than those without internal spiral confinement, although the use of HSS spirals increases the steel volume by only a small percentage. It is also shown that the axial load-axial strain curve of a CCFST column can be conservatively predicted by summing the axial load-axial strain curves of the hollow steel tube without local buckling, the HSS spiral-confined concrete core, and the sandwiched concrete between the two.

Behaviour of hybrid fibre reinforced concrete-filled steel tubular beams and columns

ABSTRACT This paper presents the flexural performance of newly developed hybrid fiber reinforced concrete-filled steel tubular sections. The test parametres are fiber volume fraction and fiber hybridation ratio. Initially mechanical properties studied for 10 mono fiber reinforced concrete mixes using steel and Polypropylene fibres with 0.5%, 1.0%, 1.5%, 2.0% and 2.5% volume fraction. Based on the performance optimum fiber dosage was determined in each fiber, with the same volume fraction three different fiber hybridation was developed. Developed hybrid fiber reinforcement concrete, conventional concrete and optimum mono fiber reinforced concrete was used in the concrete-filled steel tubular beams and columns to determine the structural performance. The test results shows that, fiber reinforced concrete-filled steel tubular beams display significant improvement in the flexural performance.

Experimental assessment of the flexural behaviour of concrete-filled steel tubular beams with octagonal sections

This paper presents an experimental investigation into concrete-filled steel tubular beams with octagonal cross-sections (OCFST) under monotonic or cyclic flexural loading. A total of eight tests, including four cyclic specimens and four monotonic counterparts, were conducted. Three concrete grades with measured compressive cylinder strength (fc′) varying from 54.5 MPa to 105.6 MPa were used to infill the OCFST specimens. The failure modes, ultimate bending moments, effective flexural stiffness, cumulative dissipated energy and deteriorations of test specimens were discussed. Test results indicate that OCFST beams exhibit a ductile plastic mode and excellent energy dissipation. Concrete grades seem to have limited influence on the ultimate strength and energy dissipation capacity. The comparison results of the ultimate bending moments and effective flexural stiffness between predictions using EN 1994-1-1 and AISC 360-16 and test results reveal their applicability to the design of OCFST beams. An energy-based hysteretic rule was adopted to assess the strength and stiffness deteriorations of the OCFST beams, and the results indicate that the predictions match well with the test observations.

Bending failure mechanism and strengthening of concrete-filled steel tubular support

Concrete-filled steel tubular (CFST) support is an innovative and effective support technology, which has been broadly applied in underground roadways. However, failure is still possible, and the primary failure locations are commonly the vault and the springline. In order to reveal the failure mechanism and provide an effective solution, the inner forces of a circular CFST support were investigated analytically. To improve the flexural performance, firstly, the flexural behavior of CFST beams strengthened by welded steel at soffit was investigated experimentally. A total of 6 beams were tested under a four-point bending load, and the investigated parameter was welded steel diameter. The corresponding nonlinear three-dimensional finite element model was established in the ABAQUS software and was validated using the experimental results. Then, the effect of welded range and welded location on the support bearing performance was investigated numerically. Finally, the proposed method was validated by an application example. Results show that CFST support bearing performance is greatly improved when the lateral pressure coefficient is equal to 1.0, because there is no bending moment, and the thrust force is equally distributed along the perimeter of the support. When the lateral pressure coefficient is not equal to 1.0, the sections at vault, invert, and springline, where the maximum bending moments located, are the most vulnerable. Welding steel at locations dominant by the bending moment is an effective way to improve flexural bearing capacity. The optimal flexural strengthening method for the support is welding steel at the vault and the springline, respectively, named ‘three-arcs welding.’

Experimental and Numerical Investigations of Bolted CFST Beams–CFST Columns Connections

The effectiveness of concrete-filled steel tubular (CFST) sections as compression and flexural members has been investigated by many authors. The combined utilization of CFST columns and steel beams has increased in composite building construction in recent years, but combined use of CFST columns and CFST beams is still unexplored. To explore their potential as part of CFST frame structures, two types of connections using extended end plate and seat angle are proposed for exterior joints of CFST beams and CFST columns. The performance of the proposed bolted connections subjected to static loads has been investigated by executing an experimental program involving eight specimens of exterior beam-to-column joints subjected to monotonically increasing load applied at the free end of the beam. The performance is appraised in terms of load deformation behaviour and failure modes of joints with varying test parameters like the beam section type, length and diameter of bolts. Finite element analysis for the applied load has been performed using ATENA-3D software, and the behaviour of the connections is investigated. The FE simulation results are compared with experimental test data to verify the FE model.

Behaviour of concrete-filled steel tubular members under pure bending and acid rain attack: Test simulation

As infrastructure in China continues to develop rapidly, concrete-filled steel tubular structures are increasingly attracting interest for use in construction engineering owing to their high section modulus, high strength and good seismic performance characteristics. However, acid rain occurs throughout much of China, and the mechanical behaviour of concrete-filled steel tubular members may be affected by the corrosive environment created by acid rain. In this study, a total of 14 circular and square-shaped concrete-filled steel tubular members made of different types of concrete (general and recycled) and with varying corrosion rates (0%, 10%, 20% and 30%) were tested under four-point bending. After testing, the flexural behaviour of the corroded and uncorroded concrete-filled steel tubular beams were analysed in detail in terms of their failure modes, moment versus deformation curves and ultimate strength. The results indicate that the corrosion leads to an evident decrease in yield strength, elastic modulus and tensile strain capacity of steel plates and also to a noticeable deterioration in the ultimate strength of the concrete-filled steel tubular members. The replacement of general concrete aggregate with recycled concrete aggregate has little impact on the flexural performance of corroded and uncorroded concrete-filled steel tubular beams. Finally, comparisons were made between the experimental results and predicted ultimate strengths from four existing codes (GB 50936-2014, DB36/J001-2007, AIJ and EN 1994-1-1:2004).

Experimental study on the flexural behavior of large-scale rectangular concrete-filled steel tubular beams

ABSTRACT This paper presents a series of test results of large-scale rectangular concrete-filled steel tubular (CFST) beams to explore their performance under pure bending. Concrete-filling tests were initially carried out on two beam specimens of 12-m in length to investigate the feasibility of casting horizontally large-scale rectangular tubes. A total of six 6-m long specimens were subjected to flexural test afterward, including four CFST beams and two steel hollow section (SHS) beams for comparison. The test results showed that the rectangular CFST beams behaved in a relatively ductile manner. The concrete infilling enhanced the flexural behavior and performance of the steel tubes. Finally, the rigid-plastic theory showed suitable to predict the moment capacity of CFST compact beams.

Experimental investigation of cementitious material-filled square thin-walled steel beams

It was recently found that concrete-filled steel tubular (CFST) beams have some advantages in resisting forces compared to steel tubular beams. By using the CFST beams, the flexural capacity, stiffness, ductility and energy dissipation ability are improved due to the presence of the filled-in concrete. Currently, bending tests are performed on glass-fibre reinforced cementitious material-filled square steel tubular (GFCMFST) beams. For comparison, unreinforced cementitious material-filled square steel tubular (CMFST) beams with a higher compressive strength are also prepared and tested. Two shear span-to-depth ratios are considered in the tests. The tests showed that all beams have a significant yielding plateau, hence, exhibit adequate ductility. Furthermore, it was found that the first crack of the filling material took place for CMFST beams at lower load levels compared to the GFCMFST beams in spite having higher compressive strength. Also, it was found that the stresses of the CMFST beams were always greater than the corresponding values of GFCMFST beams in both the tensioned and the compressed zones. The experimental moment capacities were then compared with the available design methods. It is recommended to use the design strength as estimated following EC4. Comparisons between the experimental results of the initial and the serviceability-level section flexural stiffness and its corresponding values from literature and design specifications were also made. Among the different methods, the AIJ standard was found to give the most suitable results for the initial section flexural stiffness and the serviceability-level section flexural stiffness of the current composite beams.

Rehabilitation and strengthening of high-strength rectangular CFST beams using a partial wrapping scheme of CFRP sheets: Experimental and numerical study

Recently, concrete-filled steel tubular (CFST) members are becoming highly recommended by engineers for use in composite structures. Lately, carbon fiber-reinforced polymer (CFRP) composite materials have been used for strengthening steel and CFST flexural members. Both experimental and numerical studies are adopted in this research to further investigate the behavior of various sections of rectangular CFST beams strengthened and/or repaired with unidirectional CFRP sheets. The effects of the U-shaped wrapping scheme using multiple CFRP layers applied at various beam lengths was studied. In general, the results demonstrated that the moment capacity of strengthened CFST beams increased significantly with increasing CFRP layers; for example, it increased by approximately 40% and 55% for beams with compact and slender sections, respectively, when wrapped with 4 CFRP layers. The capacity of damaged CFST beams has improved over 65% when repaired with 2 CFRP layers. Moreover, the effects of wrapping the beam along 75% of its length achieved a similar enhancement ratio and behavior as that of the beam with wrapping along 100% of its length for the same CFRP layers.

Behaviour of bolted connections in concrete-filled steel tubular beam-column joints

Many authors have established the usefulness of concrete filled steel tubular (CFST) sections as compression members while few have proved their utility as flexural members. To explore their prospective as part of CFST frame structures, two types of connections using extended end plate and seat angle are proposed for exterior joints of CFST beams and CFST columns. To investigate the performance and failure modes of the proposed bolted connections subjected to static loads, an experimental program has been executed involving ten specimens of exterior beam-to-column joints subjected to monotonically increasing load applied at the tip of beam, the performance is appraised in terms of load deformation behaviour of joints. The test parameters varied are the beam section type, type and diameter of bolts. To validate the experimental behaviour of the proposed connections in CFST beam-column joints, finite element analysis for the applied load has been performed using software ATENA-3D and the results of the proposed models are compared with experimental results. The experimental results obtained agree that the proposed CFST beam-column connections perform in a semi-rigid and partial strength mode as per specification of EC3.

Performance of hexagonal CFST members under axial compression and bending

The compressive and flexural behavior of the hexagonal concrete-filled steel tubular (CFST) members is investigated in this study. Tests are carried out on the specimens under axial compression and bending. The parameter of the experimental work is the steel ratio of the cross section, and the hollow tubular members are designed for comparison. The failure modes, load versus deformation relations and strain developments are studied. A finite element analysis (FEA) model is established and verified with the test results. The FEA model is then used to conduct the full range analysis on the load versus deformation relation, internal force distribution and stress development. The parametric study on the compressive and flexural behavior is conducted to investigate the effects of various parameters, e.g. the steel ratio, the steel yield strength and the concrete strength. Simplified models are proposed to calculate the ultimate compressive and flexural strength. It is found that the ultimate compressive strength of hexagonal CFST stub columns could be calculated accurately referring to the equations of rectangular CFST columns in DBJ/T13-51-2010 and EC4, and the plastic stress distribution method and the fiber model method achieve good predictions of the ultimate flexural strength of the hexagonal CFST beams.

Numerical modeling on post-local buckling behavior of circular and square concrete-filled steel tubular beam columns

In concrete-filled steel tubular (CFST) beam-columns under cyclic loadings, local buckling of restrained steel tube, crushing and slipping of the confined concrete could have significant influences on strength and stiffness degradation. This paper presents a numerical model based on fiber element discretization for simulating the deterioration behaviour of both rectangular and circular CFST beam columns under axial load and cyclic loading. Uniaxial stress-strain relationships for steel and concrete in CFST beam columns are proposed by introducing unified parameters to control the extent of deterioration. Axial contraction caused by local buckling was accounted by defining an inelastic region that promises plasticity and strength degradation. The numerical model was validated by comparing the simulation results and the associated experimental results. The influences of diameter-to-thickness ratio, concrete and steel strength ranging from normal strength to high strength on the deterioration behavior of circular and rectangular CFST beam-columns were analyzed. Finally, through recognizing the limit state to trigger global strength degradation of a CFST frame, the developed numerical model is applicable for simulating global behavior of CFST frames under collapse-level deformations.

DESIGN OF CIRCULAR COMPOSITE BEAMS WITH A DIFFERENT CONCRETE CORE CONSIDERING THE EFFECT OF CONCRETE IN TENSION

Currently, the bending resistance of composite steel and concrete circular beams and beam-columns may be analysed assuming the interaction between the steel shell and concrete core but ignoring the behaviour of the part of the concrete core in tension. Some natural and numerical experiments show that an influence of this part of the concrete core on the total value of bending resistance of a composite member may be rather important. Therefore, this paper pre- sents the method developed for the design of hollow and solid concrete-filled steel tubular beams based on the test data. It takes into account an effect of the part of the concrete core in tension and gives a better agreement with test results than EC4 (EN 1994-1-1 2004). Also, the paper presents the results of carried out analytical, experimental and numerical investigations of the hollow centrifuged and solid concrete-filled steel tubular beams.

Analysis of behaviour for hollow/solid concrete-filled CHS steel beams

Interaction between the external thin-walled steel tube and the internal concrete core significantly increases the bending resistance of composite beams and beam-columns in comparison with the steel or concrete members. There is presented a developed method for design of hollow and solid concrete-filled steel tubular beams based on test data, which gives better agreement with test results than EC4 because its limitation to take an increase in strength of concrete caused by confinement contradicts the recommendation of 6.7.2(4) that full composite action up to failure may be assumed between steel and concrete components of the member. Good agreement between the results of carried out experimental, numerical and theoretical investigations allows recommending the proposed method to use in design practice.

A study on bending strength of reinforced concrete filled steel tubular beam

The mechanical characteristic of reinforced concrete filled steel tubular (RCFT) structures are differed from that of concrete filled tubular steel (CFT) structures because the reinforcement in RCFT largely affects the performance of core concrete such as ductility, strength and toughness, and hence the performance of RCFT should be evaluated differently from CFT. To examine the effect axial reinforcement on bending performance, an investigation on RCFT beams with varying levels of axial reinforcement is performed by the means of numerical parametric study. According to the numerical simulation results with 13 different ratios of axial reinforcement, it is concluded that the reinforcement has obvious effect on bending capacity, and the neutral axis of RCFT is different from CFT, and an evaluation equation in which the effect of axial reinforcement is considered for ultimate bending strength of RCFT is proposed.