Concrete-filled Circular Hollow Section Research Articles

Design equations for maximum stress concentration factors for concrete-filled steel tubular K-joints

Stress concentration factors (SCFs) for welded tubular joints can be decreased by filling the chord with concrete leading to a longer fatigue life. However, there are currently no design formula available in guidelines to predict the SCF of concrete-filled circular hollow section (CFCHS) K-joints, thus limiting their applicability in bridge design. To address this gap, finite element models for CFCHS K-joints were developed and compared against test results to ensure their accuracy. Then, a comprehensive parametric study was conducted to establish relationships between maximum SCFs and four variables: brace-to-chord diameter ratio (β), chord diameter-to-thickness ratio (2γ), brace-to-chord thickness ratio (τ), and the angle between braces and chord (θ). A total of 480 FE models were examined under three loading conditions including brace and chord loading: balanced axial force, chord axial force, and chord bending. Design equations to predict the maximum SCF for CFCHS K-joints were established by multiple regression analyses of the numerical results. A comparison of maximum SCFs between circular hollow section (CHS) and CFCHS K-joints was made, and it was concluded that average reductions of 42% and 33% in maximum SCFs in CFCHS K-joints at the locations of the chord and brace were found compared to CHS joints for balanced axial force, respectively. Finally, a case study illustrating how to use the proposed equations for fatigue safety verification was presented.

Monte Carlo simulation study on the reliability of concrete-filled HSS beam-column design provisions

Revisions were recently proposed to the way in which concrete-filled hollow structural section (HSS) members are handled in CSA S16. These revisions were based on previous research, comparisons to experiments, and a first-order reliability method analysis of existing provisions for compression and flexural members. In this paper, this topic is further expanded by using Monte Carlo Simulations (MCS) to determine the inherent reliability of the previous and new design rules for concrete-filled rectangular hollow section (RHS) and circular hollow section (CHS) beam-columns. A representative set of concrete-filled RHS and CHS members with variations in concrete strength, wall slenderness, effective length, and loading eccentricity are analyzed. Using MCS, the reliability index (β+) of each is determined over a range of live-to-dead load (L/D) ratios. Inherent β+ values are compared to the code-specified target (i.e., β+ = 3.0 per Annex B of CSA S16) and the CSA S16:19 and AISC 360-16 provisions.

Cyclic Inelastic Behavior of Plate Welded to Concrete-Filled Circular Hollow Section

Cyclic Inelastic Behavior of Plate Welded to Concrete-Filled Circular Hollow Section

Behavior of partially concrete-filled X-joints with CHS braces and SHS chord under axial compression

The paper experimentally studies the static behavior of partially concrete-filled X-joints with circular hollow section (CHS) braces and square hollow section (SHS) chord under axial compression. Totally 40 partially concrete-filled CHS-SHS X-joints were designed, including 20 X-joints with concrete-filled CHS braces and hollow SHS chord (CFCHS-SHS) and 20 X-joints with hollow CHS braces and concrete-filled SHS chord (CHS-CFSHS). The impacts of the brace diameter to chord width ratio (β = d/b) and chord width to double chord thickness ratio (γ = b/2 T) on the static behavior of CHS-SHS X-joints were experimentally studied. At the same time, the corresponding finite element analysis (FEA) models were established and validated by comparison with test results, and then an extensive parametric investigation was performed to study the effects of three main influential factors (τ, concrete filling position, and in-filled concrete strength) on the ultimate strength of CHS-SHS X-joints under axial compression. The test results show that the CFCHS-SHS X-joints and CHS-CFSHS X-joints exhibit completely different failure modes under axial compression. All CFCHS-SHS X-joints are failed with chord plasticization, while the local buckling of the brace members is the primary failure mode for the CHS-CFSHS X-joints. The ultimate strength and axial stiffness of CHS-CFSHS X-joints are significantly higher than those of CFCHS-SHS X-joints. However, the ductility of CHS-CFSHS X-joints is slightly superior to that of CFCHS-SHS X-joints. Based on the analyses of test results, the design equations derived from the simplified yield line model were put forward for the estimation of the ultimate strength of CFCHS-SHS X-joints, which was verified to be reasonable and accurate. In addition, the study also revealed that the total cross-section yield strength of the brace could be utilized to accurately predict the ultimate strength of CHS-CFSHS X-joints.

Formulation of stress concentration factors for concrete-filled steel tubular (CFST) K-joints under three loading conditions without shear forces

Concrete-filled steel tubular (CFST) K-joints have been widely applied to CFST trussed arch bridges in China, which are comprised of a concrete-filled circular hollow section (CHS) chord and two CHS braces. It has been experimentally revealed that hot spot stress (HSS) of CFST K-joints is significantly lower than those of empty tubular K-joints in the reported researches. However, no parametric formulae on stress concentration factors (SCFs) of CFST K-joints have been established. In present study, three-dimensional FE models for determining the SCF distributions around the chord-brace intersections of CFST K-joints were developed first. The validity of the FE modelling has been examined by comparing with the published experimental results. Then 272 FE models of CFST K-joints with different geometric dimensions were prepared and provided for the parametric study to demonstrate the influence of four key geometric parameters, i.e. diameter ratio (β), diameter to thickness ratio of chord (2γ), thickness ratio (τ) and the angle (θ) between the axis of the chord and brace, on SCFs around the chord-brace intersection. The analysis was performed under three loading conditions, i.e. the basic balanced axial forces, axial compressive force in the chord and in-plane bending in the chord. Finally, parametric formulae to determine the SCFs in CFST K-joints were proposed by the multiple regression analysis, and their accuracy was demonstrated through the comparison of SCFs obtained by the proposed formulae and FEA.

Stress concentration factor in concrete-filled steel tubular K-joints under balanced axial load

Steel tubular K-joints are widely used in railway and highway bridges. In this study, stress concentration factor (SCF) has been investigated in uniplanar K-joints made of steel circular hollow section (CHS) braces welded to concrete-filled circular hollow section (CFCHS) chord (CHS-to-CFCHS) under balanced axial loading. Experimental SCF measurements at 45° interval on the chord and two braces are performed in one CHS-to-CFCHS K-joint specimen under balanced axial loading. A finite element (FE) model has been developed using ABAQUS. The FE model was verified with experiments by the author and other experiments reported in the literature. Parametric study on the SCF in the brace and chord has been conducted. Parametric equations for predicting the SCF in the brace and chord have been derived. The predicted SCF by the derived equations are comparable with the experimental stress concentration factors.

Formulae for hot-spot stress concentration factors of concrete-filled CHS T-joints based on experiments and FE analysis

Fatigue of welded concrete-filled tubular joints is a new subject to be investigated, compared with fatigue of welded tubular joints without concrete which has been studied well. Fatigue cracking is usually located at the hot spot stress of a welded concrete-filled tubular joint. A method of calculating its hot spot stress is essential for fatigue strength assessment. Hot spot stress concentration factors (SCF) of welded concrete-filled circular hollow section (called as CFCHS) T-joints respectively under axial force and in-plane bending are investigated experimentally and numerically in this paper. SCF testing was carried out on ten CFCHS T-joints with different non-dimensional geometric parameters β (ratio of brace to chord diameter), 2γ (ratio of chord diameter to thickness) and τ (ratio of brace to chord thickness). The experimental findings shows in most cases, a remarkable decrease in SCFs of CFCHS T-joints, compared with circular hollow section T-joints without concrete. Finite element (FE) modelling with 3D quadratic hexahedral solid element was developed and verified by the experimental data. It is found that the conversion relationship SCF= 1.2 SNCF (strain concentration factor) can be adopted for CFCHS T-joints. Using the reliable FE modelling, parametric study was conducted to know the effect of non-dimensional parameters on SCFs of CFCHS T-joints. Based on multiple regression analysis, a series of formulae are recommended for calculating SCFs in the chord and brace of CFCHS T-joints subjected to the axial force and in-plane bending in the brace and in the chord respectively, which are validated by both the experimental data and FE modelling.

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.

EXPERIMENTAL STRESS CONCENTRATION FACTOR IN CONCRETE-FILLED STEEL TUBULAR T-JOINTS

An experimental investigation of stress concentration factor (SCF) in Steel circular hollow section brace welded to concrete-filled circular hollow section chord (CHS-to-CFCHS) T-joints has been performed under axial tension, axial compression, in-plane bending and out-of-plane bending. The distribution of SCF around the welded brace-to-chord intersection on both the brace and chord has been investigated using three CHS-to-CFCHS T-joint specimens. The experimental SCF results have been compared with the predicted SCF in empty T-joints. The relationship between the maximum SCF in relation to parameter β, with fixed other geometrical parameters, has been investigated for the basic load conditions. The experimental maximum SCF under axial tension has been compared with the predicted maximum SCF from parametric equations for CHS-to-CFCHS T-joints previously developed by the authors. The results show that the concrete has a significant effect in reducing the SCF, mostly under axial tension and the parametric equations for predicting SCFs in empty T-joints are not suitable for CHS-to-CFCHS T-joints. The effect of parameter β on the maximum SCF in CHS-to-CFCHS T-joints is significant under axial tension and out-of-plane bending moment.

Behaviour of Concrete-Filled Double-Skin Circular Hollow Section Cross Joints Under Axial Compression

This paper presents an experimental investigation on concrete-filled double-skin circular hollow section (CHS) cross joints under axial compression. A total of twenty-two right-angled CHS cross joints with different brace to chord diameter ratio (β), inner tube to outer tube thickness ratio of chord (ω) and hollow ratio of chord (χ) were tested, in which eighteen concrete-filled double-skin CHS cross joints were studied for different shapes of inner tube of chord, two traditional empty CHS cross joints and two traditional concrete-filled CHS cross joints were tested for comparison. The joint strengths, failure modes, load-deformation curves and strain distribution curves of all specimens are reported. The effects of brace to chord diameter ratio (β), inner tube to outer tube thickness ratio of chord (ω), hollow ratio of chord (χ), shape of inner tube of chord and concrete strength on the behaviour of concrete-filled double-skin CHS cross joints under axial compression were evaluated. It is shown from the comparison that the ultimate load and initial stiffness of CHS cross joints are significantly enhanced by strengthening the chord member with inner tube and concrete infill. Furthermore, the ultimate strengths are increased with the increase of the β ratio, whereas the ultimate strengths are decreased with the increase of the χ ratio for all types of concrete-filled double-skin CHS cross joints. On the other hand, the ultimate strengths are enhanced with the increase of the ω ratio and concrete strength for all types of concrete-filled double-skin CHS cross joints, but the enhancement is insignificant. The corresponding finite element analysis was also performed and calibrated against the test results. The design equations are proposed based on the test and finite element analysis results for concrete-filled double-skin CHS cross joints, which are verified to be more accurate.

Experimental study of concrete-filled CHS stub columns with inner FRP tubes

An experimental study into the axial compressive behaviour of concrete-filled circular hollow section (CHS) steel columns with internal fibre reinforced polymer (FRP) tubes is presented in this paper. A total of 17 concrete-filled steel tubular (CFST) columns were tested, 15 with an inner FRP tube and 2 with no inner tube. Complementary material tests and tests on 15 FRP-confined concrete (FCC) columns were also carried out. The varied test parameters included the concrete strength, the ratio of the diameter of the steel tube to that of the FRP tube, the diameter to wall thickness ratio of the inner FRP tube and the type (influencing principally the rupture strain) of the FRP. It was found that the presence of the inner FRP tube led to considerably improved axial compressive behaviour due to the greater levels of confinement afforded to the ‘doubly-confined’ inner concrete core; the load-bearing capacity was increased by between about 10% and 50% and the ductility was also enhanced. Greater benefits arose with (1) increasing diameter of the inner FRP tube due to the increased portion of the cross-section that is doubly-confined and (2) increasing wall thickness of the inner FRP tube due to the increased level of confinement afforded to the inner concrete core. The load-deflection responses of all tested specimens were reported, revealing that failure was generally gradual with no sharp loss in load-bearing capacity, implying that the embedment of the inner FRP tube within the concrete enables it to continue to provide a reasonable degree of confinement even after the initiation of fibre rupture; this is different to the sudden loss of confinement typically observed in FRP externally jacketed concrete columns.

Fatigue behavior and design of welded tubular T-joints with CHS brace and concrete-filled chord

Concrete-filled welded tubular structures have been increasingly used in large-span constructions such as truss bridges where fatigue failure is always a critical issue that should be focused on. This paper deals with fatigue behavior of CHS-CFSHS T-joints, which are made up of circular hollow section (CHS) braces and concrete-filled square hollow section (CFSHS) chords. Experimental results on stress concentration factor (SCF) were briefly summarized. Fatigue tests were conducted under cyclic tensile force in the brace, so that cracking patterns, failure modes and fatigue data of such joints were recorded. The tested CHS-CFSHS T-joints suffered from one of the following three failure modes: (i) brace-90°-side failure, (ii) chord-90°-side failure, (iii) chord-0°-side failure. Based on the fatigue data in terms of hot spot stress range (Sr,hs) versus number of cycles (N), fatigue design Sr,hs-N curves were determined using the deterministic and least-squares methods, respectively. It was found that most of the existing Sr,hs-N curves are unsafe for fatigue design of CHS-CFSHS T-joints, except for the X´ Sr,hs-N curve recommended by American Petroleum Institute [32] and American Welding Society [36]. In addition, the fatigue data of the CHS-CFSHS T-joints were found comparable with CHS-CFCHS T-joints (T-joints with CHS brace and concrete-filled CHS chord), hence a new fatigue design Sr,hs-N curve was proposed for the both types of concrete-filled joints. A comparison between CHS-CFSHS T-joints and empty CHS-SHS T-joints showed that CHS-CFSHS T-joints generally have better fatigue behavior compared to their empty counterparts except for the case of extremely large ratio of brace diameter to chord width (β).

Numerical analysis and punching shear fracture based design of longitudinal plate to concrete-filled CHS connections

The mechanical behaviour of longitudinal plate-to-concrete-filled circular hollow section (CHS) connections under axial tension, eccentric tension and in-plane bending is extensively studied by the experimentally validated finite element analysis (FEA) in this paper. A total of 336 connections with a wide range of parameters on geometrical configurations, material properties and load positions was conducted to investigate (a) the general applicability of the experimental conclusion for the governing limit state, (b) the shear stress profile on the failure face and (c) the design equations based on fracture analytical models under various loading conditions. FEA extended the validity of experimental conclusion that the only governing limit state was punching shear failure instead of the deformation limit of 3% chord diameter (D). With an aim of proposing design equations based on ductile fracture mechanics, the stress distributions on the fracture failure face and the inner concrete were investigated by the parametric study, and then were adopted in the analytical models. Finally, design equations based on semi-theoretical models for the ultimate strength of longitudinal plate-to-concrete-filled CHS connections under three investigated loads were proposed. It is found the connection-capacity predictions agreed with both test and FEA results well.

Group Behavior of Double-Headed Anchored Blind Bolts within Concrete-Filled Circular Hollow Sections under Cyclic Loading

AbstractSeven full-scale curved T-stubs connected to concrete-filled circular hollow sections (CFCHSs) using groups of double-headed anchored blind bolts (DHABBs) have been cyclically tested in thi...

Component model for pull-out behaviour of headed anchored blind bolt within concrete filled circular hollow section

This paper presents a simplified approach to estimate the pull-out behaviour of headed anchor blind bolts (HABB) from concrete filled circular hollow section (CFCHS). The approach was developed based on a multi-linear component spring model representing the behaviour of the blind bolt’s components, i.e. the bolt’s shank, the embedded head, and the washer bearing on the inside of the tube wall. Extensive finite element analyses (FEA) were used to develop and calibrate the nonlinear behaviour of each component for monotonic loading only, in which the yield and ultimate strength, as well as the initial and secant stiffness of each component were taken into account. Important parameters such as the diameter of the HABB, diameter and thickness of the circular hollow section (CHS), embedment depth of the embedded head, and concrete strength are also incorporated in defining the component spring behaviour. The proposed method was able to yield a good match with both the FEA and test results for the individual component behaviour in terms of the initial and secant stiffness as well as the ultimate strength. Moreover, the load transfer and failure sequences of the HABB within a CFCHS are discussed, and the assembly procedures of the component springs are proposed, by which the combined component behaviour is identified and matches well with both the FEA and test results.

Mechanical behaviour of concrete-filled CHS connections subjected to in-plane bending

This paper presents an investigation on the mechanical behaviour of concrete-filled circular hollow section (CHS) connections. Four large-scale connection specimens were tested to failure under in-plane bending. The main test parameters included chord-wall slenderness and diameter ratio between brace and chord. The main failure mode observed from the tests was chord-wall punching shear failure. Meanwhile the chord-wall deformation was also investigated to determine the governing limit state. Complementary finite element (FE) methodology was validated against the experimental findings and the validated FE models were used to further study the mechanical behaviour and the strength design of the connections. The modified Mohr-Coulomb criterion (MMC) was introduced into the finite element models to capture steel fracture failure. Based on both experimental and numerical investigations, a theoretical analytical model with the consideration of the inner concrete was established and an ultimate strength design equation was proposed accordingly to predict the punching shear strength for concrete-filled CHS connections under in-plane bending.

Experimental analysis of a mechanical shear connector in concrete filled steel tube column

ABSTRACT This work includes an analytical and experimental study of the structural behavior of shear connectors in composite columns, composed of concrete-filled circular hollow section. For this study was adopted a structural bolt like a shear connector in order to verify the validity of the analytical expressions in ABNT NBR 16239: 2014 [1]. Was carried out a series of push-out tests, fixing the outer diameter of the hollow section and varying the thickness, the bolt diameter, the strength of concrete and the hole dimension. Analysis of the results shows that is possible to use this type of shear connector. The Brazilian prescriptions results are conservative and may be adjusted to provide strength capacity value closest to the experiment.

Flexural behaviour of concrete-filled CHS X-joints with curved chord under out-of-plane bending

This paper presents experimental and numerical investigations on empty and concrete-filled circular hollow section (CHS) X-joints with curved chord under out-of-plane bending. A total of 16 specimens were fabricated by hot-rolled bending the chord members into curvature with three different radii to apply out-of-plane bending to the brace members, in which 6 specimens were tested with empty curved chord, 6 specimens were tested with concrete filled in the curved chord only, and 4 traditional empty and concrete-filled CHS X-joints with straight chord were also tested for comparison. The typical failure modes, axial load-vertical displacement curves, bending moment-chord deformation curves, bending moment-rotation curves and strain distribution curves of all specimens are reported. The effects of curvature radius of chord member and concrete filled in the chord member on the joint strength and behaviour under out-of-plane bending were evaluated. It is shown from the comparison that the ultimate strengths of the CHS X-joints with curvature radius of chord less than 12d0 under out-of-plane bending are closer to those of the traditional CHS X-joints with straight chord with the increase of the curvature radius of chord member. Whereas, the ultimate strengths of the CHS X-joints with curvature radius of chord less than 12d0 under out-of-plane bending are generally similar to those of the traditional CHS X-joints with straight chord. Furthermore, the out-of-plane flexural behaviour of CHS X-joints are improved with the increase of the β ratio. Whereas, filling the concrete in the chord member cannot enhance the ultimate strengths of CHS X-joints under out-of-plane bending. The joint strengths obtained from the experimental investigation are compared with the design strengths calculated using the current design rules for traditional CHS X-joints with straight chord under out-of-plane bending, which were demonstrated to be quite conservative. In addition, the corresponding finite element analysis was also performed and calibrated against the test results. The design equations are proposed based on the test and numerical results for empty and concrete-filled CHS X-joints with curved chord under out-of-plane bending, which were verified to be more accurate.

Cyclic behaviour of individual double headed anchored blind bolts within CFST

When bolted connections are preferred and concrete-filled steel tubes are chosen as the columns, blind bolts can be used instead of ordinary structural bolts to eliminate the need for access to the inside of the tube to tighten the nut. In order to achieve a stiff and strong connection, a modification of the blind bolt has previously been made by extending the bolt shank into the concrete and inserting another nut at the end of the bolt shank, the so-called headed anchored blind bolt (HABB). A further advance has been instigated here to achieve the maximum possible pull-out stiffness of this type of blind bolted connection by installing an additional head at the middle of the bolt shank anchor, called the first embedded head. This paper reports the results of an experimental investigation of the cyclic behaviour of individual double headed anchored blind bolts (DHABBs), which have two embedded heads, within concrete-filled circular hollow sections (CFCHS) using plain and steel fibre reinforced concrete (SFRC). After achieving a good agreement between the experimental and FE results, the relative contributions to the pull-out force of the first embedded head, end embedded head, and the tube wall were derived. In addition, parametric studies have been conducted using FEA to consider the influence of the concrete strength and type of concrete, the D/t ratio of the steel tube, the removal of the first embedded head, the location of the first embedded head, and the blind bolt diameter.

Structural fire performance of restrained composite columns made of concrete-filled square and circular hollow sections

Most of the previous studies on concretefilled steel hollow section columns at high temperatures addressed the effect of depth-tothickness ratio, column slenderness, initial applied load level, load eccentricity, and local buckling of concrete-filled steel tubes on the fire resistance of these columns. For this reason, it important and required to study the influence of the axial and rotational restraint on the buckling behavior of these types of columns subjected to fire. The results of a series of fire resistance tests on these types of columns inserted in a steel frame are presented and discussed in this paper. The primary test parameters taken into account were column slenderness, type of section geometry, and axial and rotational restraint level imposed by a surrounding steel frame to the columns. The specimens were then uniformly exposed to the ISO 834 standard fire curve, and the critical time (fire resistance), failure temperature distribution and respective failure modes were assessed. Finally, the results of this research study showed most of all that the fire resistance of identical semi-rigid ended columns may be not significantly affected by the stiffness of the surrounding structure but, on the contrary, their post-buckling behavior may be affected.