Concrete-filled Square Hollow Section Research Articles

Experimental and numerical investigations on stress concentration factors of concrete filled steel tube X-joints.

An SHS-CFSHS X-joint is fabricated by welding two square hollow section (SHS) braces to a concrete-filled square hollow section (CFSHS) chord. In this paper, the stress concentration factors (SCFs) of SHS-CFSHS X-joints are investigated through experimental tests and finite element analysis (FEA), with the hot spot stress method serving as the analytical approach. Eight specimens are designed and manufactured, with FE models built in software ANSYS. These FE models are validated against the test results. The specimens are tested under brace axial tension to determine the SCFs of the X-joints. It shows that the concrete filled in the chord effectively reduces the SCFs of the X-joints. To further explore various load conditions and the influence of the parameters, FEA is carried out and a total of 64 FE models are built. Based on the FEA results, multiple regression analysis is used to obtain the SCF formulae of SHS-CFSHS X-joints under axial tension load and in-plane bending load in the brace, respectively. Comparison and analysis of the SCF results obtained from experimental tests, the proposed formulae, and FE simulations reveal that the formulae presented in this study are both conservative and suitable for predicting SCFs.

Tests of concrete-filled high strength steel RHS and SHS beams

This paper presents an experimental investigation of concrete-filled high strength steel tubular (CFHSST) beams. A total of 35 four-point bending tests were conducted, covering high-strength steel square and rectangular hollow sections with nominal 0.2% proof stresses of 700 MPa and 900 MPa. Each high strength steel tubular section was filled with concrete materials of strengths equal to 40, 70 and 110 MPa, respectively. Hollow steel tubes were also tested for comparison purpose. Moment capacity of the test specimens was measured, and ductility of the test specimens was calculated, in order to evaluate the structural benefit associated with the composite action between the two materials. It was shown that both moment capacities and ductility of the CFHSST beams were significantly increased compared to the corresponding hollow sections without concrete infill. It should be noted that the very high strength steel material of 900 MPa is not covered in any design provisions. The current design equations were derived based on normal steel and concrete grades. The suitability of the current design rules, including American Specification (AISC) and European Code (EC4), was assessed for concrete-filled high strength steel tubular sections subjected to bending.

Experimental, numerical and parametric studies on stress concentration factors of T-joints under tension

This paper investigates the behaviour of square hollow section (SHS) T-joints under static axial tension for the determination of stress concentration factors (SCFs) at the hot spot locations. Five empty and corresponding concrete-filled SHS-SHS T-joint connections were tested experimentally and numerically. The experimental investigation was carried out by attaching strain gauges onto the SHS-SHS T-joint specimens. The numerical study was then conducted by developing three-dimensional finite element (FE) T-joint models using ABAQUS finite element analysis software for capturing the distribution of the SCFs at the hot spot locations. The results showed that there is a good agreement between the experimental and numerical SCFs. A series of formulae for the prediction of SCF in concrete-filled SHS T-joints under tension were proposed, and good agreement was achieved between the maximum SCFs in SHS T-joints calculated from FE T-joint models and those from the predicted formulae.

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.

Flexural behavior of concrete-filled SHS and RHS aluminum alloy tubes strengthened with CFRP

Conventional concrete-filled aluminum alloy tube (CFAT) can effectively delay the inward local buckling of aluminum alloy tube (AA). This paper experimentally and numerically investigates the suitability of strengthening square and rectangular hollow section (SHS and RHS) CFAT beams with a layer of carbon-fiber reinforcement polymer (CFRP) under four-point bending. Among 40 beams, 30 square and rectangular CFAT beams were strengthened with CFRP comprising three arrangement schemes of CFRP and 10 conventional square and rectangular CFAT beams were treated as reference beams. Flexural stiffness of square and rectangular CFAT beams are remarkably enhanced by external bonded CFRP, while the ductility is decreased. The bottom flange-bonded CFRP scheme is less effective in enhancing ultimate strength of CFAT beams than that of four sides-bonded CFRP scheme. New design approaches for evaluating both initial and post-yield flexural stiffness of square and rectangular CFAT beams strengthened with CFRP are proposed. FE models are correctly simulated to analyze the flexural behavior of square and rectangular CFAT beams strengthened with CFRP. CFRP strengthening technique using in this study can remarkably enhance the ultimate strength of square and rectangular CFAT beams and effectively delay the outward local buckling of AA tubes.

Tensile behaviour of double headed anchored blind bolt in concrete filled square hollow section under cyclic loading

This paper presents the tensile behaviour of individual Double Headed Anchored Blind Bolts (DHABBs) which are anchored within Concrete Filled Square Hollow Section (CFSHS) tubes. The DHABB consists of a conventional Headed Anchored Blind Bolt (HABB) with one additional middle head, between the existing end head in the embedded region and head next to the tube wall. A series of experiments were conducted on the pull-out of single DHABBs under monotonic and cyclic loading using different tube sizes, bolt sizes and concrete types. Detailed results from the experimental work are presented in this paper. A comprehensive three dimensional finite element model was developed and the results were compared with full scale pull-out test results. A good agreement was obtained between the FE and experimental results.

Stress concentration factors in concrete-filled square hollow section joints with perfobond ribs

A novel tubular joint of concrete-filled square hollow section (CFSHS) chord welded to an SHS brace was studied experimentally and numerically to establish design equations for Stress Concentration Factors (SCFs) for fatigue design purposes. The chord was stiffened with internal perfobond ribs longitudinally welded to the inner faces. A series of joints with varying member sizes and wall thicknesses were first tested under axial force in the brace and the hot spot stresses were carefully measured. Three-dimensional finite element models were developed and validated with the experimental results. A comprehensive parametric study was then conducted on 90° T- and X-joints with perfobond ribs, to establish correlations between SCF and three key non-dimensional geometric parameters: brace width-to-chord width (β), width-to-wall thickness ratio of the chord (2γ), and brace wall thickness-to-chord wall thickness ratio (τ). Four basic loading conditions were examined: axial tension and in-plane bending in the brace, and axial tension and in-plane bending in the chord. Finally, a series of SCF formulae and design charts were proposed based on multiple regression analysis. It was shown that under brace axial tension stress concentrations reduced by one third and two thirds in CFSHS chords without perfobond ribs and with perfobond ribs, respectively, compared to a SHS chord. Current design guides which are based on SHS sections, overestimate SCFs significantly for CFSHS sections with perfobond ribs.

Stress concentration factors in joints of square hollow section (SHS) brace and concrete-filled SHS chord under axial tension in brace

It has been shown that concrete filling of square hollow section (SHS) chords in trusses increases fatigue life at the joint between the chord and the SHS brace. Nonetheless, currently there are no expressions for the stress concentration factor (SCF) at these joints. In this study, a robust finite element investigation is carried out to examine SCF in 90° joints made up of empty SHS braces and concrete-filled SHS (CFSHS) chords under axial tension in the brace. Results were validated against experimental data. A comprehensive parametric study was then carried out on 80 FE models to generate a large database for SCFs, considering the effects of key non-dimensional parameters, namely; width ratio between brace and chord of 0.4–1.0, width to wall thickness ratio of the chord of 12.5–25, and wall thickness ratio between brace and chord of 0.25–1.0. Using multiple regression analyses, a series of formulae for SCF were developed for practical design applications. A comparison of SCFs in joints with hollow and concrete filled chords shows a 10–26% reduction in SCFs in concrete-filled chords, leading to better fatigue behavior.

Effects of weld size on stress concentration factors of CHS-CFSHS joints

This paper aims to disclose the effects of weld size on hot spot stress in the calculation of fatigue performance of the joints. For this purpose, the author explored the hot spot stress of CHS-CFSHS T-joints, which consists of circular hollow section (CHS) braces and concrete-filled square hollow section (CFSHS) chords. After reviewing the previous studies and the relevant specifications on weld size, the author probes into the effects of weld size on the stress concentration factor (SCF) of CHS-CFSHS joints via finite-element analysis. The analysis show that the weld size directly affected the hot spot stress in both conditions, and the influence laws were largely the same; with the increase of weld size, the brace-side SCF plunged when the chord-size weld size remained the same, but the chord-side SCF changed slightly when the brace-side weld size was constant; the brace-side and chord-side SCF declined when the brace-side and chord-side weld sizes increased by the same amount. This research successfully determined the weld sizes that are consistent with the relevant specifications, and safe and simple to apply in actual engineering.

Stress concentration factors in CHS-CFSHS T-joints: Experiments, FE analysis and formulae

A CHS-CFSHS T-joint is fabricated by welding a circular hollow section (CHS) brace to a concrete-filled square hollow section (CFSHS) chord. This paper presents an investigation into stress concentration factors (SCFs) of CHS-CFSHS T-joints through experimental testing as well as finite element (FE) analysis. A series of well-designed CHS-CFSHS T-joints are first tested under axial loading (tension and compression respectively) and in-plane bending in the brace so that the strain concentration factors (SNCFs) of the joints are determined. By comparing the experimental results with those extracted from previous research on empty CHS-SHS T-joints, it is found that the in-filled concrete can effectively reduce the maximum SNCF and therefore SCF. Three-dimensional FE models are subsequently developed and validated. Then an extensive parametric study is conducted to evaluate the influences of three key non-dimensional geometric parameters (i.e. β, 2γ and τ) using the validated FE modeling strategy. Finally, SCF formulae corresponding to typical load conditions are established for fatigue design of CHS-CFSHS T-joints.

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).

Behaviour of steel-concrete-steel SHS X-joints under axial compression

This paper presents the experimental investigation and finite element analysis on double-skin square hollow section (SHS) X-joints under axial compression, which were strengthened with concrete filled in the chord between inner tube and outer tube. A total of 22 orthogonal SHS X-joints with different brace to chord width ratio (β), hollow ratio of chord (χ) and shapes of inner tube of chord were tested, in which two traditional SHS X-joints and two concrete-filled SHS X-joints were tested for comparison. The effects of brace to chord width ratio (β), inner tube to outer tube thickness ratio of chord (ω), hollow ratio of chord (χ), shape of inner tube of chord and strength of concrete infill on the structural behaviour of steel-concrete-steel SHS X-joints under axial compression were evaluated. It is shown from the comparison that the inner tube and concrete infill significantly enhanced the ultimate strengths of SHS X-joints under axial compression. Furthermore, the ultimate strength and initial stiffness of steel-concrete-steel SHS X-joints under axial compression are improved with the increase of inner tube to outer tube thickness ratio of chord (ω) and strength of concrete infill. On the other hand, the ultimate strength and initial stiffness benefit from the increase of the β ratio and decrease of the χ ratio. The design equations are proposed based on the test and numerical results for steel-concrete-steel SHS X-joints, which were verified to be more accurate.

Experimental investigation of concrete-filled square hollow section columns subjected to non-uniform exposure

An experimental study was carried out to investigate the fire performance of concrete-filled square hollow section (SHS) columns subjected to non-uniform exposure. Six full-scale concrete-filled SHS columns were tested to failure and their thermal distribution, axial deformation, lateral deflection and failure mode were measured. The effects of the number of sides exposed to fire, load ratio and the load eccentricity on the fire resistance were discussed. The results indicate that the number of sides exposed to fire has significant influence on the thermal distribution. For concrete-filled SHS columns subjected to a 1-sided exposure and 3-sided exposure, the thermal distribution is uniaxial symmetric and the lowest temperature area is closer to the side unexposed to fire rather than in the cross-sectional center. The fire resistance of the concrete-filled SHS columns increases considerably with the decreasing number of sides exposed to fire and decreasing load ratio, while load eccentricity has moderate influence. A sequentially coupled thermal-stress FE model was developed to reproduce the tested columns. The experimental results were compared with existing design provisions for concrete-filled SHS columns in uniform fire, indicating that current provisions may overestimate fire resistance of columns in non-uniform fire.

Inclined concrete-filled SHS steel column to steel beam joints under monotonic and cyclic loading: Experiments

Inclined concrete-filled steel tubular (CFST) columns have been applied in some construction projects recently, and they are usually connected with either I-shape or box steel beams in buildings. This paper presents a serial of test results of joints consist of inclined concrete-filled square hollow section (SHS) steel columns and box steel beams. A total of 24 specimens modelled as exterior joints were tested in the program. The parameters investigated included the column type, the joint type and the loading type. The failure modes, the strength and stiffness degradation and the energy dissipation of these composite joints were studied. Discussions on classification of these composite joints were made referencing existing codes. The test results showed that these composite joints reached more than 0.04rad before loads dropped to 85% of the ultimate strength under monotonic loading, and the specimens with both internal and external diaphragms exhibited the most favourable seismic performance among these joints.

Capacities of Concrete-filled Steel Beams of Unequal Double Spans by Testing and Code

This paper presents an experimental investigation on flexural capacity of square hollow section (SHS) steel and concrete-filled square hollow section (SHS) steel tube. Beams with unequal double spans and subjected to point loads at the middle of each span were tested. Comparison was made on the flexural capacity and ductility of bare SHS steel and concrete-filled SHS steel tubes. The moment capacities obtained by the design method in code of practice for structural uses of steel 2005 – CoPHK were compared with experimental results reported in this paper.

Experimental Study on SNCF of Welded CHS-CFSHS T-Joints under Axial Loading

The initiation position can be predicted by studying on strain concentration factors (SNCF) and SNCF distributions. This paper concerns an experimental investigation of SNCF of welded T-joints composed of circular hollow section (CHS) braces and concrete-filled square hollow section (CFSHS) chords under axial loading on the braces with two load cases of tension and compression. Static experiments were performed on eight test specimens designed for considering different non-dimensional geometric parameters and C50 grade concrete. Compared with joints made of CHS-SHS, the experimental results indicate that the CHS-CFSHS T-joints have lower strain concentration factors which may increase fatigue strength. The position of the maximum SNCF is usually located at 00 or 900 of the intersection line between a CHS brace and a CFSHS chord.

Behaviour of steel beam to concrete-filled SHS column frames: Finite element model and verifications

This paper presents the behaviour of the composite frame with concrete-filled square hollow section (SHS) columns to steel beam. Finite element modeling (FEM) was developed to carry out the behaviour of composite frames under a constant axial load on columns and a lateral cyclic load on the frame. Accurate material and geometrical nonlinear for confined concrete and steel were considered in the analysis. The finite element program ABAQUS was adopted. A damage plastic model for concrete and elasto-plastic model for steel were used respectively. Six composite frame tests were carried out to verify the FE model. Each test frame specimen consisted of two concrete-filled SHS columns and a steel beam to represent an interior frame in a building. The results obtained from the finite element model were verified against those obtained from the experimental results.

Nonlinear analysis of concrete-filled steel SHS and RHS columns

This paper presents an accurate nonlinear finite element model for the behaviour and design of axially loaded concrete-filled square hollow section (SHS) and rectangular hollow section (RHS) steel tube columns. The nonlinear material models for confined concrete and steel tubes were carefully modeled in the finite element analysis. The column strengths and load-axial shortening curves were evaluated. The results obtained from the finite element analysis were verified against experimental results. An extensive parametric study was conducted to investigate the effects of different concrete strengths and cross-section geometries on the strength and behaviour of concrete-filled SHS and RHS steel tube columns. The study was conducted over a wide range of concrete cube strengths ranged from 30 to 110 MPa. The overall depth of the steel tube-to-plate thickness ratio ranged from 10 to 40 covering compact SHS and RHS steel tube sections. The column strengths predicted from the finite element analysis were compared with the design strengths calculated using the American, Australian and European specifications. Based on the results obtained from the parametric study, it is found that the design strengths calculated using the American Specifications and Australian Standards are conservative, while the design strengths calculated using the European Code are accurate, except for the concrete-filled RHS compact steel tube columns having the overall depth of the steel tube-to-plate thickness ratio of 40.

Concrete-filled thin-walled steel SHS and RHS beam-columns subjected to cyclic loading

The flexural force-deformation behavior of concrete-filled thin-walled steel SHS and RHS beam-columns was experimentally investigated and the results presented in this paper. The parameters in the study included the depth-to-width ratio (β), the core concrete strength ( f cu), and the axial load level ( n). Thirty concrete-filled thin-walled steel SHS and RHS beam-column specimens were tested under constant axial load and cyclically increasing flexural loading. A mechanics model is developed in this paper for concrete-filled steel SHS and RHS columns subjected to constant axial load and cyclically increasing flexural loading, and is a development of the analysis used for monotonically loading condition (Han et al., 2001) [1]. The predicted cyclic responses for the composite columns are in good agreement with test results. Based on the theoretical model, parametric analysis was performed on the behaviors of moment ( M) versus curvature ( φ) response, lateral load ( P) versus lateral displacement (Δ) relationship, as well as ductility coefficient (μ) for the composite beam-columns. Finally, simplified models for the moment ( M) versus curvature ( φ) response, and the lateral load ( P) versus lateral displacement (Δ) relationship were suggested. Formula should be suitable for incorporation into building code, for the calculation of the ductility coefficient (μ) of the composite beam-columns under constant axial load and cyclically increasing flexural loading was developed.

Influence of concrete compaction on the strength of concrete-filled steel RHS columns

Abstract Concrete-filled steel tubular columns have better structural performance than that of bare steel or reinforced concrete (R.C.). The use of concrete-filled SHS (Square Hollow Section) and RHS (Rectangular Hollow Section) have become widespread in the past few decades. However, these members are susceptible to the influence of concrete compaction. It is expected that the influence of concrete compaction on R.C. columns and concrete-filled steel tubes is different. For R.C. columns, concrete compaction only affects the mechanical properties of concrete. But for concrete-filled steel tubes, it is well known that the interaction between steel tube and concrete is the key issue to understand the behaviour of this kind of column. The concrete compaction not only affects the properties of the core concrete itself, but also may influence the interaction between the steel tube and its concrete core, and thus influences the behaviour of the composite columns. The present study is an attempt to study the influence of concrete compaction on the strength of concrete-filled steel RHS columns. Tests on 35 concrete-filled steel RHS columns to investigate the influence of concrete compaction methods on the member capacities of the composite columns are reported. The main parameters varied in the tests are: (1) column section depth-to-width ratio, from 1.0 to 2.0, (2) tube depth to thickness ratio, from 34 to 136, (3) load eccentricity (e), from 0 to 31 mm and (4) column slenderness (λ), from 21 to 62. The main objectives of these tests were twofold: firstly, to report a series of tests on composite columns under different concrete compaction methods; and secondly, to investigate the influence of concrete compaction on the member capacities of the composite columns. Comparisons are made with predicted column strengths using the existing codes such as LRFD, American Institute of Steel Construction Inc-1997, AIJ-Architectural Institute of Japan-1997, Eurocode 4-1996 and GJB4142-2000. It was found that better compaction of concrete resulted in higher member capacities in concrete-filled steel RHS columns. The tests showed the importance of good concrete compaction for concrete-filled steel RHS columns.