Circular Hollow Section Research Articles

Experimental and numerical investigation on short CHS steel columns restrained with HFRP under axial compressive loading

Fiber-reinforced polymer (FRP) is a practical approach to delay or even suppress the deformation of steel structures. However, their application has been constrained by brittle damage and low strengthening efficiency. This paper presents an investigation of the mechanical behavior of short circular hollow section (CHS) steel columns restrained with hybrid fiber-reinforced polymer (HFRP) composed of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) under axial compression. The results show that the restraint effect of HFRP composed of one layer of transverse CFRP as the middle layer and two layers of transverse GFRP as the inner and outer layers is better than two layers of transverse CFRP, with peak load and ductility increased by 7.29 % and 12.36 % over the latter. Then, a simplified method for simulating HFRP and single FRP restrained short CHS steel columns is proposed and verified by experimental results. Moreover, a parametric study is performed to discuss the effect of the diameter-to-thickness ratio on the bearing capacity. Finally, a universal method is proposed to calculate the ultimate bearing capacity of HFRP and single FRP restrained short CHS steel columns, for which the average error and standard deviation from the measured values are 2.27 % and 0.024, respectively.

Assessment of design models for aluminium alloy CHS beam-columns with stocky cross-sections

There is little information on the prediction accuracy of the international design standards for aluminium alloy circular hollow section (CHS) beam-columns. To bridge this gap, this paper presents a comprehensive numerical investigation on the performance of aluminium alloy CHS beam-columns with stocky cross-sections. A non-linear finite element (FE) model was developed and validated against experimental results. The validated FE model was employed to conduct an extensive parametric study of aluminium CHS members subjected to combined axial compression and bending over a range of lengths, cross-sections and applied eccentricities. The effect of these parameters was investigated for two material grades, 6061-T6 and 6082-T6, resulting in a total of 168 FE models. The numerically obtained capacities were used to assess the accuracy of the strength predictions of Eurocode 9 (EC9), the Aluminium Design Manual (AA) and the Direct Strength Method (DSM). On the basis of the results, EC9, with the exponent of the interaction formula, ψc, calculated as a function of the buckling reduction factor χ, was found to provide the most accurate strength predictions with a mean value equal to 1.01 for both examined material grades. The most conservative predictions were given by the DSM, whilst the most scattered predictions by the AA, with a Coefficient of Variation (COV) equal to 0.17. Finally, based on the DSM buckling strength and the plastic cross-sectional moment resistance, a new interaction curve is proposed for CHS beam-columns with stocky cross-sections.

Numerical simulation of the crack propagation effect on the hysteretic behavior of CHS-X joints

The circular hollow section (CHS) X-joint hysteresis behavior significantly affects the seismic behavior of the steel tubular structure using this type of joint. The hysteresis behavior of this type of joint considering crack growth is studied by tests and numerical simulation. Two CHS X-joints were fabricated and tested under axial cyclic loading until the joints were cracked. The hysteresis curves of the joints, considering the influence of damage and fracture, were obtained, and the hysteresis performance of the joints was evaluated according to the curves. The VUSDFLD subroutine based on the ductile fracture model CVGM (cyclic void growth model) was embedded into ABAQUS, and the FE simulation of the test joints considering crack occurrence and growth was carried out. The test results are compared with the results of FE analysis, revealing that the proposed method accurately simulates the hysteresis behavior of these joints and the crack propagation behaviors under cyclic loads. The difference between the test and simulated crack initiation loads for the two joints is 10.8 % and 5.7 %, respectively.

Fire Resistance of Ultra-High-Strength Steel Columns Using Different Heating Rates

Ultra-High-Strength Steel (UHSS) offers several advantages over normal carbon steel, promoting exceptional strength, reducing self-weight, improving fire resistance, enhancing durability, and reducing material consumption. These advantages result in cost savings and sustainable engineering construction. The 3D numerical model is based on Geometrical and Materially Nonlinear Imperfection Analysis (GMNIA) and determines the fire resistance of different cross-section columns. The model is validated with experimental tests, with a maximum relative error of 11%. A parametric analysis is presented, based on 252 simulations, assuming three heating rates, two different cross-sections, two different thicknesses, three lengths, and seven load levels. The fire resistance depends on the heating rate, but the critical temperature is almost equal and independent of the heating rate, if one assumes implicit creep in the constitutive material model. The fire resistance decreases with the load level, as expected. The thickness effect of the hollow section is almost negligible in the fire resistance of UHSS columns. The fire resistance decreases more in higher load levels for slender columns. Columns with Circular Hollow Sections (CHSs) generally show higher fire resistance than hybrid columns in longer columns, but the hybrid columns are subject to much higher loads. New design formulas are presented for the critical temperature of UHSS columns, depending on the load level and slenderness of two different cross-sections.

A component method approach for single-sided beam-to-column joints with CHS column and welded double-tee beam

This paper focuses on predicting the flexural stiffness and strength of beam-to-column joints with Circular-Hollow-Section (CHS) columns and externally welded double-tee beams using the Component Method approach. Currently, Eurocode 3 Part 1.8 lacks guidelines for assessing the stiffness of these joints, resulting in their practical modelling using extreme assumptions like pin or full restraint. Nevertheless, the literature demonstrates that these joints can exhibit semi-rigid behaviour due to the relatively high local deformability of the tube in the joint area. Another concern relates to the strength of these joints. In fact, recent studies have shown that Eurocode 3 Part 1.8 formula for predicting the yield resistance of these joints is over-conservative.Within this framework, the purpose of this study is to fill the current knowledge gaps (over-conservative strength prediction and lack of rules for determining the stiffness) by undertaking a thorough investigation involving numerical and analytical activities. Specifically, the research presents a component-based modelling approach for predicting stiffness and strength of CHS to double-tee beam connections.In this regard, the work presented in this paper has involved the selection of experimental tests from literature to validate a Finite Element (FE) model, specifically focusing on X- and T-joints and internal and external beam-to-column joints. The validated numerical models have been then employed to simulate the response under monotonic loading conditions of additional sets comprising forty T-joints and thirty external beam-to-column joints, respectively. The currently available formulations for predicting the resistance of T-joints were applied to the forty cases simulated, evaluating their accuracy. This allowed the individuation of the most accurate literature formula for the strength prediction of T-joints, showing that the recent proposal of Voth and Packer (2012) yields sufficiently accurate results. Instead, analytical formulations for predicting the stiffness of T-joints were properly derived in a closed-form solution and were validated against the results of the parametric study. Finally, drawing upon knowledge regarding the stiffness and strength of T-joints, the flexural response of external beam-to-columns connections was evaluated proposing a component model, whose accuracy was verified against the cases simulated in the parametric analysis.

Compressive load capacity of CHS X-joints: The Efficacy of doubler plates

This study explores the effects of doubler plates and alterations in joint configuration, on the static characteristics of X-joints made from circular hollow sections (CHS) with slender walls, concentrating on how they withstand compressive forces applied to the braces. A detailed finite element analysis (FEA) was launched. Its accuracy verified through experimental tests carried out by the research team and by comparing it with existing studies. The investigation included an extensive parametric analysis (by generating 204 models) to evaluate changes in initial stiffness, load capacity, and modes of failure, with a focus on the importance of interactions between the chord and plates and the impact of geometric and material nonlinearities. Findings revealed that the doubler plates significantly improve the maximum load bearing capacity and failure modes under various joint geometrical scenarios. While the benefits of doubler plates in enhancing the durability of X-joints are clear, their effectiveness under axial load was not studied. Based on these insights, the research introduces a new theoretical design equation, based on yield volume theory and nonlinear regression, to accurately forecast the ultimate load capacity of the joints.

Optimization of fibre orientation for composite reinforcement of circular hollow section KT-joints

PurposeComposite materials are effective alternatives for rehabilitating critical members of offshore platforms, bridges, and other structures. The structural response of composite reinforcement greatly depends on the orientation of fibres in the composite material. Joints are the most critical part of tubular structures. Various existing studies have identified optimal reinforcement orientations for a single load component, but none has addressed the combined load case, even though most practical loads are multiplanar.Design/methodology/approachThis study investigates the optimal orientation of composite reinforcement for reducing stress concentration factors (SCF) of tubular KT-joints. The joint reinforcement was modelled and simulated using ANSYS. A parametric study was carried out to determine the effect of the orientations of reinforcement in the interface region on SCF at every 15° offset along the weld toe using linear extrapolation of principal stresses. The impact of orientation for uniplanar and multiplanar loads was investigated, and a general result about optimum orientation was inferred.FindingsIt was found that the maximum decrease of SCF is achieved by orienting the fibres of composite reinforcement along the maximum SCF. Notably, the optimal direction for any load configuration was consistently orthogonal to the weld toe of the chord-brace interface. As such, unidirectional composites wrapped around the brace axis, covering both sides of the brace-chord interface, are most effective for SCF reduction.Originality/valueThe findings of this study are crucial for adequate reinforcement of tubular joints using composites, offering a broader and universally applicable optimum orientation that transcends specific joint and load configuration.

Experimental and numerical investigations of S890 and S960 ultra-high strength steel circular hollow section columns

The experimental and numerical investigations of the buckling behaviour and load-carrying capacities of S890 and S960 ultra-high strength steel (UHSS) circular hollow section (CHS) columns were conducted and reported in this paper. The experimental program encompassed tensile coupon tests, initial global and local geometric imperfection measurements and pin-ended column tests. Parallelly, the numerical modelling program was carried out, where the developed finite element (FE) models were validated with reference to the experimental observations and then employed in parametric studies to result in additional numerical data, considering various cross-sectional dimensions and member lengths. Based on the obtained FE and test data, the applicability of the relevant codified design buckling curves in the current European, Australian and American specifications to S890 and S960 UHSS CHS columns was evaluated, since the aforementioned standards only cover the design rules with steel material yield strength no greater than 700 MPa. The assessment results showed that the European code and Australian standard can generally accurately predict the resistances of S890 and S960 UHSS CHS columns, but result in some unsafe data points, while the American specification yielded accurate and consistent resistance predictions. Finally, a revised Eurocode design buckling curve was proposed, accompanied by the reliability assessment through statistical analyses, with the improved design accuracy and consistency revealed.

Cyclic modelling of T-joints with CHS chord members and passing-through plates

This paper focuses on modelling the cyclic response of welded T-joints between Circular-Hollow-Section (CHS) chords and passing-through plates. Recent works have highlighted that, in practical applications, this specific joint typology can be regarded either as a standalone connection or as a component representing one of the main sources of deformability and energy dissipation capacity in more complex beam-to-column joints with CHS columns and passing-through I-beams. Currently, research efforts have primarily focused on predicting the strength and stiffness of this type of connection. However, there is a knowledge gap regarding the characterization of the cyclic response of joints with passing-through members, a crucial aspect for modelling steel structures with these connection types in seismic areas.In light of this, the paper aims to characterize the cyclic response of T-joints with CHS members and passing-through plates using a comprehensive approach that incorporates experimental, numerical, and analytical methods. Initially, the results of an experimental study examining the cyclic behaviour of T-joint specimens with passing-through plates are presented. Subsequently, models of the tested joints are implemented in Finite Element software and validated against the experimental outcomes. The validated models are used to conduct a parametric study, considering 44 different configurations to obtain their force-displacement cyclic responses. These responses are mathematically modelled using the hysteretic uniaxial material from the OpenSees library and a Genetic Algorithm based tool previously developed by the authors (MultiCal). Finally, regression analyses are performed to develop formulations for predicting the mathematical parameters of the hysteretic uniaxial material model starting directly from the geometrical and mechanical properties of the connection.This approach enables designers to model the cyclic response in structural analysis with a straightforward approach, without the need for new calibrations or tests, if the modelled joints fall within the proposed range of calibration. From the point of view of the component modelling, the approach presented aligns with the larger research strand devoted to the extension of the component method codified in Eurocode 3 part 1.8 to cyclic loading conditions.

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.

Experimental and Numerical Research on the Behavior of Steel Columns with Circular Hollow Cross Sections

A circular, hollow tubular steel column is introduced for experimental and analytical analysis in this study. A series of axial compression tests for the variation of static schemes are reported in this study. All theoretical, numerical, and experimental analyses are based on the European Standards for the steel structure, respectively EN 1993-1-1. The experimental models of steel columns are conducted on actual steel columns with a length of 3000 mm and a circular hollow section of 114.3/2.8 mm. To assess the behavior and stress values of the columns, various schematically supported systems are modeled, starting from the axial-centered columns to the symmetrical eccentric load and asymmetrical loaded columns. 3D modeling of the steel columns using the finite element program SEISMOSOFT is also developed for such elements. The accuracy of the model is compared with the experimental results using numerical analysis by the finite element method. Finally, the numerical comparison of the results provides a recommendation for the engineers regarding the design and construction of such columns. Doi: 10.28991/CEJ-2024-010-05-014 Full Text: PDF

SCFs for CHS-to-CHS T- and X-type moment-loaded end connections: Proposal for formulae

The research presented in this paper is part of a series of efforts to develop design recommendations for hollow structural section (HSS) connections situated at the end of a truss (or frame) under fatigue loading. Full-sized circular hollow section (CHS)-to-CHS regular and end connections (with open chord ends and chord-end cap plates) were tested under branch in-plane bending to measure strain concentrations at the critical locations. This experimental data was then used to verify a finite element (FE) modelling approach. An FE parametric study (totaling 1120 FE models, including 560 T-type connections and 560 X-type connections) was then carried out to investigate the effects of different chord end boundary conditions (i.e., varied chord end lengths, and open versus cap-plated chord ends). The effects of branch-to-chord width (β), chord slenderness (2γ) ratios and branch-to-chord thickness (τ) were also investigated. It was found that the stress concentration factors (SCFs) in end connections are sensitive to the boundary conditions (i.e., open chord end versus with chord-end cap plate) and all three non-dimensional parameters (β, 2γ and τ). Therefore, SCF correction factors based on e/b0, β, 2γ and τ, are derived for use with existing SCF formulae in CIDECT Design Guide 8.

Plastic collapse behavior of plate to CHS connection under biaxially symmetric load

This study investigates the plastic collapse behavior of a non-diaphragm type connection with flange plates connected to a circular hollow section (CHS) column, where four flanges were joined from two orthogonal directions. The analysis was conducted using the finite element method (FEM). The focus was on determining the collapse load of the flange joint under a biaxially symmetric load from the four flanges. In an initial numerical analysis, the fundamental collapse behavior of the cylindrical wall was explored. Finite element models of CHS tubes with perfectly elasto-plastic material properties were employed. The analysis revealed that the plastic collapse mechanism of the cylindrical wall was influenced by the axial force in the transverse flanges. Prediction equations for the collapse load of the joint were formulated, assuming four representative collapse mechanisms and employing the principle of virtual work based on the numerical analysis results. Additionally, the ultimate strength of the joint, determined by the plastic collapse of the cylindrical wall, was computed using finite element models of CHS tubes with realistic hardening properties. A comparison between predictions from the equations based on perfectly elasto-plastic CHS tubes and numerical solutions through finite element analysis (FEA) of CHS tubes with realistic hardening properties validates the accuracy of the proposed prediction equations, even when applied to realistic steel tubes.

Comparative experimental analysis of local buckling in mild and high-strength steel (HSS) CHS subjected to various galvanization techniques

This article reports on an extensive experimental study on galvanized and ungalvanized cold-formed circular hollow sections (CFCHS). All samples were produced by cold-forming thermomechanically rolled steels (TM-steels) in S355, S550, S600, S700, and S960 with high-frequency induction welding (HFI). The test specimens were examined without galvanization, with a pre-galvanization and a post-galvanization. Hardness measurements and microstructural analysis were conducted on seven samples, while the mechanical behavior was examined through 82 tensile and 78 stub-column tests with corresponding imperfection measurements. Despite being classified as prone to local elastic buckling according to international standards, the S960 samples in this study did not exhibit such failure. This suggests that the classification by the codes was conservative. The effects of pre-galvanization on CHS have not been investigated so far, and current findings from literature regarding the impact of post-galvanization on load-bearing behavior suggest a severe capacity decrease for CFCHS in high steel grades. Contrary to previous findings, the current study reveals capacity-enhancing effects resulting from galvanization. Both galvanization types enhanced the stiffness and the cross-section capacity under compression. Galvanization induces changes in material properties, including yield and tensile strength, ductility, and the characteristics of the stress-strain curve. This study serves as a validation base for less conservative yet reliable approaches for the cross-section design of high-strength steel CHS and the utilization of beneficial effects due to galvanization.

Experimental and numerical investigations of S960 hot-rolled ultra-high strength steel seamless circular hollow section beam–columns

The rapid development of manufacturing techniques and material science enables advanced hot-rolled ultra-high strength steel (UHSS) with material grades up to S960 to be available recently. The structural performance of S960 hot-rolled UHSS seamless circular hollow section (CHS) beam–columns manufactured through the advanced fabrication approach was experimentally and numerically investigated and reported in this paper. In physical testing, totally ten beam–column specimens were tested, with the complementary material testing and initial global geometric imperfection measurements through the innovative digital image correlation (DIC) method performed. FE modeling was then carried out; FE models of S960 hot-rolled UHSS seamless CHS beam–columns were established and then validated the test observations in terms of the failure loads and the load–mid-height lateral displacement curves. Parametric studies were thereafter performed, with cross-section dimensions, member lengths and loading combinations varying within a certain range, resulting in the additional 300 data to be acquired. Finally, the applicability of existing design methods, as prescribed in the European code, the Australian standard and the American specification, to S960 hot-rolled UHSS seamless CHS beam–columns was evaluated. It can be concluded from the evaluation results that all the three design codes provide accurate and consistent failure load predictions for S960 hot-rolled UHSS seamless CHS beam–columns.

A cradle-to-site life cycle analysis of laser-cut passing-through tubular joints under different seismic conditions

Steel circular hollow sections (CHS) offer a number of advantages compared to their open section counterparts such as better resistance under tension, compression, bending in all directions and an overall reduction of structural weight, required material for corrosion and fire protection. Nevertheless, the traditional methods to connect open section beams to CHS columns often demand a significant amount of fabrication work, leading to high expenditure and resource consumption – therefore limiting their use in the current industry. These issues can be solved with an automatized joint fabrication. To that purpose, this article proposes two innovative “passing-through” I-to-CHS joints, achieved via laser cutting technology (LCT). Experimental and numerical studies were conducted to assess the joint fabrication aspects as well as the structural performance of the proposed joints. The results showed that the proposed joints can offer respectively 2.5- and 10-times larger strength and stiffness properties compared to a conventional directly welded I-to-CHS joint having similar section properties for the beam and the column. However, this study focuses on the environmental sustainability and economic feasibility of the proposed joints. A cradle-to-site life cycle analysis is conducted using multi-storey frame structures. The environmental and economic impact of the proposed joints are compared with different types of existing I-to-CHS joints in order to assess their possible benefits and limitations. Thanks to the savings in the raw material, surface treatment and transportation resources, it could be stated that, the passing-through approach with LCT offers approximately 32−42% reduction in the construction and fabrication costs of the multi-storey structures while reducing the CO2 emissions by 24−28%, compared to the directly welded I-to-CHS joints.

Global buckling and resistances of austenitic stainless steel semi-oval hollow section beam-columns

As advanced manufacturing technologies become increasingly mature in civil engineering, novel tubular sections, characterised by appealing architectural appearance and prominent structural efficiency, can be fabricated, with an example being semi-oval hollow section. The present paper reports an experimental and numerical investigation into the global buckling behaviour and resistances of austenitic stainless steel semi-oval hollow section beam-columns under combined compression and bending. An experimental programme was firstly performed on ten austenitic stainless steel semi-oval hollow section beam-column specimens and included initial geometric imperfection measurements and eccentric compression tests. The obtained test failure loads, load–mid-height lateral deflection curves and failure modes were fully reported. Following the experimental programme, a numerical modelling programme was conducted, including validation studies to validate developed finite element models against the test results and parametric studies to generate additional numerical data. Given that the existing European code and American specification do not cover the design of stainless steel semi-oval hollow section components, the applicability of the codified design interaction curves for austenitic stainless steel circular hollow section beam-columns to their semi-oval hollow section counterparts was evaluated based on the obtained test and numerical data. The European code was found to result in relatively accurate and consistent resistance predictions for austenitic stainless steel semi-oval hollow section beam-columns with Class 1 and 2 cross-sections, but the resistance predictions for those with Class 3 cross-sections were conservative and scattered. The American specification was shown to offer overall accurate and consistent resistance predictions, but some resistance predictions were marginally unsafe.

Full scale testing of fatigue resistant composite joints for offshore wind Jacket and Floating structures

Complex welds in the joint region reduce the fatigue resistance of structural joints of circular hollow sections with factors that results in increased wall thicknesses of the tubes and overspending of steel in the multi-membered part of the jacket and floating support structures for offshore wind turbines. The wrapped composite joint is a breakthrough technology utilising bonding and fatigue resistant composite material to connect steel tubular members instead of welding. The main technical advantage is that the superior fatigue life of wrapped composite joints. Full-scale wrapped composite joint is tested by cyclic out-of-plane bending loading in resonance-based Cronos testing rig at OCAS N.V. in Belgium. Results reveal exceptional fatigue life of wrapped joints to combined loading when compared to welded joints, showcasing a potential that can radically reduce steel use for offshore structures.

Axial compressive behaviors of multi-cavity concrete-filled double-skin tubular stub columns

The axial compressive behaviors of multi-cavity concrete-filled double-skin steel tubular (MCFDST) stub columns are investigated using experimental and numerical methods in this study. The MCFDST columns comprise two concentric steel tubes (circular hollow section (CHS) inner and square hollow section (SHS) outer), sandwiched concrete filled between them, and four strips connecting outer and inner tubes. Fourteen MCFDST stub columns and two concrete-filled double-skin steel tubular (CFDST) (CHS inner and SHS outer) stub columns are fabricated and tested. Experimental results are used to validate the accuracy of finite element models of MCFDST and CFDST stub columns, and based on which, extensive parameter analyses are conducted. The analysis results show that the strips delay the local buckling and change the buckling mode of outer steel tube, as well as improve the bearing capacity and ductility of stub columns. Observed failure modes are controlled by the local buckling of the outer tube associated with the crushing of filled concrete. In addition, the axial compressive performance of MCFDST stub columns can be improved by reducing the width-to-thickness ratio of outer tube, increasing the steel yield strength of outer tube and concrete strength, and adding strips. The hollow ratio has a negligible effect on the axial compressive behaviors when it ranges from 0.26 to 0.56. Furthermore, the predicted results by European, Japanese, British, and Chinese codes are compared with the experimental and FE results, suggesting that the formula in the European code has the best accuracy when predicting the load-bearing capacity of MCFDST stub columns.

Experimental study on CFRP reinforcement and static performance of CHS KT-joints

Owing to the engineering application demands for the reinforcement of complex joints in multiplanar circular hollow section (CHS) trusses, this study investigated the reinforcement methods and static performance of KT-joints strengthened with carbon fibre-reinforced polymer (CFRP) sheets. First, CFRP reinforcement strategies for KT-joints were studied. Experimental comparisons in static performances were then conducted between bare and CFRP-strengthened CHS KT-joints. Meanwhile, CFRP reinforcement mechanisms were analysed. Finally, CFRP effective strain index η was proposed for quantitative assessment of CFRP strength utilisation. The results indicated that CFRP effectively restrained chord depression and expansion deformation, prevented local buckling of the compressive brace, and reduced strain concentration near the intersection lines. Compared with the bare joint, the ultimate load-bearing capacity and ductility of the joint reinforced with 4 layers of CFRP sheets on the chord and 3 layers on the braces increased by 24% and 25%, respectively. The initial stiffness of the joint at compressive and tensile brace locations increased by 48% and 375%, respectively. The η varied at different regions of the joint, with the maximum η found near the K-shaped gap and the intersection line of compressive brace. Based on the distribution of η, strategies for improving CFRP strength utilisation were proposed.