Circular Hollow Section Research Articles

Experimental test setup design for ultimate limit state evaluation of ductile wrapped composite KK‐joints

AbstractComplex circular hollow section double KK‐joints are widely used in, e.g. offshore wind turbine jackets. As an alternative to conventional welded joints of the CHS multi‐membered structures, wrapped composite joints have shown superior structural performance in previous pilot tests. Experimentally characterizing the joint behaviour under uniplanar loading can be complicated, because of the ductile failure mechanisms of wrapped composite joints and second‐order bending moments. This paper presents the design of a test frame in combination with a conventional tensile 2.5 MN test rig. Ductile joint failure, joint imperfections and second‐order effects are considered in the design. Structural finite element software SOFiSTiK is used to analyse the effects of wrapped joint behaviour and imperfections on the test system and interaction of joint, frame and loading rig behaviours. Non‐linear joint behaviour and misalignments of the braces increased the second‐order effects significantly. Mitigation strategies have been explored to reduce additional bending moments in the KK‐joint. A complete test setup is proposed to perform static experiments on double KK‐joints in uniplanar axial loading conditions.

Fatigue behaviour of hybrid grouted joints under axial loading

AbstractSteel constructions made of circular hollow sections are commonly joined by welding or ring‐flange bolting. An additional, novel joining method is the so‐called hybrid grouted joint. The hybrid grouted joint is characterised by its multi‐layered composition. In contrast to state‐of‐the‐art grouted joints, hybrid grouted joints include thin adhesive layers, which are applied on the steel surfaces prior to grouting. Sand grains are embedded in the yet uncured adhesive. After the curing of the adhesive, the surface has a high degree of roughness. This allows a high‐performance interlock between fine grain grout and the adhesively bonded sand granules across the contact surfaces in the overlap area. The ability to transfer loads across a wide area with low stress concentrations makes the hybrid grouted joint a promising joint alternative especially regarding the fatigue performance. This paper summarises the main findings concerning fatigue behaviour of the hybrid grouted joint under axial loading. Wöhler diagrams were determined in numerous fatigue tests for two combinations of adhesive and grout.

Numerical investigation and design method of cold-formed sorbite stainless steel circular tubes under uniaxial compression

The recently developed novel high-strength sorbite stainless steel (S600E) combines the advantages of exceptional mechanical properties, high corrosion resistance, effortless weldability, and cost-effectiveness, granting remarkable competitiveness in stainless steel structures. The authors have conducted and published experiment results on S600E circular hollow section (CHS) columns under uniaxial compression, which serve as a foundation for the subsequent numerical analysis presented in this study. An extensive parametric study encompassed key variable parameters such as slenderness ratio, section size, and diameter-to-thickness ratio, culminating in 748 numerical models. Meanwhile, the ultimate strength obtained through finite element analysis (FEA) was utilized to evaluate the calculation methods prescribed by current European, Australian/New Zealand, and Chinese stainless steel structural design standards and the direct strength method (DSM) available in the literature. The result shows that the DSM can accurately predict the ultimate strength of CHS columns with an average error of 0.2%, while other specifications are slightly conservative. Then, the effective area calculation method was proposed in this paper, whose rationality was verified by meso finite element analysis. Moreover, a revised equation in Chinese code was proposed, which can improve the prediction accuracy by 9.2%. Finally, a reliability analysis was conducted to assess the safety levels of the design standards and DSM. The findings of this study can not only benefit the adoption of the high-strength stainless steel in the construction industry and therefore potentially mitigate carbon-emission-related environmental issues.

Buckling-restrained brace with multiple steel bar cores using SHS outer tube

To overcome the limited maximum strength of buckling-restrained braces (BRBs) using single roll-threaded processed core bars, a BRB with multiple steel bar cores was proposed in a recent study. Its mechanical behavior was revealed by experimental studies; however, improvements are required in the structure of the BRB owing to its complexity and heavy weight. This study proposes an improved BRB with multiple steel bar cores in which the dual buckling-restraining member consists of a square hollow section (SHS) outer tube and two sets of circular hollow section (CHS) inner tubes. The outer tube is used to improve the flexural stiffness and solve the weight issues associated with the previous type. Three full-scale specimens were tested to determine their mechanical behaviors, such as the hysteresis behavior and internal force variation of the restraining members. The test results indicate that the proposed BRB can exhibit a stable hysteresis response and satisfactory energy dissipation if it fulfills the requirements of restraining members stipulated in the design method. Furthermore, finite element analysis (FEA) was used to determine the cyclic behavior of the proposed BRB. Based on the analysis results, the inner tube failure mode, core buckling response, and variation in the core strain distribution are presented to complement the observations obtained from the tests.

Seismic performance evaluation of steel circular hollow section bridge piers with corroded ends

Seismic performance evaluation of steel circular hollow section bridge piers with corroded ends

Understanding In-Line Connections Behavior from Experimental and Numerical Analyses on Rectangular and Circular Hollow Section Elements

Depending on the connection type, especially semi-rigid connections, the analyses of building structures offer accurate results function of the rigidity and ductility. The present paper analyzes the in-line connection of rectangular and circular hollow sections, categorized as semi-rigid connections, suitable for an architectural design of invisible joints. For such connection the standards do not cover an explicit design method. Experimental bending tests were performed on rectangular and circular hollow sections having the end plate fixed inside the profile and bolted by four and one high-strength bolts, respectively. The joint separation represents a serviceability criterion which was monitored using digital image correlation technique. Based on experimental results, a numerical model was validated using the finite element method. After the validation of the numerical model based on the experimental results, a parametric investigation was conducted to study the influence of the access hole, the preload level, the end plate thickness, and the axial force. The results show the small influence of the bolt preload, but the end plate thickness was of major importance. A reduction of the assembly rigidity was also caused by the manhole. The study shows the feasibility of the connection configuration with the end plate positioned inside the hollow profile.

Static strengths of cold-formed steel elliptical hollow section X-joints

The newly emerged elliptical hollow section (EHS) has a vast of structural and architectural merits and is favorable for engineers and designers. This paper presents an experimental investigation concerning the structural behavior of cold-formed steel EHS X-joints. Thirty elliptical hollow section X-joints with three different brace-chord orientations were carefully designed, fabricated by robotic gas metal arc welding and tested with the braces axially compressed. The width ratio between brace and chord (β) ranged from 0.29 to 1. The brace-to-chord thickness ratio (τ) ranged from 0.58 to 1.83 and the chord width-to-thickness ratio (2γ) ranged from 10.1 to 52.6. The static strengths, failure modes and load-deformation responses are discussed. The current codified design provisions do not cover the design of cold-formed steel elliptical hollow section X-joints investigated in this study. As the performance of EHS lies between the circular and rectangular hollow sections, the feasibility of the design provisions initially calibrated for conventional X-joints as specified in the CIDECT and Eurocode 3 with incorporation of equivalent section methods as well as the design recommendations previously proposed for hot-rolled steel elliptical joints was evaluated. In general, these design methods provide accurate strength predictions for cold-formed steel elliptical hollow section X-joints. Nevertheless, the scattering of predictions is still a concern and the design method shall be further improved.

Buckling resistance of hot‐finished CHS beam‐columns using FE modelling and machine learning

AbstractThe use of circular hollow sections (CHS) has increased in recent years owing to its excellent mechanical behaviour including axial compression and torsional resistance as well as its aesthetic appearance. They are popular in a wide range of structural members, including beams, columns, trusses and arches. The behaviour of hot‐finished CHS beam‐columns made from normal‐ and high‐strength steels is the main focus of this article. A particular attention is given to predict the ultimate buckling resistance of CHS beam‐columns using the recent advancement of the artificial neural network (ANN). Finite element (FE) models were established and validated to generate an extensive parametric study. The ANN model is trained and validated using a total of 3439 data points collected from the generated FE models and experimental tests available in the literature. A comprehensive comparative analysis with the design rules in Eurocode 3 is conducted to evaluate the performance of the developed ANN model. It is shown that the proposed ANN‐based design formula provides a reliable means for predicting the buckling resistance of the CHS beam‐columns. This formula can be easily implemented in any programming software, providing an excellent basis for engineers and designers to predict the buckling resistance of the CHS beam–columns with a straightforward procedure in an efficient and sustainable manner with least computational time.

Experimental study of the behavior under axial compression of steel tube confined concrete with a circular hollow section (STCC-CHS)

A new type of steel tube confined concrete columns (STCC) in which the outer steel tube only confines the core concrete and does not directly bear the axial load. Based on this, a new type of steel tube confined concrete with a circular hollow section (STCC-CHS) column was presented in this article. It demonstrated high strength and relatively light weight, high economic efficiency and promises easy installation. Axial compression tests were conducted to quantify the effects of the diameter-thickness ratio and the hollow ratio on the failure modes, the ultimate strength in axial compression, strain and stiffness. Constraint effects and ductility were also analyzed. The test results show that the outer steel tube effectively delays local buckling failure. The smaller the hollow ratio and the smaller the diameter-thickness ratio, the greater a column's bearing capacity. The observed ultimate compressive strength is compared with STCC and CFDST components. The experimental data are used to test the utility of the recommended formulas for predicting the axial bearing capacity of such columns, with several different models of the confined concrete strength.

Strength enhancement and stub-column behavior of cold-formed stainless steel elliptical hollow sections

This paper aims to investigate the strength enhancement, local geometric imperfections, and stub-column behavior of cold-formed (CF) stainless steel elliptical hollow sections (EHS) by using finite element methods. In the present study, an advanced numerical approach was used to conduct a comprehensive parametric study for CF stainless steel EHS with a wide range of cross-section dimensions and slenderness values as well as different stainless steel alloys. By analyzing the generated numerical data from the parametric study, the effects of cold-forming and material parameters on the strength enhancement were examined. Predictive models of the strength enhancement as well as the local geometric imperfections for CF stainless steel EHS are proposed. The slenderness limits for CF stainless steel circular hollow sections (CHS) and CF steel EHS in the existing design methods were assessed for CF stainless steel EHS. New slenderness limits are proposed for CF stainless steel EHS. In addition, the existing equivalent diameter method, the Continuous Strength Method (CSM) and the Direct Strength Method (DSM) were evaluated for the design of CF stainless steel EHS. Modifications on these existing design methods are proposed for CF stainless steel EHS, which can improve their accuracy.

Fire design of stainless steel columns with hollow circular and elliptical sections

One of the advantages of stainless steel is its better behaviour under elevated temperatures when compared to the conventional carbon steel, which can be important for structural applications where the fire action should be considered. The second generation of Part 1–2 of Eurocode 3 (prEN 1993-1-2:2021) proposes new calculation formulae resulting from studies on profiles with I-sections. However, it is still important to evaluate the accuracy and safety of the same formulae for columns with circular and elliptical hollow sections (CHS and EHS respectively), whose application in stainless steel structures is common. Hence, this work presents a numerical parametric study on the fire resistance of columns subjected to axial compression, with CHS and EHS, considering different stainless steel grades, cross-sections and member slendernesses, and elevated temperatures. The ultimate load capacities of the columns are obtained by geometrically and materially nonlinear analyses including imperfections, using the SAFIR finite element program and considering the constitutive law based on the two-phase Ramberg-Osgood formulation proposed in prEN 1993-1-2:2021. Based on the comparisons between the numerical results and the methodology of prEN 1993-1-2:2021, small suggestions are proposed to improve the fire design formulae for the cases of CHS and EHS profiles subjected to axial compression.

Cold-formed high strength steel CHS-to-RHS T- and X-joints: Performance and design after fire exposures

The post-fire structural performance of cold-formed high strength steel (CFHSS) T- and X-joints with circular hollow section (CHS) braces and square and rectangular hollow section (SHS and RHS) chords is numerically investigated in this study. The tubular members had the nominal 0.2% proof stress of 900 MPa. The CHS-to-RHS T- and X-joints were subjected to compression loads through brace members. The nominal values of peak fire temperatures (ψ) were 300°C, 550°C, 750°C and 900°C. Tests carried out by the authors investigating the post-fire static response of cold-formed S900 steel grade CHS-to-RHS T- and X-joints were formed the basis of the numerical investigation carried out in this study. The test results reported by the authors were used to develop accurate finite element (FE) models. The validated FE models were used to perform extensive numerical parametric studies comprising 768 FE specimens. The validity ranges of critical geometric parameters were extended beyond current limits mentioned in international codes and guides. The residual strengths of test and FE specimens were compared with the nominal resistances predicted from design equations given in Eurocode 3 and Comité International pour le Développement et l’ Etude de la Construction Tubulaire (CIDECT) Design Guide 3 using the measured post-fire residual material properties. Generally, design rules given in Eurocode 3 and CIDECT are shown to be quite conservative with very dispersed and unreliable predictions. As a result, accurate and reliable design rules are proposed in this study.

Design of cold-formed high strength steel CHS-to-RHS T- and X-joints at elevated temperatures

The static resistances of cold-formed S900 steel grade tubular T- and X-joints at elevated temperatures have been numerically investigated in this study. Circular hollow sections (CHS) were used as the braces, while square and rectangular hollow sections (SHS and RHS) were used as the chords for both T- and X-joints. In this study, both T- and X-joints were subjected to compression loads. The mechanical properties of cold-formed S900 steel grade hollow section members at elevated temperatures were used to perform the numerical investigation. The static resistances of CHS-to-RHS T- and X-joints were investigated at 400°C, 500°C, 600°C and 1000°C. The finite element (FE) models developed and validated by the authors for ambient temperature and post-fire investigations of cold-formed S900 steel grade CHS-to-RHS T- and X-joints were used in this study to perform numerical investigation at elevated temperatures. A comprehensive FE parametric study, including a total of 768 CHS-to-RHS T- and X-joints, was performed in this study using the validated FE models. Both CHS-to-RHS T- and X-joints were failed by chord face failure and a combination of chord face and chord side wall failure mode. The nominal resistances predicted from design rules given in European code and CIDECT, using mechanical properties at elevated temperatures, were compared with the resistances of CHS-to-RHS T- and X-joints at elevated temperatures. It is shown that the predictions from design rules given in European code and CIDECT are quite conservative but unreliable. As a result, economical and reliable design equations are proposed in this study for predicting the resistances of the investigated joints.

Global instability of cold-formed steel elliptical hollow section members under combined compression and biaxial bending

The present study numerically investigated the global instability of cold-formed (CF) steel elliptical hollow section (EHS) members under combined compression and biaxial bending. A finite element (FE) model for CF tubular members under combined compression and biaxial bending was first developed, in which the strength enhancement, geometric imperfection and residual stresses due to roll-forming were incorporated. The FE model was validated against the existing test results. The validated FE model was employed to carry out a parametric study for CF steel EHS members, which covers an extensive range of cross-section geometries, material grades (including both high strength and normal grade steels), member slenderness, loading eccentricities and loading angles. Based on the generated FE data, the application of the current interaction design formulae for steel members under combined loading in EN 1993–1-1, together with the codified slenderness limits for circular hollow section (CHS) under combined loading, were assessed for the design of CF steel EHS members under combined compression and biaxial bending. It can be found that the design formulae in EN 1993–1-1, together with the slenderness limits for CHS, can give very conservative predictions for CF steel EHS members under combined loading. In addition, the influences of different parameters, including the loading eccentricity, loading angle, loading level and the reduction factor for lateral-torsional buckling, on the member behavior were also examined. Finally, new design recommendations are proposed for the design of CF steel EHS members under combined compression and biaxial bending.

Bolted CHS Flange-Plate Connections under Bending

Most research on circular hollow section (CHS) flange-plate connections deals with axial tension loading, yet bending action is a common load case. Prior researchers have studied both ring and blank-flange-plate connections, using a four-point-bending setup, generating a database of 10 experimental tests and five numerical models on bolted blank-flange-plate connections under pure bending. This limited data is extended herein by two large-scale laboratory tests with blank-flange-plate connections having a large span-to-tube diameter ratio, and 120 numerical models. Flange-plate plastification and bolt fracture are the valid failure modes for this splice, so all data was meticulously assessed to exclude experimental tests or numerical models not governed by connection failure. Existing design models for calculating the connection capacity under pure bending are evaluated and revised. Moreover, based on rectangular flange-plate connections, a novel method is proposed to design its circular counterpart under pure bending. For all methods, the difference in using the ultimate stress for flange-plate moment capacity—instead of the conventional yield stress—is also analyzed. Lastly, a reliability analysis is conducted to support the proposed design method.

Numerical investigation of branch plate-to-CHS connection under eccentric shear loading

Circular Hollow Section (CHS) columns are usually used in high-rise buildings. International design specifications usually overestimate the design of the beam-to-CHS column branch plate connections. This paper numerically investigates the behavior and strength of beam-to-CHS column branch plate connections using a finite element (FE) model. The FE model is verified using different experimental results available in previous literature. A parametric study is carried out on specimens subjected to eccentric shear loading on the branch plate with a constant column length and end conditions. The parameters included the CHS column diameter, wall thickness, branch plate length and the effect of load eccentricity. A notable effect of these parameters is observed on the connection capacity. The mode of failure of the studied specimens ranged from chord plastification for small thicknesses followed by chord face punching to excessive column ovalization at the vicinity of the connection followed by either chord punching or tearing out in the weld. For CHS columns with large thicknesses, punching failure in the column face or premature weld failure is observed. Finally, the results of the parametric study indicated a prominent effect of load eccentricity on the in-plane bending capacity. Therefore, a proposed improvement to the current design guidelines for the strength of CHS column branch plate connection subjected to in-plane bending is presented in this paper.

Experimental Assessment of the Behavior of Tubular Truss Girder Joints Fabricated with Laser Cutting Technology

Directly welded truss girder joints result in high-stress development on the tube wall, which leads to increased chord thicknesses, requires extra welding, and presents detailing difficulties. Laser cutting technology (LCT) allows us to make precise cuts and pass through the truss members, welding the connection on the two sides. This can reduce geometrical imperfections, and, as a result, it increases joint strength. This paper presents truss girder joints developed with LCT that can develop higher resistance to static gravity loads with respect to conventional connections thanks to the pass-through solution. An extensive experimental campaign (31 specimens) has been carried out involving the monotonic gravity loading of several truss specimens to simulate their in-service conditions. Different truss typologies are tested (Warren and Pratt) using circular and square hollow section profiles, at different scales (full-scale and small-scale). The results showed that Warren-type structures with LCT connection solutions had a 20% higher strength with respect to the equivalent structures with directly welded joints. Furthermore, the correlation between the different failure modes and joint parameters (e.g., angles and distance between brace members) has been discussed.

Development of Hybrid Machine Learning Models for Predicting Permanent Transverse Displacement of Circular Hollow Section Steel Members under Impact Loads

The impact effect is a crucial issue in civil engineering and has received considerable attention for decades. For the first time, this study develops hybrid machine learning models that integrate the novel Extreme Gradient Boosting (XGB) model with Particle Swam Optimization (PSO), Grey Wolf Optimizer (GWO), Moth Flame Optimizer (MFO), Jaya (JA), and Multi-Verse Optimizer (MVO) algorithms for predicting the permanent transverse displacement of circular hollow section (CHS) steel members under impact loads. The hybrid machine learning models are developed using data collected from 357 impact tests of CHS steel members. The efficacy of hybrid machine learning models is evaluated using three performance metrics. The results show that the GWO-XGB model achieves high accuracy and outperforms the other models. The values of R2, RMSE, and MAE obtained from the GWO-XGB model for the test set are 0.981, 2.835 mm, and 1.906 mm, respectively. The SHAP-based model explanation shows that the initial impact velocity of the indenter, the impact mass, and the ratio of impact position to the member length are the most sensitive parameters, followed by the yield strength of the steel member and the member length; meanwhile, member diameter and member thickness are the parameters least sensitive to the permanent transverse displacement of CHS steel members. Finally, this study develops a web application tool to help rapidly estimate the permanent transverse displacement of CHS steel members under impact loads.

Buckling capacity of steel circular hollow section considering the restraint of welded hollow spherical joints

An integral finite element model of steel circular hollow section and welded hollow spherical joints is established to consider the restraint of welded hollow spherical joints on steel circular hollow section in this paper. The buckling modes are used as the initial geometrical imperfection forms of the steel circular hollow section. Based on the statistics method, a method for calculating the buckling capacity design value of the steel circular hollow section with uniform reliability considering the restraint of welded hollow spherical joints is established, and the formula for calculating error of this value at certain confidence is deduced. The relationships between the buckling capacity design value of steel circular hollow section and the structural parameters of the member and joint are determined quantitatively by linear regression method. A practical formula for the buckling capacity design value of steel circular hollow section considering the restraint of welded hollow spherical joints is derived. Compared to the results calculated by the design code indicates that if the difference of restraint effect which is caused by different welded hollow spherical joint is ignored, the buckling capacity value of steel circular hollow section calculated based on the unified effective length coefficient may be bigger or smaller than that of the actual case. Therefore, refined consideration should be placed on the different restraint of welded hollow spherical joints of different sizes on steel circular hollow section, and the restraint of the end joints should be accurately considered when determining the buckling capacity of steel circular hollow sections.

Performance of externally reinforced CHS X-joints subjected to axial loads

Circular hollow sections (CHS) have evolved as the primary choice for the tubular lattice structures, due to their excellent mechanical properties and aesthetic appearance. However, enhancing the joints capacities in such structures poses as challenge to structural designer. Recent research addressed several external reinforcing schemes that can be utilized for strengthening CHS X-joints. Although each of these schemes was studied thoroughly, no collective study was conducted to compare between their efficiencies in enhancing CHS X-joints strengths and stiffnesses. This study focuses on evaluating the structural performance of these techniques relative to each other. The results showed that CHS X-joints strengthened using outer flanged pipe to confine brace-chord intersection achieved the highest strength and stiffness enhancement among the examined joints. Also, it proved that joints strengthened with single or double outer stiffeners yielded acceptable capacity enhancement with the advantages of easy fabrication and acceptable aesthetically appearance.