Imperial College London

ProfessorLeroyGardner

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Structural Engineering
 
 
 
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Contact

 

+44 (0)20 7594 6058leroy.gardner

 
 
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Location

 

435Skempton BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

523 results found

Afkhami S, Amraei M, Gardner L, Piili H, Wadee MA, Salminen A, Björk Tet al., 2022, Mechanical performance and design optimisation of metal honeycombs fabricated by laser powder bed fusion, Thin Walled Structures, Vol: 180, Pages: 1-17, ISSN: 0263-8231

Honeycomb structures have a wide range of applications, from medical implants to industrial components. In addition, honeycombs play a critical role when passive protection is required due to their low density and high energy absorption capabilities. With the transition of additive manufacturing from a rapid prototyping approach to a manufacturing process, this technology has recently offered designers and manufacturers the ability to fabricate and modify lattice structures such as honeycombs. The current study presents the application of laser powder bed fusion, a common additive manufacturing process for producing industrial metal components, for fabricating metal honeycombs. In addition, this study examines three modified designs that can only be practically fabricated using additive manufacturing and compares them with conventional honeycombs. For this purpose, quasi-static and dynamic compression tests are conducted to evaluate and compare the performance of the honeycomb structures. The results show that the structures produced by additive manufacturing have acceptable performance compared to conventional honeycomb structures, and laser powder bed fusion can be considered to be a reliable manufacturing method for honeycomb production. Furthermore, the honeycombs produced according to the modified designs generally outperformed their counterparts made from the typical hexagonal cells. Ultimately, the use of triangular cells as a design modification is proposed toproduce honeycombs with promising performance characteristics in all of their principal axes and under various pressure scenarios, from quasi-static to dynamic loading rates. Finally, this study also investigates the applicability of a newly developed maraging steel for additive manufacturing of honeycombs. Microstructural analysis and quasi-static tensile tests have confirmed the material properties for this purpose.

Journal article

Zhang W, Gardner L, Wadee MA, Chen KS, Zhao WYet al., 2022, On the uniform torsional rigidity of square concrete-filled steel tubular (CFST) sections, Structures, Vol: 43, Pages: 249-256, ISSN: 2352-0124

There are a number of scenarios in which structural members experience torsion, including under the direct application of torsional loading, when transverse loading is applied at an eccentricity to the shear centre and as a second order effect arising from lateral torsional instability. To date, the torsional rigidity of concrete-filled steel tubular (CFST) sections has yet to be fully explored; hence a study into the uniform torsional rigidity of square CFST sections is presented herein. First, the strain energy of square CFST sections is formulated, in which the longitudinal warping displacement is assumed to have an undetermined constant. The undetermined constant is then deduced by means of the principle of minimum strain energy, and thus an analytical expression for the uniform torsional rigidity of square CFST sections is obtained. The accuracy of the derived formula is verified against existing theoretical solutions for simplified scenarios, test data and the results of numerical simulations. Finally, the influence of the key parameters in the derived formula for the torsional rigidity of square CFST sections are analysed, and a simplified design formula is presented.

Journal article

Huang C, Meng X, Buchanan C, Gardner Let al., 2022, Flexural buckling of wire arc additively manufactured tubular columns, Journal of Structural Engineering, Vol: 148, ISSN: 0733-9445

ire arc additive manufacturing (WAAM) is a metal 3D printing method that enables large-scale structural elements with complex geometry to be built in a relatively efficient and cost-effective manner, offering revolutionary potential to the construction industry. Fundamental experimental data on the structural performance of WAAM elements, especially at the member level, are however lacking. Hence, an experimental study into the flexural buckling response of WAAM tubular columns has been conducted and is presented in this paper. A total of stainless steel square and circular hollow section (SHS and CHS) columns were tested under axial compression with pin-ended boundary conditions. Regular SHS and CHS profiles were chosen to enable direct comparisons against equivalent, conventionally manufactured sections and hence to isolate the influence of the additive manufacturing process, while the cross-section sizes and column lengths were varied to achieve a broad spectrum of member slendernesses. Owing to the geometricundulations inherent to the WAAM process, 3D laser scanning was used to determine the as-built geometry and global geometric imperfections of the specimens; digital image correlation (DIC) was employed to monitor the surface deformations of the specimens during testing. Full details of the column testing program, together with a detailed discussion of the experimental results, are presented. The applicability of the current column design provisions in EN 1993-1-4 and AISC 370 to WAAM stainless steel members was assessed by comparing the test results with the codifiedstrength predictions. The comparisons emphasized the need to allow for the weakening effect of the inherent geometric variability of WAAM elements, in order for safe-sided strength predictions to be achieved

Journal article

Guo X, Kyvelou P, Ye J, Teh L, Gardner Let al., 2022, Experimental investigation of wire arc additively manufactured steel single-lap shear bolted connections, Thin Walled Structures, ISSN: 0263-8231

Journal article

Kyvelou P, Buchanan C, Gardner L, 2022, Numerical simulation and evaluation of the world’s first metal additively manufactured bridge, Structures, Vol: 42, Pages: 405-416, ISSN: 2352-0124

Recent disruptive technological advances, including wire arc additive manufacturing (WAAM) and the concept of digital twins, have the potential to fundamentally transform the way in which we design, build and manage structures. WAAM is a method of metal 3D printing that is well suited to the price-sensitive construction industry and has been used to manufacture the MX3D bridge – the world’s first metal additively manufactured bridge. The intricate geometry, undulating surface finish and particular material properties rendered the bridge outside the scope of any existing structural design standards; hence, physical testing and advanced numerical modelling were carried out for its safety assessment. The key features of the finite element model of the bridge, and its validation against in-situ structural tests, are described herein. Subsequent numerical studies undertaken to verify the structural performance of the bridge under various loading scenarios are presented, while the basis for the development of the smart digital twin of the bridge is also introduced. The presented research provides insight into the use of advanced computational simulations for the verification and ongoing assessment of structures produced using new methods of manufacture.

Journal article

Wang Z, Li M, Han Q, Yun X, Zhou K, Gardner L, Mazzolani Fet al., 2022, Structural fire behaviour of aluminium alloy structures: Review and Outlook, Engineering Structures, ISSN: 0141-0296

Aluminium alloys are gaining increasing use in the construction industry, underpinned by extensive research and the growing availability of codified structural design rules at room temperature. More recently, considering that the material properties of aluminium alloys degrade significantly at elevated temperatures, a substantial number of studies have also been conducted to investigate the behaviour and design of aluminium alloy structures exposed to fire. This paper presents a review of recent studies on the mechanical characteristics of aluminium alloys in fire and after fire, as well as the structural behaviour of aluminium alloy structures in fire conditions, considering members, connections, joints and overall systems. In addition, possible passive and active fire protection measures for aluminium alloy structures are introduced and discussed. Lastly, recommendations for future work on the structural fire behaviour of aluminium alloy structures are set out, providing insight into aspects that require further investigations to promote the more widespread use of aluminium alloys in structural applications.

Journal article

Castanheira DS, de Lima LRO, da S Vellasco PCG, Cashell K, Gardner Let al., 2022, Compressive behaviour of double skin sections with stainless steel outer tubes and recycled aggregate concrete, Structures, Vol: 41, Pages: 750-763, ISSN: 2352-0124

An experimental and numerical study into the behaviour of concrete-filled double skin tubular (CFDST) stub columns is presented. A total of eight axial compression tests were carried out, four utilising conventional concrete and four with recycled aggregate concrete. The stub columns were circular in cross-section and each comprised an austenitic stainless steel outer tube and a carbon steel inner tube, of varying dimensions. Accordingly, hollow ratios of 0.67 and 0.55 were considered. The recycled coarse aggregate was made by crushing test specimens from a previous research project, and a replacement ratio of 50% was adopted. During the experiments, similar structural behaviour and failure modes were observed between the specimens with conventional and recycled aggregate concrete. To investigate the behaviour further, a finite element model was developed in ABAQUS; validation of the model against the experimental results from the current work as well as data available in the literature is described. The finite element model was employed to conduct a parametric study to examine the load-bearing contributions of the constituent components of CFDST sections and to assess the influence of the hollow ratio on the structural behaviour. The experimental and numerical ultimate loads are compared with the capacity predictions determined using available design procedures. Overall, the results show that CFDST stub columns with recycled aggregate concrete can achieve similar capacities to their conventional concrete counterparts, demonstrating the potential for the wider use of recycled aggregate concrete, towards more sustainable structural solutions.

Journal article

Zhong Y, Zhao O, Gardner L, 2022, Experimental and numerical investigation of S700 high strength steel CHSbeam-columns after exposure to fire, Thin Walled Structures, Vol: 175, Pages: 1-12, ISSN: 0263-8231

This paper presents an experimental and numerical investigation into the post-fire behaviour and residual capacity of S700 high strength steel circular hollow section (CHS) beam-columns. The experimental investigation was performed on ten S700 high strength steel CHS beam columns and included heating and cooling of the specimens as well as post-fire material testing, initial global geometric imperfection measurements and pin-ended eccentric compression tests. A subsequent numerical investigation was conducted, where finite element models were developed and validated against the test results and then employed to carry out parametric studies to generate further numerical data over a wide range of cross-section dimensions, member lengths and loading combinations. In view of the fact that there are no specific provisionsfor the design of steel structures after exposure to fire, the relevant room temperature design interaction curves were evaluated, using post-fire material properties, to assess their applicability to S700 high strength steel CHS beam-columns after exposure to fire, based on the test and numerical data. The evaluation results revealed that the interaction curves provided in the American Specification and Australian Standard result in a high level of design accuracy and consistency, while the Eurocode interaction curve leads to more conservative and scattered failure load predictions. Finally, a revised Eurocode interaction curve, with more accurate end points, was proposed and shown to offer improved failure load predictions for S700 high strength steel CHS beam-columns after exposure to fire.

Journal article

Yun X, Zhu Y, Wang Z, Gardner Let al., 2022, Benchmark tests on high strength steel I-section frames, Engineering Structures, Vol: 258, ISSN: 0141-0296

Physical experiments are widely used in structural engineering for the validation of finiteelement models and the establishment and assessment of design provisions. However, whilethere exists an abundance of member-level test data, there is a clear need for furtherbenchmark frame-level test data, particularly given the growing trend for advanced systemlevel simulation and design. Towards meeting this need, and exploiting state-of-the-artmeasurement techniques, tests on eight high strength steel frames are presented herein. Thetested frames were fixed-base, single storey, unbraced in-plane and made up of welded S690steel I-section members. Two section sizes, of different local slenderness, were employed toassess the influence of local buckling on cross-section strength and the potential for plasticredistribution of forces within the frames. All the frames were laterally restrained out-ofplane using a bespoke lateral bracing system, but free to deform in-plane. The frames weresubjected to different combinations of horizontal and vertical loading, in order for beam,sway and combined mode mechanisms to form. Extensive use was made of digital imagecorrelation in order to obtain detailed information on the deformation of the frames and theircomponents. The test setup, instrumentation, loading procedures and frame structuralYun X, Zhu YF, Wang ZX, Gardner L. Benchmark tests on high strength steel I-sectionframes. Engineering Structures, (Accepted).2response, including load-deformation characteristics, failure modes and connection rotationalstiffnesses are fully reported. The experimental data indicated that, despite the lower ductilityand strain hardening of HSS relative to normal strength steel (NSS), HSS frames with stockycross-sections have the ability to form plastic hinges with sufficient rotation capacity toachieve a considerable amount of inelastic moment redistribution; the potential for extendingplastic design to HSS structures has therefore been demonstrated. The

Journal article

Yang X, Yang H, Gardner L, Wang Yet al., 2022, A continuous dynamic constitutive model for normal- and high-strength structural steels, Journal of Constructional Steel Research, Vol: 192, Pages: 1-13, ISSN: 0143-974X

The use of numerical models in the advanced analysis and design of steel structures,particularly under extreme loading conditions, is becoming increasingly widespread. Acrucial component of such models is an accurate description of the material response. Asystematic study into the dynamic constitutive modelling of structural steels is presentedherein. The key features of the dynamic stress–strain characteristics of structural steels atvarious strain rates, i.e., test methods, material strength, strain-rate effect index and strainrate effect models, are examined and discussed. Supplementary SHPB tests on bothnormal- and high-strength structural steels (Q235, Q355, Q460, and S960) under a widerange of strain rates up to 5000 s-1, filling gaps in existing datasets, are then carried out. Adatabase of dynamic test results, containing 453 stress–strain curves, is systematicallyestablished and analyzed. Finally, a continuous dynamic constitutive model, capturing thedependency of both yield strength and strain rate, is proposed to predict the dynamic stress–strain response for structural steels, from normal- to high-strength material (235–960 MPa),and from static to high strain rate loading conditions (up to 5000 s-1).

Journal article

Ye J, Quan G, Kyvelou P, Teh L, Gardner Let al., 2022, A practical numerical model for thin-walled steel connections and built-up members, Structures, Vol: 38, Pages: 753-764, ISSN: 2352-0124

The interaction between the cold-formed steel members and fasteners in a structural assembly often involves complex stress states, material plasticity, significant in-plane deformations and curling, rendering computational modelling of the resulting structural system challenging. In addition, the presence of initial geometric imperfections and the susceptibility of cold-formed steel members to local instabilities adds a further degree of complexity to the numerical modelling. The behaviour of bolted connections between cold-formed steel members has traditionally been assessed based on physical tests, though these can be time consuming and expensive. In this paper, a practical numerical approach to the simulation of bolted cold-formed steel connections, capturing all the key behavioural features including bolt preload, slippage and bearing, is introduced. Starting from a typical shell finite element (FE) model of a cold-formed steel plate, a small number of solid elements is introduced around the bolt holes, to allow accurate replication of the bolt-ply interaction, with contact pairs defined between the bolts and the surrounding material. An explicit dynamic solver is then employed to solve the geometrically and materially nonlinear problem, both with and without bolt slippage. Validation of the FE model is conducted using benchmark tests reported in the literature. It is shown that the proposed numerical method can accurately capture the structural behaviour of bolted cold-formed steel connections and cold-formed steel built-up sections, while being computationally efficient and numerically stable; the proposed approach is therefore recommended for the numerical modelling of cold-formed steel systems assembled using fasteners.

Journal article

Huang C, Kyvelou P, Zhang R, Britton T, Gardner Let al., 2022, Mechanical testing and microstructural analysis of wire arc additively manufactured steels, Materials and Design, Vol: 216, ISSN: 0264-1275

Wire arc additive manufacturing (WAAM) is a metal 3D printing method that allows the cost-effective and efficient production of large-scale elements, and has thus gained great interest from architects and structural engineers. Integration of this novel technology into the construction industry, however, requires the development of a clear understanding of the mechanical behaviour of WAAM materials. To this end, a comprehensive experimental study into the mechanical properties and microstructure of WAAM plates made of normal- and high-strength steels has been undertaken and is reported herein. A total of 137 as-built and machined tensile coupons were tested, extracted in various directions relative to the print layer orientation from WAAM plates of two nominal thicknesses, built using different deposition strategies. Theinfluence of the geometric undulations inherent to the WAAM process and deposition strategy on the resulting mechanical properties was investigated. Microstructural characterisation was also performed by means of optical microscopy (OM) and electron backscatter diffraction (EBSD). The WAAM normal-strength steel plates exhibited a principally ferritic-pearlitic microstructure, while the WAAM high-strength steel plates displayed a mixed microstructure featuring ferrite, bainite and martensite. The EBSD analysis revealed a weak crystallographic texture, which explained the observed mechanical properties being almost isotropic. No significant differences in tensile properties were observed with the different deposition strategies, except for some variation in ductility. The geometric undulations of the as-built coupons resulted in some reduction in effective mechanical properties and a degree of anisotropy. Overall, the examined WAAM material exhibited consistent mechanical properties, a Young’s modulus comparable to conventionally-produced steel plates, marginally lower strength, reflecting the slower cooling conditions than is customary, and good

Journal article

Zhong Y, Yao S, Zhao O, Gardner Let al., 2022, Structural response and residual capacity of S700 high strength steel CHS columns after exposure to elevated temperatures, Journal of Structural Engineering, Vol: 148, Pages: 1-12, ISSN: 0733-9445

The structural behavior and residual capacity of S700 high strength steel circular hollow section (CHS) columns after exposure to elevated temperatures have been studied through testing and numerical modeling. The testing program was conducted on ten S700 high strength steel CHS columns and comprised heating and cooling of the specimens as well aspost-fire material testing, measurements of initial geometric imperfections and pin-ended column tests. Numerical simulations were subsequently performed, where finite element models were built and validated with reference to the experimental results and afterward used to conduct parametric studies to obtain further numerical data. Given that there are no specific provisions for the design of steel structures after exposure to elevated temperatures, the relevant room temperature design buckling curves were evaluated, using post-fire material properties, for their applicability to S700 high strength steel CHS columns after exposure to elevated temperatures, based on the experimental and numerical data. The evaluation results indicated that the buckling curves prescribed in the American Specification and Australian Standard lead to accurate and consistent residual capacity predictions for S700 high strength steel CHScolumns after exposure to elevated temperatures, while the Eurocode buckling curve yields conservative predictions of residual capacity. A revised Eurocode buckling curve was then proposed and results in a higher level of accuracy than its original counterpart.

Journal article

Behzadi-Sofiani B, Gardner L, Wadee MA, 2022, Testing, numerical analysis and design of stainless steel equal-leg angle section beams, Structures, Vol: 37, Pages: 977-1001, ISSN: 2352-0124

The stability and design of stainless steel equal-leg angle section members subjected to uniaxial bending are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and physical tests on hot-rolled austenitic stainless steel equal-leg angle section beams is first presented. The test results are then used to validate shell finite element models developed within ABAQUS, which are in turn used to undertake numerical parametric studies that consider both hot-rolled and cold-formed equal-leg angle section beams in austenitic, duplex and ferritic stainless steel with a wide range of slenderness values. Recent studies have shown that for angles under major-axis bending, both lateral-torsional and local buckling can arise, while under minor-axis bending, lateral-torsional buckling and Brazier-type flattening can occur. When designing for major-axis bending according to Eurocode 3, both local and lateral-torsional buckling are considered; it is shown herein that equal-leg angle sections under major-axis bending can be designed using a normalised slenderness based on the minimum of the local and lateral-torsional elastic buckling moments, while also considering their ratio. Under minor-axis bending, however, in comparison with the current provisions in Eurocode 3 that only require cross-section checks, it is shown that both safer and more accurate resistance predictions can be achieved when account is taken for lateral-torsional buckling and Brazier-type flattening. New design proposals for stainless steel equal-leg angle section beams, covering both major- and minor-axis bending, are therefore developed. The proposed design rules offer substantially more accurate and consistent resistance predictions compared to existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor γM1 = 1.1 , is verified following the procedure provided in EN 1990.

Journal article

Long Y, Zeng L, Gardner L, Wadee MAet al., 2022, A new model for calculating the elastic local buckling stress of steel plates in square CFST columns, Thin Walled Structures, Vol: 171, ISSN: 0263-8231

Previous studies into the elastic local buckling response of the steel plates in concrete-filled steel tubular (CFST) sections have considered the effect of concrete infill on the plate boundary conditions and on the buckling mode shape (i.e. one-way buckling only), but have not considered the influence of hoop stresses. This is addressed in the current paper, where a new model is proposed in which the unloaded plate edges are assumed to be elastically restrained and account is taken of the influence of hoop stresses. Tensile hoop stresses are shown to delay the occurrence of local buckling in the steel plates of square and rectangular CFST sections, though their beneficial influence reduces with increasing rotational edge restraint. The model is verified against existing experimental results, revealing better predictions of elastic local buckling stresses compared to previous approaches. The influence of the key parameters — tensile hoop stresses, varying boundary conditions and differing width-to-thickness (b/t) ratios is subsequently explored using the developed model, where it is shown that the b/t ratio remains the dominant factor in defining the elastic local buckling stress of steel plates in CFST sections. Finally, an alternative equivalent thickness approach to allow for the influence of the hoop stresses is proposed.

Journal article

Behzadi-Sofiani B, Gardner L, Wadee MA, Behzadi-Sofiani B, Gardner L, Wadee MAet al., 2022, Testing, simulation and design of steel equal-leg angle section beams, Thin Walled Structures, Vol: 171, Pages: 1-20, ISSN: 0263-8231

The stability and design of steel equal-leg angle section members subjected to uniaxial bending are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and physical tests on hot-rolled steel equal-leg angle section beams is first presented. The test results, in combination with existing experimental data on steel equal-leg angle section beams collected from the literature, are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Next, a numerical parametric study is presented considering both hot-rolled and cold-formed steel angle section beams with a wide range of slenderness values. In major-axis bending, both lateral-torsionaland local buckling were observed, with the former characterised by lateral deflection and twist of the cross-section along the member length but no cross-section deformation and the latter by relative twist and transverse bending of the outstands. In minor-axis bending, lateral-torsional buckling and Brazier flattening were observed, with the latter characterised by splaying of the outstands. Note that, for equal-leg angles under minor-axis bending, lateral-torsional buckling is dominant when the cross-section tips are in compression, while Brazier flattening is more influential when the cross-section tips are in tension. When designing for major-axis bending according to Eurocode 3, both local and lateral-torsional buckling are considered; it is shown herein that equal-leg angle section beams under major-axis bending can be designed using a normalised slenderness based on the minimum of the local and lateral-torsional elastic buckling moments, while also considering the ratio of the local tothe lateral-torsional elastic buckling moments. For minor-axis bending, Eurocode 3 only requires cross-section checks; this is found to result in unsafe predictions in some cases. It is shown that both safer and more acc

Journal article

Xu F, Pan W-H, Chan T-M, Sheehan T, Gardner Let al., 2022, Fracture prediction for square hollow section braces under extremely low cycle fatigue, Thin Walled Structures, Vol: 171, ISSN: 0263-8231

Abstract: This paper examines the extremely low cycle fatigue (ELCF) fracture of concentrically loaded square hollow section (SHS) braces subjected to cyclic loading. Numerical analyses are presented for both individual bracing members and bracing members integrated into concentrically braced frames (CBFs). The behaviour of the individual members was predicted using solid finite element (FE) simulations that employed a ductile fracture model and a nonlinear damage evolution rule. The solid FE model, which was validated using data from experiments, could adequately predict both the hysteretic response and the ELCF fracture cracking process. The coupled effects of instabilities (i.e. local and global buckling) and fracture on the ELCF performance of the braces were assessed, and the rotation capacity prior to fracture was quantified. This quantified rotation capacity was then incorporated into fibre-based FE models of CBFs as a member-level fracture criterion. The structure-level simulations were able to accurately capture the complex interactions between the frame components, i.e. the columns, beams, brace-gusset-plate connections and beam-to-column connections, and hence replicate the overall behaviour of CBFs, specifically, two-storey chevron braced frames. The influence of cross-section and member slenderness was evaluated and the importance of considering both in the development of cross-section slenderness limits was highlighted. The combined member- and structure-level simulation approach is proposed as an accurate and efficient means of assessing the seismic performance of CBFs.

Journal article

Kanyilmaz A, Demir AG, Chierici M, Berto F, Gardner L, Kandukuri SY, Kassabian P, Kinoshita T, Laurenti A, Paoletti I, du Plessis A, Razavi SMJet al., 2022, Role of metal 3D printing to increase quality and resource-efficiency in the construction sector, Additive Manufacturing, Vol: 50, Pages: 1-17, ISSN: 2214-8604

Demand for the construction of new structures is increasing all over the world. Since the construction sector dominates the global carbon footprint, new construction methods are needed with reduced embodied carbon and high resource efficiency to realize a sustainable future. In this direction, Metal Additive Manufacturing, also known as 3D printing, can be an opportunity. Many studies are underway to answer open questions about printed metal products and processes for high-tech industries. The construction sector must join the metal 3D printing research more actively to enrich the knowledge and experience on this technology and correctly adapt the process parameters suitable to the construction sector requirements. This paper states the opinion of a research group composed of academics and practitioners from Europe, the US, Japan, and South Africa on how metal 3D printing can be a complementary tool/technology to conventional manufacturing to increase productivity rates and reduce the costs and CO2 emissions in the construction industry.

Journal article

Xing Z, Zhao O, Kucukler M, Gardner Let al., 2022, Fire testing and design of slender stainless steel I-sections in weak-axis flexure, Thin Walled Structures, Vol: 171, Pages: 1-13, ISSN: 0263-8231

The structural fire response of slender austenitic stainless steel I-sections in weak-axis flexure is studied experimentally for the first time. The presented experimental programme comprised local geometric imperfection measurements for all the test beams, room temperature material tests, a reference room temperature weak-axis bending test and a series of elevated temperature weak-axis bending tests. The experimental setup, procedure and measured responses of the test specimens are fully described. The results demonstrate that, despite the studied cross-section being slender, considerable inelastic strength reserves were displayed. Such behaviour has previously been observed in steel I-sections in weak-axis flexure at room temperature. The test results are used to assess the accuracy of existing fire design provisions in predicting the weak-axis bending moment resistances of slender stainless steel I-sections. Significantly improved capacity predictionswere achieved through the application of a plastic effective width method in which the beneficial influence of the partial spread of plasticity is captured

Journal article

Hadjipantelis N, Weber B, Buchanan C, Gardner Let al., 2022, Description of anisotropic material response of wire and arc additively manufactured thin-walled stainless steel elements, Thin Walled Structures, Vol: 171, Pages: 1-17, ISSN: 0263-8231

In contrast to conventionally-produced structural steel and stainless steel elements, wireand arc additively manufactured (WAAM) elements can exhibit a strongly anisotropicmaterial response. To investigate this behaviour, data obtained from tensile tests on machined and as-built coupons extracted from WAAM stainless steel sheets are analysed.The observed mechanical response in the elastic range is described accurately using anorthotropic plane stress material model requiring the definition of two Young’s moduli, thePoisson’s ratio and the shear modulus. In the inelastic range, the anisotropy is capturedthrough the Hill yield criterion, utilising the 0.2% proof stresses in the three different loading directions relative to the deposition direction; plastic Poisson’s ratios are also reported.The presented findings and constitutive description highlight significant variation in theproperties of the studied stainless steel with direction, which opens up opportunities toenhance the mechanical performance of WAAM structures by optimising both the locationand orientation of the printed material.

Journal article

Quan C, Walport F, Gardner L, 2022, Equivalent imperfections for the out-of-plane stability design of steel beams by second-order inelastic analysis, Engineering Structures, Vol: 251, ISSN: 0141-0296

In current structural design specifications, such as EN 1993-1-1 for steel and EN 1993-1-4 forstainless steel, the stability of members is typically assessed through the use of buckling curves,which consider the influence of initial geometric imperfections and residual stresses. Analternative, more direct, approach is to perform either an elastic or inelastic second-orderanalysis of the member or structure with imperfections. For modelling convenience, so-called‘equivalent’ imperfections are typically utilised, which consider the combined influence ofboth geometric imperfections and residual stresses. Equivalent imperfections for the design ofcolumns and beams by second-order elastic analysis, also referred to as geometrically nonlinearanalysis with imperfection (GNIA), are provided in the current design specifications. Forcolumns, equivalent imperfections for design by second-order inelastic analysis, also referredto as geometrically and materially nonlinear analysis with imperfections (GMNIA), wererecently developed, but for beams that are susceptible to lateral-torsional buckling (LTB), thereare currently no appropriate provisions. The aim of this study is therefore to develop equivalentimperfections for use in out-of-plane stability design of steel and stainless steel members byGMNIA. The proposals are calibrated against the results of benchmark finite element (FE)simulations performed on a large number of steel and stainless steel members with geometricimperfections and residual stresses subjected to major axis bending. Two proposals forequivalent imperfection amplitudes are developed: (1) e0,mod, for use with eigenmode-affine2imperfections and (2) e0,bow, for use with sinusoidal bow imperfections. The latter is appliedsolely in the lateral direction and as a summation of a half-sine wave and a full sine wave.Relative to the traditional Eurocode design calculations, employing the proposed LTBimperfections in GMNIA provides generally more accurate me

Journal article

Meng X, Gardner L, 2022, Stability and design of normal and high strength steel CHS beam-columns, Engineering Structures, Vol: 251, Pages: 1-14, ISSN: 0141-0296

An experimental and numerical study into the global buckling behaviour of structural steel circular hollow section (CHS) beam-columns with normal and high strength steel grades is presented in this paper. Two cold-formed S700 CHS profiles – 139.7×4 and 139.7×5 (diameter × thickness, in mm), were used in the beam-column testing programme. Ten CHS specimens were tested under eccentric compression, resulting in a combination of axial compressive force and uniform first-order bending moment. Digital image correlation (DIC) was used in the experiments to enable full-field monitoring of the strain and deformations in the specimens. Finite element (FE) modelling was also carried out with the aims of first replicating the test results and then generating further numerical data to complement the test data pool. Assessment of the existing EC3 design provisions was carried out against the test and FE data, which revealed that the lack of account taken for the full influence of varying yield strengths leads to inaccuracies in the EC3 resistance predictions. Improved design rules were proposed accordingly and shown to result in considerably more consistent predictions of buckling strength across a wide spectrum of steel grades. Lastly, a reliability assessment of both the EC3 and proposed design approaches was performed in accordance with EN 1990. The statistical results confirmed that the current EC3 partial safety factor is suitable for use with the new design proposals.

Journal article

Zhu Y, Yang H, Gardner L, Wan Jet al., 2022, Performance of reinforced concrete filled steel tubular (RCFST) members subjected to transverse impact loading, Journal of Constructional Steel Research, Vol: 188, Pages: 1-17, ISSN: 0143-974X

Reinforced concrete-filled steel tubular (RCFST) members possess excellent load-bearing properties and fire resistance, and have been applied in a range of practical engineering projects. In addition to static and seismic loads, RCFST members may also be subjected to transverse impact loads, such as those from vehicle/vessel collisions and terrorist attacks. An experimental and numerically study into the dynamic behaviour of RCFST members subjected to transverse impact is therefore the focus of the present paper. Nine specimens were tested using a drop-weight experimental setup, and the instantaneous impact force and deformation states were recorded. A finite element model was established, considering strain rate effects, and validated against the test results. Parametric studies were then conducted in which the effects of the material selection, cross-section characteristics, specimen scale and impact conditions were analysed. Finally, design proposals for RCFST members under transverse impact loading are presented.

Journal article

Behzadi Sofiani B, Gardner L, Wadee MA, 2021, Stability and design of fixed-ended stainless steel equal-leg angle section compression members, Engineering Structures, Vol: 249, ISSN: 0141-0296

The stability and design of fixed-ended stainless steel equal-leg angle section members subjected to axial compression are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and tests on hot-rolled austenitic stainless steel equal-leg angle section columns is first presented. The test results, in combination with existing experimental data on stainless steel equal-leg angle section columns collected from the literature, are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Validation is performed by means of comparisons between the test and numerical results, considering ultimate loads, failure modes and the load–deformation responses, all of which are shown to be generally in good agreement. A numerical parametric study is then presented considering angle section columns in the three main families of stainless steel (austenitic, ferritic and duplex) with a wide range of slenderness values. The behaviour and normalised load-carrying capacity of the studied members is shown to be dependent on not only the column slenderness, but also the ratio of the elastic torsional-flexural buckling load to the elastic minor-axis flexural buckling load. Finally, a design approach recently proposed for carbon steel angle section columns is extended for application to stainless steel, and verified against the experimental and numerical results. The proposed approach offers substantially improved accuracy and consistency in strength predictions compared to the existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor ϒM1 = 1.1, is verified following the EN 1990 procedure.

Journal article

Vella N, Gardner L, Buhagiar S, 2021, Analytical modelling of cold-formed steel-to-timber connections with inclined screws, Engineering Structures, Vol: 249, Pages: 1-18, ISSN: 0141-0296

An analytical model that can describe the load-slip response of cold-formed steel-to-timberconnections, based on three components: timber embedment, screw bending and the axial pullthrough, is presented. The model takes into consideration the screw inclination and the damageto the timber caused during the screw drilling process. The model was validated againstavailable push-out test results, where it was shown that the model can accurately describe theultimate load, the slip at ultimate load, and the two slip moduli, ks and ks,m, with mean modelto-test ratios of 1.01, 1.03, 1.09 and 0.96, respectively. The observed failure modes were alsoaccurately predicted. Comparisons between the test data and the predictions of the EN 1995-1-1 design expressions showed that the Eurocode underestimated the ultimate loads by around50% on average and overestimated the slip moduli by around 500% on average; the proposedmodel therefore offers a significant improvement over current design provisions.

Journal article

Ye J, Kyvelou P, Gilardi F, Lu H, Gilbert M, Gardner Let al., 2021, An end-to-end framework for the additive manufacture of optimized tubular structures, IEEE Access, Vol: 9, ISSN: 2169-3536

Although additive manufacturing (AM) has been maturing for some years, it has only recently started to capture the interest of the cost-sensitive construction industry. The research presented herein is seeking to integrate AM into the construction sector through the establishment of an automated end-to-end framework for the generation of high-performance AM structures, combining sophisticated optimization techniques with cutting edge AM methods. Trusses of tubular cross-section subjected to different load cases have been selected as the demonstrators of the proposed framework. Optimization studies, featuring numerical layout and geometry optimization techniques, are employed to obtain the topology of the examined structures, accounting for practical and manufacturing constraints. Cross-section optimization is subsequently undertaken, followed by a series of geometric operations for the design of free-form joints connecting the optimized members. Solid models of the optimized designs are then exported for wire arc additive manufacturing (WAAM). Following determination of the optimal printing sequence, the trusses are printed and inspected. The efficiency of the optimized designs has been assessed by means of finite element modelling and compared against equivalent conventional designs. More than 200% increases in efficiency (reflected in the capacity-to-mass ratios) were achieved for all optimized trusses (when compared to their equivalent reference designs), demonstrating the effectiveness of the proposed optimization framework.

Journal article

Quan C, Kucuckler M, Gardner L, 2021, Out-of-plane stability design of steel beams by second-order inelasticanalysis with strain limits, Thin Walled Structures, Vol: 169, Pages: 1-20, ISSN: 0263-8231

An accurate and consistent approach to the out-of-plane stability design of steel beams and structures utilising second-order inelastic analysis with strain limits is proposed.The method is implemented using computationally efficient beam elements, with the ultimate structural resistance defined either by (i) the ultimate load factor or (ii) the load factor at which a strain limit, determined on the basis of the continuous strength method (CSM), is attained, whichever occurs first. Thus far, the methodhas been established for the in-plane design of steel structures and structural components;in the present paper, its scope is extended, for the first time, to the scenarios in which out-of-plane stability effects, with a focus on lateral-torsional buckling(LTB), govern. The accuracy and safety of the method are assessed against the results of nonlinear shell finite element (FE) modelling. It is shown thatthe proposed method consistently provides more accurate results than the traditional LTB design method of prEN 1993-1-1.In addition to its accuracy, the proposed approach also streamlines the design process b eliminatingthe need for cross-section classification and member design checks.

Journal article

Kyvelou P, Huang C, Gardner L, Buchanan Cet al., 2021, Structural testing and design of wire arc additively manufactured square hollow sections, Journal of Structural Engineering, Vol: 147, Pages: 1-19, ISSN: 0733-9445

Wire arc additive manufacturing (WAAM) is a method of metal 3D printing that has the potential for significant impact on the construction industry due to its ability to produce large parts, with reasonable printing times and costs. There is currently however a lack of fundamental data on the performance of structural elements produced using this method of manufacture. Seeking to bridge this gap, the compressive behavior and resistance of WAAM square hollow sections (SHS) are investigated in this study. Testing reported in a previous study by the authors of sheet material produced in the same manner as the studied SHS is first summarized. The production, measurement and testing of a series of stainless steel SHS stub columns are then described. Regular cross-section profiles were chosen to isolate the influence of 3D printing and enable direct comparisons to be made against equivalent sections produced using traditional methods of manufacture. A range of cross-section sizes and thicknesses were considered to achieve variation in the local cross-sectional slenderness of the tested specimens, allowing the influence of local buckling to be assessed. Repeat tests enabled the variability in response between specimens to be evaluated; a total of 14 SHS stub columns of seven different local slendernesses was tested, covering all cross-section classes of AISC 370 and Eurocode 3. Advanced non-contact measurement techniques were employed to determine the as-built geometric properties, while digital image correlation measurements were used to provide detailed insight into the deformation characteristics of the test specimens. Owing to the higher geometric variability of WAAM relative to 2 conventional forming processes, the tested 3D printed stub columns were found to exhibit more variable capacities between repeat specimens than is generally displayed by stainlesssteel SHS. Comparisons of the stub column test results with existing structural design rules highlight the need to

Journal article

Meng X, Gardner L, 2021, Flexural buckling of normal and high strength steel CHS columns, Structures, Vol: 34, Pages: 4364-4375, ISSN: 2352-0124

This paper presents an experimental and numerical investigation into the stability of normal and high strength steel circular hollow section (CHS) columns. Two cold-formed S700 CHS profiles – 139.7×4 and 139.7×5 (in mm), were studied in the experimental programme, and twelve column tests (six on each profile), together with accompanying material tests, were performed. The load-deformation histories and key results from the tests are reported. Following the physical testing, a numerical simulation campaign was conducted. The developed finite element (FE) models for CHS columns were initially validated against the test results. Parametric studies were then carried out, where 2000 additional buckling resistance data were numerically generated for hot-finished and cold-formed CHS columns, covering steel grades from S355 to S900. The obtained test and FE results, together with existing experimental data from the literature, were used to assess the current Eurocode 3 stability design rules. It was shown that the normalised performance of CHS columns is influenced by the yield strength, which is not fully captured by the EC3 design approach. The somewhat conservative Class 3 slenderness limit for compression further worsen the accuracy for those with slender cross-sections. Improvements to the EC3 design rules were proposed to address these shortcomings, including (a) modified Ayrton-Perry formulae enabling a continuous transition of buckling curves across the yield strength spectrum and (b) a new Class 3 limit for CHS in compression. The modified EC3 approach was assessed and shown to improve the accuracy and consistency of resistance predictions over the current EC3 approach, while meeting the Eurocode structural reliability requirements.

Journal article

Walport F, Arrayago I, Gardner L, Nethercot Det al., 2021, Influence of geometric and material nonlinearities on the behaviour and design of stainless steel frames, Journal of Constructional Steel Research, Vol: 187, Pages: 1-10, ISSN: 0143-974X

Material nonlinearity affects the stiffness and consequently the distribution of internal forces and moments in indeterminate structures. This has a direct impact on their behaviour and design, particularly in the case of stainless steel, where material nonlinearity initiates at relatively low stress levels. A method for accounting for the influence of material nonlinearity in stainless steel frames, including making due allowance for the resulting amplified second order effects, is presented herein. Proposals have been developed for austenitic, duplex and ferritic stainless steels. The method was derived based on benchmark results calculated through second order inelastic analysis with strain limits, defined by the Continuous Strength Method, using beam finite element models. A comprehensive set of frames was modelled and the proposed assessment of second order effects in the plastic regime was also verified against the results of four full-scale stainless steel frame tests. The proposed method is due to be included in the upcoming revision to Eurocode 3 Part 1.4

Journal article

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