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

499 results found

Behzadi-Sofiani B, Gardner L, Wadee MA, 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

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

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

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

Long Y, Zeng L, Gardner L, Wadee MAet al., 2021, A new model for calculating the elastic local buckling stress of steel plates in square CFST columns, Thin Walled Structures, 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

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, 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

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., 2021, Role of metal 3D printing to increase quality and resource-efficiency in the construction sector, Additive Manufacturing, 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

Yun X, Meng X, Gardner L, 2021, Design of cold-formed steel SHS and RHS beam–columns considering the influence of steel grade, Thin-Walled Structures, Pages: 108600-108600, ISSN: 0263-8231

The structural behaviour and design of cold-formed steel square and rectangular hollow section (SHS and RHS) beam–columns, made of both normal and high strength steels, are investigated in the present study through a comprehensive numerical modelling programme. Refined finite element (FE) models were first established and calibrated against a collection of existing cold-formed steel member test results from the literature. Following this, an extensive parametric study was undertaken to expand the cold-formed steel beam–column data pool, covering a wider range of steel grades, cross-section geometries, member slendernesses and loading combinations. Results from the parametric study were then used to examine the accuracy of the current codified beam–column design methods, provided in the European Standards EN 1993-1-1 (2005) and EN 1993-1-12 (2007) as well as the American Specification AISC 360-16 (2016). The comparisons revealed that the codified design rules provide varying levels of accuracy in predicting the ultimate resistances of cold-formed steel beam–columns depending on the steel grade. An improved design approach, compatible with current codified design rules in EN 1993-1-1 (2005), is proposed that features (1) recently developed column buckling curves that reflect the increasing normalised column buckling resistance with increasing steel yield strength; (2) bending moment resistances calculated using the Continuous Strength Method (CSM) and (3) new interaction curves for cold-formed steel beam–columns, which are anchored to these more accurate end points. The newly proposed design approach is shown to yield more accurate and consistent resistance predictions over current design provisions for cold-formed steel SHS and RHS beam–columns made of different steel grades, and its reliability has also been statistically verified in accordance with EN 1990.

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

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

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

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

Xu F, Pan W-H, Chan T-M, Sheehan T, Gardner Let al., 2021, Fracture prediction for square hollow section braces under extremely low cycle fatigue, Thin Walled Structures, 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

Wang F, Young B, Gardner L, 2021, Testing and numerical modelling of circular CFDST cross-sections with stainless steel outer tubes in bending, Engineering Structures, Vol: 247, Pages: 1-14, ISSN: 0141-0296

The structural performance of circular concrete-filled double skin tubular (CFDST) cross-sections with stainless steel outer tubes has been examined herein based on experiments and numerical modelling. A laboratory testing programme comprising a total of 22 four-point bending tests was performed on seven CFDST cross-sections with varying concrete grades. The details of the test rig and procedures, as well as the key test observations, including the failure moment capacities, moment–curvature curves and failure modes, are fully reported. A numerical modelling programme was then carried out; a finite element (FE) model was first established to replicate the test observations, and then adopted to conduct a parametric study to acquire further FE data over a broader spectrum of material strengths and cross-section slendernesses. Based on the combined set of test and FE results, the general design provisions for concrete-filled carbon steel members in the current European and American design codes were evaluated for their applicability to the studied CFDST cross-sections. Overall, the results revealed that both of the examined design codes yield unduly conservative (less so for the higher concrete grades) and scattered moment resistance predictions, though some moment resistances predicted from the European code were on the unsafe side. Modifications, including a concrete reduction factor η to reflect the reduced relative effectiveness of using higher concrete grades and a modified stress distribution considering the partial spread of plasticity, were proposed and shown to improve the consistency of the resistance predictions. Finally, the reliability of the current and modified design rules was demonstrated through statistical analyses.

Journal article

Xing Z, Zhao O, Kucukler M, Gardner Let al., 2021, Fire testing and design of slender stainless steel I-sections in weak-axis flexure, Thin Walled Structures, 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

Gardner L, Kucuckler M, 2021, In-plane structural response and design of duplex and ferritic stainless steel welded I-section beam-columns, Engineering Structures, Vol: 247, Pages: 1-17, ISSN: 0141-0296

In this paper, the in-plane behaviour and design of duplex and ferritic stainless steelwelded I-section beam-columns fabricated through the welding of individual hot-rolled stainless steel plates are explored. Finite element models able to replicate the structural responseof stainless steel I-section members are created and validated against experimental resultsfrom the literature. Using the validated finite element models, extensive numerical parametric studies are performed, the results of which are utilised to investigate the accuracyand reliability of existing design provisions for duplex and ferritic stainless steel I-sectionbeam-columns. Scope for improvement is revealed, prompting the development of new beamcolumn design rules that are compatible with those provided for carbon steel members inEN 1993-1-1. The new proposals are shown to offer improved accuracy and consistency overexisting design provisions for duplex and ferritic stainless steel I-section members under combined axial compression and bending, and are recommended for inclusion in the upcomingrevision to EN 1993-1-4. The reliability of the proposed design rules, with a partial safetyfactor γM1 = 1.1, is demonstrated.

Journal article

Jiang K, Tan KH, Zhao O, Gardner Let al., 2021, Block tearing of S700 high strength steel bolted connections: Testing, numerical modelling and design, Engineering Structures, Vol: 246, Pages: 1-10, ISSN: 0141-0296

Abstract: The block tearing behaviour and resistance of S700 high strength steel (HSS) bolted connections in tension have been investigated through tests and numerical simulations and reported in this paper. The experiments were performed on sixteen S700 high strength steel bolted connections, including seven 2-bolt connections (with each comprising two bolts arranged perpendicular to the loading direction) and nine 4-bolt connections (with each comprising four bolts arranged in two rows). All specimens were carefully designed, with a series of geometric parameters, including the end and edge distances as well as the longitudinal and transverse pitches, varied and studied. The experimental setup and procedures, together with the key observed results, including the failure loads, the load–elongation curves and the block tearing failure modes, are fully reported and discussed. The experiments were supplemented by numerical simulations; finite element models were firstly developed to replicate the experimental behaviour and then employed to perform parametric studies to generate further numerical data on S700 high strength steel bolted connections susceptible to block tearing over a wide range of geometric dimensions. On the basis of the test and numerical 2 data, the existing design methods for high strength steel bolted connections susceptible to block tearing, as given in the European code, American Specification and Australian Standard, were evaluated. The evaluation results revealed that all three considered design codes lead to consistent and safe-sided but conservative predictions of the failure loads, due principally to the lack of appropriate consideration of the location and area of the shear failure plane. The proposals of Teh and Uz [1], developed for normal strength steel bolted connections, were then investigated and shown to result in substantial improvements of block tearing resistance predictions, although some of the resistances are unsafe. Fina

Journal article

Dodwell T, Flemming L, Buchanan C, Kyvelou P, Detommasoe G, Gosling P, Scheichl R, Kendall W, Gardner L, Girolami M, Oates Cet al., 2021, A data-centric approach to generative modelling for 3D-printed steel, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 477, ISSN: 1364-5021

The emergence of additive manufacture (AM) for metallic material enables components of near arbitrary complexity to be produced. This has potential to disrupt traditional engineering approaches. However, metallic AM components exhibit greater levels of variation in their geometric and mechanical properties compared to standard components, which is not yet well understood. This uncertainty poses a fundamental barrier to potential users of the material, since extensive post-manufacture testing is currently required to ensure safety standards are met. Taking an interdisciplinary approach that combines probabilistic mechanics and uncertainty quantification, we demonstrate that intrinsic variation in AM steel can be well described by a generative statistical model that enables the quality of a design to be predicted before manufacture. Specifically, the geometric variation in the material can be described by an anisotropic spatial random field with oscillatory covariance structure, and the mechanical behaviour by a stochastic anisotropic elasto-plastic material model. The fitted generative model is validated on a held-out experimental dataset and our results underscore the need to combine both statistical and physics-based modelling in the characterization of new AM steel products.

Journal article

Hadjipantelis N, Weber B, Buchanan C, Gardner Let al., 2021, Description of anisotropic material response of wire and arc additively manufactured thin-walled stainless steel elements, Thin Walled Structures, 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

Walport F, Kucukler M, Gardner L, 2021, Stability design of stainless steel structures, Journal of Structural Engineering, Vol: 148, Pages: 1-19, ISSN: 0733-9445

The direct analysis method (DAM), featuring second order elastic analysis with two stiffness reduction factors - τb and τg, is the primary means of stability design for steel structures in AISC 360 and AISI S100. The equivalent provisions for stainless steel structures, which are due to be incorporated into the upcoming AISC 370 and ASCE-8 Specifications are developed herein. Stainless steel exhibits a rounded stress-strain response, typically described by the Ramberg-Osgood formulation. The slope of this function (i.e. the tangent modulus), adjusted to consider the influence of residual stresses, is used to define the stiffness reduction factor τb at a given axial load level to be applied to members in compression to allow for the adverse influence of the spread of plasticity and residual stresses. The dependency of the degree of stiffness reduction on the roundedness of the stress-strain curve, which varies between the different grades of stainless steel is also directly captured through the strain hardening exponent n that features in the Ramberg–Osgood formulation. Values of 0.7 for AISC 370 and 0.9 for ASCE-8 are proposed for the general stiffness reduction factor τg to be applied to all member stiffnesses to account for the development and spread of plasticity, and to ensure a suitable reduction in stiffness for slender members with low axial load levels. The different τg values between the two specifications is required to reflect the different buckling curves and axial-bending interaction expressions employed. The accuracy of the proposed method for the design of stainless steel members and frames is assessed through comparisons with benchmark shell finite element results. Comparisons are also made against the new provisions in AISC for design by second order inelastic analysis. The reliability of the design proposals is demonstrated through statistical analyses, where it is shown that a resistance factor ϕ of 0.9 can be adopted.

Journal article

Zhang R, Buchanan C, Matilainen V-P, Daskalaki-Mountanou D, Britton T, Piili H, Salminen A, Gardner Let al., 2021, Mechanical properties and microstructure of additively manufactured stainless steel with laser welded joints, Materials and Design, Vol: 208, Pages: 1-20, ISSN: 0264-1275

Powder bed fusion (PBF) is a commonly employed metal additive manufacturing (AM) process in which components are built, layer-by-layer, using metallic powder. The component size is limited by the internal build volume of the employed PBF AM equipment; the fabrication of components larger than this volume therefore requires mechanical joining methods, such as laser welding. There are, however, very limited test data on the mechanical performance of PBF metal with laser welded joints. In this study, the mechanical properties of PBF built 316L stainless steel parts, joined together using laser welding to form larger components, have been investigated; the microstructure of the components has also been examined. 33 PBF 316L stainless steel tensile coupons, with central laser welds, welded using a range of welding parameters, and with coupon half parts built in two different orientations, were tested. The porosity, microhardness and microstructure of the welded coupons, along with the widths of the weld and heat-affected zone (HAZ), were characterised. The PBF base metal exhibited a typical cellular microstructure, while the weld consisted of equiaxed, columnar and cellular dendrite microstructures. Narrow weld regions and HAZs were observed. The PBF base metal was found to have higher proof and ultimate strengths, but a similar fracture strain and a lower Young’s modulus, compared with conventionally manufactured 316L stainless steel. The strengths were dependent on the build direction – the vertically built specimens showed lower proof strengths than the horizontal specimens. The laser welds generally exhibited lower microhardness, proof strengths and fracture strains than the PBF base metal which correlated with the observed structure. This work has demonstrated that PBF built parts can be joined by laser welding to form larger components and provided insight into the resulting strength and ductility.

Journal article

Lapira L, Wadee MA, Gardner L, 2021, Nonlinear analytical modelling of flat and hyperbolic paraboloidal panels under shear, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 477, Pages: 1-19, ISSN: 1364-5021

The hyperbolic paraboloid (hypar) form has been widely used in long-span roof structures and the subject of much research under out-of-plane loading. However, the behaviour of hypars under in-plane loading has been less keenly studied and there is no suitable guidance for their design in current codes of practice. A nonlinear analytical model treating the hypar as a deliberate imperfection applied to a flat plate is presented. A Rayleigh-Ritz formulation using appropriate shape functions is developed and the resulting equations are solved using numerical continuation techniques. The results are verified with nonlinear finite element models, showing good correlation across a range of thicknesses and degrees of initial curvature. Key moda lcontributions that influence the behaviour of the hypar are identified, providing insight into the nonlinear behaviour of hypars subject to in-plane shear. The main differences in behaviour between the flat plate and the hypar panel are shown to be most prevalent in the early stages of loading, where the influence of the initial geometry is at its greatest.

Journal article

Kyvelou P, Slack H, Wadee MA, Buchanan C, Gardner Let al., 2021, Material testing and analysis of WAAM stainless steel, Sheffield, UK, Eurosteel 2020

Conference paper

Kyprianou C, Kyvelou P, Gardner L, Nethercot Det al., 2021, Experimental study of sheathed cold-formed steel beam-columns, Thin Walled Structures, Vol: 166, ISSN: 0263-8231

An experimental study of sheathed cold-formed steel C-lipped wall studs, with service holes, subjected to compression and major axis bending is presented in this paper. A total of 17 experiments were performed with both oriented strand board (OSB) and plasterboard used as sheathing and with varying connector spacing employed between the sheathing panels and the steel members. The tested specimens comprised a single 2.4 m long column sheathed on both sides and secured at the ends to top and bottom tracks. The member tests were complemented by material tests, stub column tests and initial geometric imperfection measurements. The specimens were tested in a dual-actuator rig where axial compression was applied by means of a vertical actuator through the top track, while bending was applied through the application of four lateral point loads. Eight pure compression tests with both plasterboard and OSB sheathing and with the spacing of the connectors varying between 75 mm and 600 mm were initially performed. Specimens with OSB sheathing were then tested under pure bending and combined loading. The full load–deformation responses and failure modes of the member test specimens are reported. The compressed studs connected to the plasterboard sheathing at wider spacings exhibited pull-through failure of the connectors, followed by flexural torsional buckling, while the specimens with denser connector spacings, failed by local buckling at the member ends. The OSB sheathed specimens under pure compression failed by local and distortional buckling, those under combined loading exhibited local failure at the service openings, while for those under pure bending, local buckling and stud-to-track connector failure occurred. Reducing the spacing of the connectors from 600 mm to 75 mm resulted in up to 20% and 30% increases in capacity for the studs sheathed with OSB and plasterboard respectively.

Journal article

Behzadi Sofiani B, Gardner L, Wadee MA, Dinis P, Camotim Det al., 2021, Behaviour and design of fixed-ended steel equal-leg angle section columns, Sheffield, UK, Eurosteel 2020

Conference paper

Meng X, Gardner L, 2021, Testing, modelling and design of normal and high strength steel tubular beam-columns, Journal of Constructional Steel Research, Vol: 183, ISSN: 0143-974X

The influence of yield strength on the in-plane stability and design of structural steel square and rectangular hollow section (SHS and RHS) beam-columns under compression plus uniform bending is investigated in this study. Ten buckling tests were carried out on hot-finished S690 SHS 100×100×4 (in mm) specimens under combined compression plus uniaxial bending. Finite element (FE) models of SHS and RHS beam-columns were established, validated and employed to perform parametric studies, where supplementary buckling resistance data were numerically derived. Through comparisons with the test and FE data, shortcomings of the current EC3 beam-column interaction curves were revealed, particularly in the higher yield strength domain. A modified EC3 approach was developed accordingly, featuring more accurate compression end points and re-calibrated interaction factors, and was shown to significantly improve the design accuracy and consistency for both hot-finished and cold-formed SHS and RHS beam-columns over a wide spectrum of steel grades. Subsequent reliability analysis confirmed the applicability of the current EC3 partial safety factor to the proposed modified EC3 design approach.

Journal article

Behzadi-Sofiani B, Gardner L, Wadee MA, 2021, Fixed-ended stainless steel equal-leg angle section columns - behaviour and design, 8th international conference on coupled instabilities in metal structures (CIMS 2020)

Conference paper

Behzadi-Sofiani B, Gardner L, Wadee MA, Dinis P, Camotim Det al., 2021, Behaviour and design of fixed-ended steel equal-leg angle section columns, Journal of Constructional Steel Research, Vol: 182, ISSN: 0143-974X

The mechanical behaviour and design of fixed-ended steel equal-leg angle section members subjected to axial compression are addressed in this study. First, the critical buckling behaviour is described. Experimental data on 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 test and numerical results, considering ultimate loads and failure modes, all of which are shown to be generally in good agreement. A numerical parametric study is then presented considering steel angle section columns with a wide range of slenderness values. The behaviour and load-carrying capacity of the columns 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, the collected experimental and generated numerical results are used to develop a new design approach, suitable for incorporation into future revisions of Eurocode 3, for fixed-ended steel equal-leg angle section columns, reflecting the observations made. The proposed approach offers improved accuracy and consistency in strength predictions compared to the existing codified design rules. The reliability of the new design approach, with a recommended partial safety factor γM1 = 1.0, is verified following the EN 1990 procedure.

Journal article

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