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

487 results found

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

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

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, ISSN: 0141-0296

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, 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

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

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

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, 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

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, 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

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

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, 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

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

Arrayago I, Real E, Gardner L, Mirambell Eet al., 2021, The Continuous Strength Method for the design of stainless steel hollow section beam-columns, Engineering Structures, Vol: 238, Pages: 1-14, ISSN: 0141-0296

The Continuous Strength Method (CSM) is a deformation based design approach that provides accurate cross-section resistance predictions by making rational allowance for the interaction between cross-section elements, the partial spread of plasticity and the beneficial effects of strain hardening. The CSM can be used in conjunction with advanced analysis for the design of members and frames, but, for hand calculations, member-level stability checks are currently limited to stainless steel hollow section columns failing by flexural buckling. Extension to the design of stainless steel members subjected to combined compression and bending moment is presented in this paper. The analysis is based on numerical results and existing experimental data collected from the literature on stainless steel hollow section members, including members with stocky and slender cross-sections. Comparisons demonstrate that the adoption of the CSM design equations in conjunction with both current and revised interaction factors considerably improves the accuracy of beam-column capacity predictions for members with stocky cross-sections. The analysis on beam-columns with slender sections shows that similar resistance predictions are obtained using Eurocode 3 and the CSM. The reliability of the proposed approach is demonstrated through statistical analyses performed in accordance with EN1990.

Journal article

Walport F, Kucukler M, Gardner L, 2021, Stability design of stainless steel structures, Journal of Structural Engineering, 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

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

Xing Z, Zhao O, Kucukler M, Gardner Let al., 2021, Fire testing of austenitic stainless steel I-section beam–columns, Thin-Walled Structures, Vol: 164, Pages: 1-15, ISSN: 0263-8231

With the increasing use of stainless steel elements in construction, the need for comprehensive rules to enable their efficient structural design is clear. To date, the fire behaviour of stainless steel I-section beam–columns has been the subject of relatively little research. In particular, there is an absence of experimental data. To address this gap in knowledge, full-scale anisothermal fire tests on six grade 1.4301 austenitic stainless steel I-section beam–columns have been carried out; the test procedure and results are reported herein. The test specimens were subjected to eccentric axial compression with two eccentricity values so as to achieve different combinations of axial compression and uniform minor axis bending. Complementary initial local and global geometric imperfection measurements, room temperature tensile coupon tests and room temperature beam–column tests were also carried out. Based on the obtained experimental results, together with additional numerical results from a previous study, the existing design rules in the European structural steel fire design standard EN 1993-1-2 and the new design method of Kucukler et al. (2021) for stainless steel beam–columns in fire, which will be incorporated into the next version of EN 1993-1-2, are assessed.

Journal article

Wang Z, Wang YQ, Yun X, Gardner L, Teh Let al., 2021, Experimental study of swage-locking pinned aluminium alloy shear connections, Thin Walled Structures, Vol: 163, Pages: 1-12, ISSN: 0263-8231

This paper presents an experimental investigation into the structural behaviour and strength of swage-locking pinned aluminium alloy double-shear connections. It reports the failure modes (shear-out, bearing, block shear and net section failures), ultimate resistances and load-deformation histories. A total of twenty-three tests with four different aluminium alloy grades and various geometric variables, including end distances, edge distances and pin spacings, were carried out. The experimental results were utilized to assess the accuracy of the current American, Australian/New Zealand and European design provisions, as well as recent proposals by Teh and co-workers, extended to the design of aluminium alloy shear connections. The current code equations are shown to provide overly conservative strength predictions for the tested specimens. The equations of Teh and co-workers provide significantly more accurate strength predictions than the codes, though the test results indicate that there is still scope for more efficient design equations for swage-locking pinned aluminium alloy shear connections.

Journal article

Xing Z, Kucukler M, Gardner L, 2021, Local buckling of stainless steel I-sections in fire: finite element modelling and design, Thin Walled Structures, Vol: 161, ISSN: 0263-8231

The structural response of stainless steel I-sections in fire is investigated in this paper. Finite element models of stainless steel I-section members, capable of replicating their cross-section behaviour at elevated temperatures, are created and validated against existing experimental data from the literature. The validated finite element models are then utilised to perform comprehensive numerical parametric studies, where over 1000 numerical simulations of the response of stainless steel I-sections in fire are carried out, considering different cross-section dimensions, loading conditions, stainless steel grades and elevated temperature levels. On the basis of the findings from the parametric studies, the existing design rules of the European structural steel fire design standard EN 1993-1-2 and the recent design recommendations of Xing et al. [1], together with the plastic effective width method of Bambach and Rasmussen [2], are assessed in terms of their accuracy and reliability. It is observed that relative to the existing fire design rules set out in EN 1993-1-2, the design methods of Xing et al. [1] and Bambach and Rasmussen [2] are able to provide more accurate and reliable ultimate cross-section resistance predictions for stainless steel I-sections in fire, providing further verification of the suitability of the design provisions of Xing et al. [1] for inclusion in the next revision of EN 1993-1-2.

Journal article

Kyprianou C, Kyvelou P, Gardner L, Nethercot DAet al., 2021, Characterisation of material and connection behaviour in sheathed cold-formed steel wall systems - Part 1: Experimentation and data compilation, STRUCTURES, Vol: 30, Pages: 1161-1183, ISSN: 2352-0124

Journal article

Xing Z, Kucukler M, Gardner L, 2021, Local buckling of stainless steel I-sections in fire: Finite element modelling and design, Thin-Walled Structures, Vol: 161, ISSN: 0263-8231

The structural response of stainless steel I-sections in fire is investigated in this paper. Finite element models of stainless steel I-section members, capable of replicating their cross-section behaviour at elevated temperatures, are created and validated against existing experimental data from the literature. The validated finite element models are then utilised to perform comprehensive numerical parametric studies, where over 1000 numerical simulations of the response of stainless steel I-sections in fire are carried out, considering different cross-section dimensions, loading conditions, stainless steel grades and elevated temperature levels. On the basis of the findings from the parametric studies, the existing design rules of the European structural steel fire design standard EN 1993-1-2 and the recent design recommendations of Xing et al. [1], together with the plastic effective width method of Bambach and Rasmussen [2], are assessed in terms of their accuracy and reliability. It is observed that relative to the existing fire design rules set out in EN 1993-1-2, the design methods of Xing et al. [1] and Bambach and Rasmussen [2] are able to provide more accurate and reliable ultimate cross-section resistance predictions for stainless steel I-sections in fire, providing further verification of the suitability of the design provisions of Xing et al. [1] for inclusion in the next revision of EN 1993-1-2.

Journal article

Kyprianou C, Kyvelou P, Gardner L, Nethercot Det al., 2021, Characterisation of material and connection behaviour in sheathed cold-formed steel wall systems - Part 2: analytical modelling, Structures, Vol: 30, Pages: 1184-1199, ISSN: 2352-0124

Analytical models to describe the material and connection behaviour of the key components of sheathed cold-formed steel wall systems are developed and assessed in the present paper. The experimental data generated and collected in the companion paper (Kyprianou et al. [1]) are utilised for the calibration of the developed models. The assembled experimental database comprises the results of more than 400 physical tests, featuring material tests on plasterboard and oriented strand board (OSB), screw connector tests as well such as pull-through and push-out tests. The Ramberg–Osgood model (Ramberg and Osgood [2]) was shown to accurately describe the stress–strain behaviour of both plasterboard and OSB in the longitudinal and transverse direction in both tension and compression, while the Mander model (Mander et al. [3]) was also shown to accurately capture the compression behaviour for both materials and to follow the post peak unloading response. A generalised Ramberg–Osgood curve with linear post-peak unloading was adopted for describing the pull-through load-deformation behaviour of screws in OSB and plasterboard, while a similar generalised Ramberg–Osgood formulation, but with different exponents for the initial and subsequent parts of the curve was shown to accurately capture the shear load-slip behaviour of screws in steel-to-board connections. Predictive expressions for the ultimate capacities and recommended values for the remaining model parameters are provided herein. The developed predictive models are suitable for use in numerical simulations and advanced design methods.

Journal article

Yun X, Wang Z, Gardner L, 2021, Full-range stress-strain curves for aluminum alloys, Journal of Structural Engineering, Vol: 147, Pages: 1-15, ISSN: 0733-9445

Aluminum alloys are being increasingly used in a wide range of construction applications owing to their sound mechanical properties, lightness in weight, strong corrosion resistance, ability to be formed into complex and efficient cross-sectional shapes, and natural aesthetics. Aluminum alloys are characterized by a rounded stress–strain response, with no sharply defined yield point. Such behavior can be accurately represented using Ramberg–Osgood-type equations. In the present study, use of a two-stage Ramberg–Osgood model to describe the full-range stress–strain behavior of aluminum alloys is proposed and, following careful analysis of a comprehensive database of aluminum alloy coupon test data assembled from the literature, standardized values or predictive expressions for the required input parameters are derived. The experimental database includes over 700 engineering stress–strain curves obtained from 56 sources and covers five common aluminum alloy grades: 5052-H36, 6061-T6, 6063-T5, 6082-T6, and 7A04-T6. The developed model is shown to be more accurate in predicting the full-range stress–strain response of aluminum alloys than existing expressions, and is suitable for use in the analytical modeling, numerical simulation, and advanced design of aluminum alloy structures.

Journal article

de J dos Santos J, Liang Y, Zhao O, de Andrade SAL, de Lima LOR, Gardner L, da S Vellasco PCGet al., 2021, Testing and design of stainless steel staggered bolted connections, Engineering Structures, Vol: 231, Pages: 1-14, ISSN: 0141-0296

The present paper reports a thorough experimental investigation into the net section failure behaviour and capacity of stainless steel staggered bolted connections in tension. The testing programme was carried out on 31 stainless steel staggered bolted connection specimens, with 18 made of austenitic stainless steel (grade EN 1.4301), 7 made of duplex stainless steel (grade EN 1.4462) and 6 made of ferritic stainless steel (grade EN 1.4016). The geometric parameters, including the transverse and staggered pitches, and the staggered bolt hole patterns of the connection specimens, were varied. The test setup and procedures, as well as the key experimentally observed results, including the net section failure modes and loads, are reported in detail. The experimentally obtained net section failure loads and modes are analysed and discussed, and then utilised to assess the accuracy of the established design rules for stainless steel staggered bolted connections, given in the European, American and Australian/New Zealand standards. All three examined standards consider (i) net section fracture and (ii) gross section yielding in the design of stainless steel staggered bolted connections, and specify that the design failure load shall be taken as the minimum value calculated from all potential failure modes. It was found that the current design standards lead to overly conservative and scattered failure load predictions as well as inaccurate failure mode predictions. A new design approach based on the continuous strength method (CSM) is proposed, and shown to result in substantially improved predictions of both failure loads and failure modes.

Journal article

Quan C, Fieber A, Gardner L, 2021, Elastic local buckling of three-flanged cross-sections, Thin Walled Structures, Vol: 160, Pages: 1-16, ISSN: 0263-8231

In current structural steel design specifications, the local buckling of cross-sections is typically treated on an element-by-element basis, with the boundary conditions along the adjoined longitudinal edges of the individual plates assumed to be simply-supported. In reality, cross-sections buckle locally as a whole and the individual plate elements interact. As a result, the boundary conditions along the adjoined longitudinal edges of the critical isolated plate (i.e. that with the lowest elastic local buckling stress) lie between lower and upper bounds of simply-supported and fixed, respectively. Based on this concept, explicit formulae to predict the elastic local buckling stress of full cross-sections of common profiles, including I-sections, have recently been developed[1]. In the present paper, the formulae for single I-sections set out in[1] are extended to cover the case of three-flanged cross-sections that arise in longitudinally-stiffened plate girders and in the haunch and apex regions of portal frames. The geometry and loading of the studied cross-sections are assumed to remain constant along the member length, i.e. the influence of tapering and moment gradients on local buckling are not considered herein, but has been evaluated in parallel work [2]. The proposed formulae are calibrated against results from finite strip analysis performed usingCUFSMv4.05[3]on a range of three-flanged sections, and provide predictions of elastic local buckling stresses that are typically within 5% of the numerically obtained values.

Journal article

Kyprianou C, Kyvelou P, Gardner L, Nethercot Det al., 2021, Characterisation of material and connection behaviour in sheathed cold-formed steel wall systems - Part 1: experimentation and datacompilation, Structures, ISSN: 2352-0124

The material and connection behaviour in sheathed cold-formed steel wall systems are investigated in the present paper through experimentation. A total of 103 material and component tests was performed, including six cold-formed steel tensile coupon tests, nine tests on screws in tension, nine tests on screws in shear, 36 material tests on plasterboard and orientated strand board (OSB), 25 pull-through connection tests and 18 push-out (shear) connection tests. The plasterboard and oriented strand board were tested in both compression and tension, as well as both longitudinally and transversely to the production direction of the board. The main objective of the study was to measure and characterise the nonlinear response of all the materials and sheathing-to-steel connection components that are used in typical cold-formed steel wall systems, to support the ongoing and future development of accurate numerical simulations and structural design provisions for such systems. The present paper focuses on the experimental investigation and the collection of existing test data from the literature; a description of all the tests performed and a discussion of the results obtained are provided. The companion paper focuses on the establishment and assessment of predictive models to describe the responses of the material and connection components.

Journal article

Walport F, Gardner L, Nethercot D, 2021, Design of structural stainless steel members by second order inelastic analysis with CSM strain limits, Thin Walled Structures, Vol: 159, ISSN: 0263-8231

System-level advanced analysis is now a viable tool for widespread use in structuraldesign. By directly capturing frame and member level instability effects, plasticity, initialgeometric imperfections and residual stresses in the analysis, the need for subsequentindividual member checks can be eliminated. The analysis of structural members and framesis typically carried out using beam elements, which are unable to capture the effects of localbuckling. However, local buckling dictates the strength and ductility of cross-sections and theextent to which plastic redistribution of forces and moments can be exploited; it cannottherefore be disregarded. A proposal is made herein, in which strain limits, defined by thecontinuous strength method, are applied to simulate local buckling in beam element models,thereby controlling the degree to which spread of plasticity, force and moment redistributionand strain hardening can be utilised in the design of structural elements and systems. Strainsare averaged over a defined distance along the member length to reflect the fact that localbuckling requires a finite length over which to develop and to allow for local moment gradienteffects. Design is based directly on the application of strain limits to all cross-sections in thestructure. The accuracy of the proposed method for the design of stainless steel members isassessed through comparisons with benchmark shell finite element results; both I-section and hollow section members are considered. Comparisons against current design methods confirmthe significant benefits of applying the proposed approach in terms of both the accuracy andthe consistency of the resistance predictions. The reliability of the design approach isdemonstrated through statistical analyses performed in accordance with EN 1990. Applicationof the proposed method is particularly appropriate for stainless steel structures due to the highmaterial value and the complexities presented by the nonlinear material stress&

Journal article

Zhang R, Gardner L, Buchanan C, Matilainen VP, Piili H, Salminen Aet al., 2021, Testing and analysis of additively manufactured stainless steel CHS in compression, Thin Walled Structures, Vol: 159, ISSN: 0263-8231

Additive manufacturing, also referred to as 3D printing, has the potential to revolutionise the construction industry, offering opportunities for enhanced design freedom and reduced material use. There is currently, however, very limited data concerning the performance of additively manufactured metallic structural elements. To address this, an experimental and numerical investigation into the cross-sectional behaviour of circular hollow sections (CHS),produced by powder bed fusion (PBF) from Grade 316L stainless steel powder, is presented. The experimental programme comprised tensile coupon tests, initial geometric imperfection measurements and five axially loaded stub column tests on specimens with a range of diameter to-thickness (D/t) ratios. Similar cross-sectional behaviour to that of conventionally produced stainless steel CHS was observed, with the more slender cross-sections displaying increased susceptible to local buckling. In parallel with the experimental study, numerical simulations were carried out initially to replicate the experimental results and then to conduct parametric studies to extend the cross-sectional capacity data over a wider range of D/t ratios. The generated experimental and numerical results, together with other available test data on stainless steel CHS from the literature, were used to evaluate the applicability of existing design approaches for conventionally formed sections to those produced by additive manufacturing. Keywords: Additive manufacturing; Circular hollow sections; Current design approaches; Digital image correlation (DIC); Powder bed fusion (PBF); Stainless steel; Stub column testing; Tensile coupon tests; 3D printing.

Journal article

Xing Z, Zhao O, Kucukler M, Gardner Let al., 2021, Testing of stainless steel I-section columns in fire, Engineering Structures, Vol: 227, ISSN: 0141-0296

Although there have been significant developments in the testing, simulation and design of stainless steel structural elements at room temperature, the structural response of stainless steel members in fire has received significantly less attention. In particular, full-scale fire tests on stainless steel I-section members, which are becoming increasingly widely used in structural engineering applications to meet growing load-carrying capacity demands, are currently scarce. In this paper, the results of eight full-scale anisothermal fire tests on grade 1.4301 laser-welded austenitic stainless steel I-section columns are reported. Complementary initial local and global geometric imperfection measurements, room temperature tensile coupon tests and room temperature column buckling tests are also described. On the basis of the findings from the fire experiments, the accuracy and safety of the European fire design standard EN 1993-1-2 and the recent design recommendations of Kucukler et al. [1] for stainless steel columns in fire, which will be incorporated into the upcoming version of EN 1993-1-2, are assessed. It is observed that, relative to the existing column fire design rules in EN 1993-1-2, the design method of Kucukler et al. [1] provides more reliable ultimate strength predictions for austenitic stainless steel I-section columns in fire.

Journal article

Wu K, Wadee MA, Gardner L, 2021, Prestressed stayed beam-columns: sensitivity to prestressing levels, pre-cambering and imperfections, Engineering Structures, Vol: 226, ISSN: 0141-0296

The behaviour and structural performance of imperfect beam-columns with crossarms and externally prestressed cable stays are studied numerically, where the combination of bending and compression is assumed to be derived from the system self-weight acting orthogonally to the applied axial load. Both doubly-symmetric and mono-symmetric systems are studied. Sensitivity of the structural response to varying prestressing levels, pre-cambering and initial imperfections is investigated. Different initial imperfection levels and combinations are considered to facilitate the exploration of interactive buckling. The optimum prestressing force in terms of ultimate resistance and two structural efficiency indicators is also studied. It is found that relatively small crossarm lengths, stay diameters and crossarm length ratios should be avoided. Moreover, mono-symmetric cases are more sensitive to the level of pre-cambering than their doubly-symmetric counterparts. Considering both load-carrying capacity and structural efficiency, doubly-symmetric cases perform best with zero pre-cambering, but mono-symmetric cases perform best with upward pre-cambering. As for the true optimum prestressing levels, these are recommended to be significantly above the linearly obtained optimum to maximize the structural efficiency.

Journal article

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