Imperial College London


Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Structural Engineering



+44 (0)20 7594 6058leroy.gardner




435Skempton BuildingSouth Kensington Campus





Publication Type

381 results found

Meng X, Gardner L, 2020, Simulation and design of semi-compact elliptical hollow sections, Engineering Structures, Vol: 202, Pages: 1-15, ISSN: 0141-0296

Current structural steel design codes typically feature a step in the cross-sectional resistance functions at the Class 2 slenderness limit due to the abrupt switch between elastic and fully plastic capacities. To address this issue, new design rules, featuring a gradual transition between elastic and plastic resistances, have been recently developed for semi-compact (Class 3) I- and box sections to account for the partial spread of plasticity. This approach is extended herein to the cross-section and member buckling design of semi-compact elliptical hollow sections (EHS). Finite element models were first established and validated against previous test results; particular attention was given to the modelling of local geometric imperfections. Parametric studies were then conducted, where over 4000 structural performance data were numerically generated covering a wide range of cross-section aspect ratios, material properties, local and global slendernesses and load combinations. Upon completion of the numerical simulation programme, structural design rules of semi-compact EHS were developed. The classification of EHS under biaxial bending and compression plus biaxial bending was initially addressed. Following this, new design expressions featuring elasto-plastic section properties were developed to exploit partial plastification at both cross-section and member buckling levels. The accuracy of the design proposals was evaluated through comparisons between the test/numerical data and the resistance predictions; the comparisons revealed that the proposed elasto-plastic cross-section and member buckling design rules lead to both improved accuracy and consistency over the existing elastic provisions. The reliability of the proposals was verified through statistical analyses in accordance with EN 1990, demonstrating their suitability for incorporation into the next revision to EN 1993-1-1.

Journal article

Wadee MA, Hadjipantelis N, Bazzano JB, Gardner L, Lozano-Galant JAet al., 2020, Stability of steel struts with externally anchored prestressed cables, Journal of Constructional Steel Research, Vol: 164, ISSN: 0143-974X

Externally anchored prestressed cables can be employed to enhance the stability of steeltruss compression elements significantly. To demonstrate this concept, a system comprisinga tubular strut subjected to an external compressive load and a prestressed cable anchoredindependently of the strut is studied. Energy methods are utilized to define the elasticstability of the perfect and imperfect systems, after which the first yield and rigid–plasticresponses are explored. The influence of the key controlling parameters, including thelength of the strut, the axial stiffness of the cable and the initial prestressing force, on theelastic stability, the inelastic response and the ultimate strength of the system is demon-strated using analytical and finite element (FE) models. To illustrate the application of thestudied structural concept, FE modelling is employed to simulate the structural response ofa prestressed hangar roof truss. A nearly two-fold enhancement in the load-carrying capac-ity of the truss structure is shown to be achieved owing to the addition of the prestressedcable

Journal article

Wu K, Wadee MA, Gardner L, 2019, Stability and ultimate behaviour of prestressed stayed beam-columns, Engineering Structures, Vol: 201, ISSN: 0141-0296

The instability of beam-columns with crossarms and externally prestressed cable stays is studied analytically, where the combination of bending and compression is assumed to be derived from the system self-weight acting orthogonally to the applied axial load. Three principal zones of behaviour are identified with two of these each having two sub-zones that relate the critical buckling load to the initial prestressing force applied to the stay cables. The ultimate load-carrying capacity of the beam-columns is evaluated by conducting nonlinear finite element analysis within the commercial package ABAQUS. Results show that the analytically derived critical buckling loads generally provide safe predictions of the ultimate loads due to significant post-buckling strength. It is found that releasing the geometric double symmetry of the system can make for a significantly more efficient structure due to the effect of pre-cambering against the self-weight. The strength and efficiency of stayed beam-column systems opens up a range of potential applications, including lighter alternatives to conventional props to support wide excavations, which currently utilize very heavy steelwork.

Journal article

Wang Z, Wang Y, Zhang Y, Gardner L, Ouyang Yet al., 2019, Experimental investigation and design of extruded aluminium alloy T-stubs connected by swage-locking pins, Engineering Structures, Vol: 200, Pages: 1-15, ISSN: 0141-0296

The structural behaviour and design of extruded aluminium alloy T-stubs connected by swage-locking pins under monotonic loading are investigated in this study. Thirty experiments were performed and the test set-up, testing procedure and experimental results, including failure modes, ultimate load-carrying capacity, deformation capacity and load-displacement response, are reported. Component tests were also carried out on the swage-locking pins to assess their load-carrying capacities under pure tension, pure shear and combined tension and shear; these tests were complemented by tensile coupon tests on the pin and T-stub plate material. Resistance functions to predict the capacity of the swage-locking pins were developed and assessed against the test results. The EN 1999-1-1 (EC9) design rules for predicting the resistance of extruded aluminium alloy T-stubs were also evaluated and found to be safe-sided, but rather conservative relative to the experimental results. Improved resistance predictions were achieved through application of the continuous strength method (CSM).

Journal article

Walport F, Gardner L, Nethercot DA, 2019, A method for the treatment of second order effects in plastically-designed steel frames, Engineering Structures, Vol: 200, Pages: 109516-109516, ISSN: 0141-0296

The susceptibility of steel frames to global second order effects, also referred to as sway effects, ‘P–Δ’ effects and global geometric nonlinearities, is traditionally assessed through the elastic buckling load amplifier αcr. For elastic analysis, EN 1993-1-1 and other international steel design standards state that second order effects may be neglected provided αcr is greater than or equal to 10. However, when plastic analysis is employed, yielding of the material degrades the stiffness of the structure, and hence a stricter requirement of αcr ≥ 15 is prescribed in EN 1993-1-1 for second order effects to be neglected. Use of a single limit of 15 for any structural system is however considered to be overly simplistic. A more consistent and accurate approach is to determine the degree of stiffness degradation and hence the increased susceptibility to second order effects on a frame-by-frame basis. A parametric analysis to assess the stability of steel frames in the plastic regime is presented herein. A series of frames with varying geometries and load cases has been assessed. Based on the findings, a proposal for the calculation of a modified elastic buckling load factor αcr,mod, which considers the reduction in stiffness following plasticity on a frame-by-frame basis, is presented.

Journal article

Wang F, Young B, Gardner L, 2019, Experimental study of square and rectangular CFDST sections with stainless steel outer tubes under axial compression, Journal of Structural Engineering, Vol: 145, ISSN: 0733-9445

A comprehensive experimental investigation into the axial compressive response of concrete-filled double skin tubular (CFDST) sections with stainless steel square and rectangular outer tubes is presented. A total of 28 tests was carried out. The experimental setup and procedures are described, and the test observations are fully reported. The test results are employed to assess the applicability of the current European and North American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections. Modifications to the current design codes are also considered—a higher buckling coefficient k of 10.67 to consider the beneficial restraining effect of the concrete on the local buckling of the stainless steel outer tubes and a reduction factor η to account for the effective compressive strength of high strength concrete. Overall, the comparisons revealed that the existing design rules may generally be safely applied to the prediction of the compressive resistance of CFDST cross-sections with stainless steel outer tubes, while the modified design rules offered greater accuracy and consistency.

Journal article

Liu F, Wang Y, Gardner L, Varma AHet al., 2019, Experimental and numerical studies of reinforced concrete columns confined by circular steel tubes exposed to fire, Journal of Structural Engineering, Vol: 145, ISSN: 0733-9445

Reinforced concrete columns confined by steel tubes, also known as steel tube–confined reinforced concrete (STCRC) columns, are a kind of composite column in which the outer steel tube acts predominantly as hoop reinforcement. This is achieved by the provision of breaks to the longitudinal continuity of the steel tube. The compressive behavior and seismic performance of STCRC columns have been extensively studied in the last few decades. However, knowledge of the fire behavior of STCRC columns is very limited. Hence, experimental and numerical studies to investigate the response of STCRC columns under combined thermal (fire) and structural loading are presented herein. Four full-scale STCRC columns and one concrete-filled steel tubular (CFST) column were first axially loaded and then subjected to fire until failure. The measured furnace temperatures, specimen temperatures, axial displacement versus time curves, and fire resistance of the columns are presented and discussed. A nonlinear finite-element model employing a sequentially coupled thermal-stress analysis was then developed and validated against recent fire tests on STCRC and CFST columns reported in the literature. Following extensive parametric studies, a simplified method is proposed for predicting the temperatures of the steel tube, reinforcing bars, and concrete. Design rules are then proposed for predicting the load-bearing capacity of STCRC columns exposed to fire, which are consistent with the design method for STCRC columns at ambient temperature.

Journal article

Fieber A, Gardner L, Macorini L, 2019, Design of structural steel members by advanced inelastic analysis with strain limits, Engineering Structures, Vol: 199, Pages: 1-21, ISSN: 0141-0296

Structural steel design is traditionally a two step process: first, the internal forces and moments in the structure are determined from a structural analysis. Then, a series of design checks are carried out to assess the strength and stability of individual members. The structural analysis is typically performed using beam finite elements, which are usually not able to capture local buckling explicitly. Instead, the assessment of local buckling and rotation capacity is made through the concept of cross-section classification, which places class-specific restrictions on the analysis type (i.e. plastic or elastic) and defines the cross-section resistance based on idealised stress distributions (e.g. the plastic, elastic or effective moment capacity in bending). This approach is however considered to be overly simplistic and creates artificial steps in the capacity predictions of structural members. A more consistent approach is proposed herein, whereby a second-order inelastic analysis of the structure or structural component is performed using beam finite elements, and strain limits are employed to mimic the effects of local buckling, control the spread of plasticity and ultimately define the structural resistance. The strain limits are obtained from the continuous strength method. It is shown that not only can local buckling be accurately represented in members experiencing uniform cross-sectional deformations along the length, but, by applying the strain limits to strains that are averaged over a defined characteristic length, the beneficial effects of local moment gradients can also be exploited. The proposed method is assessed against benchmark shell finite element results on isolated members subjected to bending, compression and combined loading. Compared to conventional steel design provisions and even to existing advanced design approaches utilising second-order elastic analysis, the proposed design approach provides consistently more accurate capacity predic

Journal article

Toffolon A, Meng X, Taras A, Gardner Let al., 2019, The generalized slenderness‐based resistance method for the design of SHS and RHS, Steel Construction, Vol: 12, Pages: 327-341, ISSN: 1867-0520

Selected, extended paper from the SDSS 2019 special session ECCS/TC8 – Structural StabilityThe cross‐sectional strength of square (SHS) and rectangular (RHS) hollow sections loaded in compression and various degrees of uniaxial or biaxial bending are governed by local instabilities in the elastic or plastic range. Common design checks for cross‐sectional strength, e.g. those found in the Eurocodes, regularly penalize these sections through a conservative omission of various mechanical effects and a categorization of cross‐sections into distinct classes with corresponding, markedly different, design rules. This leads to discontinuities and inaccuracies in the strength representation. Such conservatism is particularly detrimental to the introduction of high‐strength steel hollow sections, which often fall into the semi‐compact and slender cross‐section classes for which local buckling is more relevant. This paper discusses the results of extensive research work carried out during the RFCS research project HOLLOSSTAB. In this project, new design rules were developed for the cross‐sectional and member design checks of hollow sections with various shapes and slenderness ratios, termed the “Generalized Slenderness‐based Resistance Method – GSRM”. This paper summarizes the experimental and numerical campaign carried out within HOLLOSSTAB and describes the new GSRM design rules and their background for the case of the cross‐sectional strength of SHS and RHS.

Journal article

Meng X, Toffolon A, Gardner L, Taras Aet al., 2019, The generalised slenderness‐based resistance method for the design of CHS and EHS, Steel Construction, Vol: 12, Pages: 342-353, ISSN: 1867-0520

Selected, extended paper from the SDSS 2019 special session ECCS/TC8 – Structural StabilityThis paper presents the development and assessment of an innovative cross‐section design method for structural steel circular and elliptical hollow sections (CHS and EHS) – the generalised slenderness‐based resistance method (GSRM). A numerical simulation programme was first conducted to expand the data pool for CHS and EHS. Finite element (FE) models were established, validated against existing test data and then utilised for parametric studies, where a total of over 3 700 cross‐section resistance data were numerically generated. The development of the GSRM for CHS and EHS is then presented. Key design parameters, including the reference resistances and the generalised local slenderness, are initially defined. The general design procedure is subsequently introduced. Two design alternatives for CHS and EHS – a strength‐based approach, and a deformation‐based approach based on the continuous strength method (CSM), are developed and presented. Finally, the proposed GSRM is assessed using the previously collected test results and freshly generated FE data, where excellent accuracy and consistency in the resistance predictions are clearly revealed for all loading scenarios. Subsequent reliability analyses demonstrate that the current partial safety factor used in EN 1993‐1‐1 can be applied to the GSRM, achieving an appropriate level of reliability.

Journal article

Meng X, Gardner L, Sadowski A, Rotter Jet al., Elasto-plastic behaviour and design of semi-compact circular hollow sections, Thin Walled Structures, ISSN: 0263-8231

Previous research has revealed shortcomings in the current Eurocode 3 (EC3) provisions for the design of semi-compact (Class 3) cross-sections. These shortcomings arise primarily from the lack of utilisation of partial plastification in bending, leading to a step in the design resistance function at the boundary between Class 2 and 3 cross-sections and an underestimation of the available capacity. This affects the accuracy of resistance predictions in bending and under combined loading, and applies at both cross-sectional and member level. To address this issue, the use of an elasto-plastic section modulus, which lies between the plastic and elastic section moduli, has been proposed and employed in the design of semi-compact I- and box sections. The aim of the present study is to develop new cross-section and member buckling design rules incorporating the elasto-plastic section modulus for semi-compact circular hollow sections (CHS), and to assess their accuracy against existing experimental and freshly generated numerical data. Firstly, an experimental database, consisting of previous cross-section and member buckling test results on steel CHS, was established. A comprehensive numerical simulation programme was subsequently carried out; in this programme, finite element (FE) models were developed, validated and used for parametric studies, where over 600 numerical structural performance data on semi-compact CHS were generated. New sets of cross-section and member buckling design expressions featuring elasto-plastic section properties were then proposed and assessed against the test and numerical data. The proposals were shown to offer improved accuracy and design efficiency over the elastic EC3 methods. The reliability of the proposed elasto-plastic design rules was then confirmed through statistical analyses in accordance with EN 1990, demonstrating their suitability for inclusion into the next revision of EN 1993

Journal article

Hadjipantelis N, Gardner L, Wadee MA, 2019, Finite-element modeling of prestressed cold-formed steel beams, Journal of Structural Engineering, Vol: 145, Pages: 04019100-1-04019100-19, ISSN: 0733-9445

The concept and structural benefits of prestressing cold-formed steel beams are explored in the present paper. In the proposed system, prestressing is applied by means of a high-strength steel cable located within the cross section of the beam at an eccentric location with respect to the strong geometric axis. The internal forces generated by the prestressing are opposite in sign to those induced under subsequent vertical loading. Hence, the development of detrimental compressive stresses within the top region of the cold-formed steel beam is delayed and thus the load-carrying capacity of the beam is enhanced. Owing to the precamber that is induced along the member during the prestressing stage, the overall deflections of the beam are also reduced significantly. In the present paper, finite-element (FE) modeling was employed to simulate the mechanical behavior of prestressed cold-formed steel beams during the prestressing and vertical loading stages. Following the validation of the FE modeling approach, a set of parametric studies was conducted, where the influence of the key controlling parameters on the structural benefits obtained from the prestressing process was investigated. The parametric results were utilized to determine how the benefits obtained from the addition of the prestressed cable can be maximized, demonstrating the significant enhancements in the performance of the cold-formed steel beam that can be achieved.

Journal article

Tan QH, Gardner L, Han LH, Song TYet al., 2019, Fire performance of steel reinforced concrete-filled stainless steel tubular (CFSST) columns with square cross-sections, Thin-Walled Structures, Vol: 143, ISSN: 0263-8231

Concrete-filled stainless steel tubular (CFSST) columns combine the advantages of composite action seen in concrete-filled steel tubular (CFST) columns with the durability benefits associated with stainless steel. An effective means of reducing the material usage in the outer stainless steel tube in CFSST columns is to embed an inner carbon steel profile. This enables the material costs to be reduced, while achieving similar load-bearing capacity and durability, as well as enhanced fire resistance. The behaviour of such steel reinforced composite columns, i.e. concrete-filled stainless steel tubular (CFSST) columns with outer square cross-sections and embedded carbon steel profiles, under ISO 834 standard fire conditions is investigated in this study by finite element (FE) analysis. Firstly, FE models are developed and validated against relevant published experimental data on CFSST and steel reinforced CFST columns under fire conditions. Based on the validated FE models, the working mechanisms of the studied steel reinforced CFSST columns under fire conditions are investigated by analysis of the temperature field, failure modes, axial deformation versus time response and internal force distribution. The fire performance of the studied steel reinforced CFSST columns is also evaluated in comparison with CFST and CFSST columns with the same total cross-sectional area of steel or the equivalent cross-sectional load-bearing capacity at ambient temperature. Finally, with respect to fire resistance, the optimal ratio of the cross-sectional areas of the inner carbon steel profile to the outer stainless steel tube is investigated.

Journal article

Kucukler M, Gardner L, 2019, Design of hot-finished tubular steel members using a stiffness reduction method, Journal of Constructional Steel Research, Vol: 160, Pages: 340-358, ISSN: 0143-974X

A stiffness reduction method (SRM) for the design of hot-finished tubular steel members is presented in this paper. Stiffness reduction functions that fully capture the adverse influence of imperfections and plasticity on member stability are developed. The proposed SRM is implemented by (i) reducing the flexural stiffness (EI) of the member using the developed stiffness reduction functions, (ii) performing elastic Linear Buckling Analysis (LBA) and Geometrically Nonlinear Analysis (GNA) of the member with reduced flexural stiffness and (iii) making cross-section strength checks and ensuring that the lowest buckling load amplifier from LBA is greater than or equal to 1.0. Owing to the full allowance for the spread of plasticity, residual stresses and geometrical imperfections through stiffness reduction and instability effects through LBA and GNA, the proposed approach offers an enhanced and more direct assessment of structural behaviour relative to traditional design where structural analysis is accompanied by member design equations, effective lengths and the notional load concept. The proposed method is verified against nonlinear finite element modelling for a large number of tubular steel members. Comparisons of the proposed approach against the methods recommended in the European structural steel design code EN 1993-1-1 for the design of tubular members are also provided.

Journal article

Fieber A, Gardner L, Macorini L, 2019, Formulae for determining elastic local buckling half-wavelengths of structural steel cross-sections, Journal of Constructional Steel Research, Vol: 159, Pages: 493-506, ISSN: 0143-974X

Formulae for determining elastic local buckling half-wavelengths of structural steel I-sections and box sections under compression, bending and combined loading are presented. Knowledge of local buckling half-wavelengths is useful for the direct definition of geometric imperfections in analytical and numerical models, as well as in a recently developed strain-based advanced analysis and design approach (Gardner et al., 2019a, 2019b). The underlying concept is that the cross-section local buckling response is bound by the theoretical behaviour of the isolated cross-section plates with simply-supported and fixed boundary conditions along their adjoined edges. At the isolated plate level, expressions for the half-wavelength buckling coefficient k Lb , which defines the local buckling half-wavelength of a plate as a multiple of its width b, taking into account the effects of the boundary conditions and applied loading, have been developed based on the results of finite strip analysis. At the cross-sectional level, element interaction is accounted for through an interaction coefficient ζ that ranges between 0 and 1, corresponding to the upper (simply-supported) and lower (fixed) bound half-wavelength envelopes of the isolated cross-section plates. The predicted half-wavelengths have been compared against numerical values obtained from finite strip analyses performed on a range of standard European and American hot-rolled I-sections and square/rectangular hollow sections (SHS/RHS), as well as additional welded profiles. The proposed approach is shown to predict the cross-section local buckling half-wavelengths consistently to within 10% of the numerical results.

Journal article

Wang F, Young B, Gardner L, 2019, Compressive testing and numerical modelling of concrete-filled double skin CHS with austenitic stainless steel outer tubes, Thin-Walled Structures, Vol: 141, Pages: 345-359, ISSN: 0263-8231

A comprehensive experimental and numerical study of concrete-filled double skin tubular (CFDST)stub columns is presented in this paper. A total of 23 tests was carried out on CFDST specimens with austenitic stainless steel circular hollow section (CHS)outer tubes, high strength steel CHS inner tubes, and three different grades of concrete infill (C40, C80 and C120). The ultimate load, load-deflection histories and failure modes of the stub columns are reported. The test results were employed in a parallel numerical simulation programme for the validation of the finite element (FE)model, by means of which an extensive parametric study was undertaken to extend the available results over a wide range of cross-section slendernesses, inner tube strengths and concrete grades. The experimentally and numerically derived data were then employed to assess the applicability of the existing European, Australian and North American design provisions for composite carbon steel members to the design of the studied CFDST cross-sections. Overall, the existing design rules are shown to provide generally safe-sided (less so for the higher concrete grades)but rather scattered capacity predictions. Use of an effective concrete strength is recommended for the higher concrete grades and shown to improve the consistency of the design capacity predictions.

Journal article

Gardner L, 2019, Stability and design of stainless steel structures – Review and outlook, Thin-Walled Structures, Vol: 141, Pages: 208-216, ISSN: 0263-8231

This paper provides a review of recent developments in research and design practice surrounding the structural use of stainless steel, with an emphasis on structural stability. The nonlinear stress-strain characteristics of stainless steel, which are discussed first, give rise to a structural response that differs somewhat from that of structural carbon steel. Depending on the type and proportions of the structural element or system, the nonlinear material response can lead to either a reduced or enhanced capacity relative to an equivalent component featuring an elastic, perfectly plastic material response. In general, in strength governed scenarios, such as the in-plane bending of stocky beams, the substantial strain hardening of stainless steel gives rise to capacity benefits, while in stability governed scenarios, the early onset of stiffness degradation results in reduced capacity. This behaviour is observed at all levels of structural response including at cross-sectional level, member level and frame level, as described in the paper. Current and emerging design approaches that capture this response are also reviewed and evaluated. Lastly, with a view to the future, the application of advanced analysis to the design of stainless steel structures and the use of 3D printing for the construction of stainless steel structures are explored.

Journal article

Kyvelou P, Gardner L, Nethercot DA, 2019, Impact statement on “Design of composite cold-formed steel flooring systems”, Structures, Vol: 20, Pages: 213-213, ISSN: 2352-0124

Journal article

Kyvelou P, Kyprianou C, Gardner L, Nethercot DAet al., 2019, Challenges and solutions associated with the simulation and design of cold-formed steel structural systems, Thin-Walled Structures, Vol: 141, Pages: 526-539, ISSN: 0263-8231

The treatment of cold-formed steel sections in design codes is very largely restricted to individual members under ideal conditions. More efficient design is possible if the complexities of the structural response caused by the thin plating and complex shapes, together with the actual conditions of load introduction and restraint arising from practical situations can be recognised. Traditionally this has only been possible by resorting to full-scale testing. This is, of course, time consuming and expensive; moreover, the impossibility of covering all variations of all the important problem parameters means that developing a comprehensive understanding of all aspects of the physical behaviour is unlikely. Numerical analysis offers the promise of an alternative approach. However, for this to be reliable there must be confidence that it accurately models the physical situation. For the past decade a programme of research has been underway aimed at the provision of a more complete understanding of the structural behaviour of cold-formed steel sections when employed in particular practical situations. Three such cases are addressed herein: purlins as used in the roofs of industrial buildings, beams used to support floors and columns forming part of a stud wall framing system. In each case the process has been to firstly identify all the important structural components including fastening arrangements, then to develop numerical models using ABAQUS that represent each of these physical features to a sufficient degree of accuracy, then to validate the models by comparison with all available test data, then to conduct parametric studies covering the full range of variables found in practice and, finally, to use the pool of results and the improved insights into behaviour as the basis for improved design approaches that, by more accurately capturing the key physical features, provide better predictions of performance. An important feature of this has been to ensure that the r

Journal article

Meng X, Gardner L, Testing of hot-finished high strength steel SHS and RHS under combined compression and bending, Thin-Walled Structures, ISSN: 0263-8231

An experimental investigation into the cross-sectional behaviour of hot-finished high strength steel tubular sections, to support the assessment and development of structural design guidance, is presented. Two grades of quenched and tempered high strength steel – S690 and S770 and eight cross-sections – seven square hollow sections (SHS) and one rectangular hollow section (RHS), covering a wide range of local slenderness, were examined. This test programme consisted of twelve tensile coupon tests, five stub column tests and 30 short beam-column tests, with various initial loading eccentricities employed to achieve a spectrum of compression and bending combinations. Local geometric imperfection measurements were carried out on each test specimen using 3D laser-scanning, and a rational approach to analysing the local imperfection distributions using the scanned data was subsequently proposed. Following the experimental study, the current Eurocode 3 cross-section design provisions were evaluated through comparisons with the results from the present research and additional results from the literature. The experimental results and findings from the present study provide a basis for the development of numerical models and the enhancement of existing design methods in the future.

Journal article

Kucukler M, Gardner L, 2019, Design of web-tapered steel beams against lateral-torsional buckling through a stiffness reduction method, Engineering Structures, Vol: 190, Pages: 246-261, ISSN: 0141-0296

A stiffness reduction method for the lateral-torsional buckling (LTB) assessment of welded web-tapered steel beams is presented in this study. The method is implemented by (i) modelling a tapered steel beam using elastic beam finite elements specifically developed to represent the elastic instability response of tapered steel members, (ii) reducing the Young's modulus E and shear modulus G of each element through a stiffness reduction function considering the bending moments and cross-section properties at the middle of each element and (iii) performing an elastic Linear Buckling Analysis of the beam with reduced stiffness, referred to as LBA-SR herein. Since the adverse influence of the development of plasticity and imperfections on the ultimate member strengths are fully accounted for through stiffness reduction, the presented method does not require any further global instability assessment using member design equations; thus, the proposed method is both direct and practical. Verification of the method is shown for a wide range of web-tapered steel beams using results from nonlinear shell finite element modelling.

Journal article

Hadjipantelis N, Gardner L, Wadee MA, 2019, Design of prestressed cold-formed steel beams, Thin-Walled Structures, Vol: 140, Pages: 565-578, ISSN: 0263-8231

Structural design rules for prestressed cold-formed steel beams, considering both the prestressing and imposed vertical loading stages, are presented herein. In the proposed approach, the cold-formed steel member is designed as a beam-column using linear interaction equations in conjunction with the Direct Strength Method (DSM), while the prestressed cable is designed by ensuring that its tensile capacity is not violated during the two loading stages. In the present paper, the design approach and the failure criteria, which define the permissible design zone for the prestressed system, are first introduced. The suitability of the design recommendations is then assessed by comparing a set of parametric finite element (FE) results for several combinations of prestress levels, beam geometries and cable sizes, with the corresponding design predictions. Finally, following reliability analysis, the implementation of the design recommendations is illustrated through a practical worked example.

Journal article

McCann F, Gardner L, 2019, Numerical analysis and design of slender elliptical hollow sections in bending, Thin-Walled Structures, Vol: 139, Pages: 196-208, ISSN: 0263-8231

The local buckling behaviour and ultimate cross-sectional resistance of slender tubular elliptical profiles in bending are examined by means of numerical modelling. After successful validation of the numerical model against previous experimental results, a parametric study comprising 240 simulations was conducted in order to investigate the influence of cross-section aspect ratio, axis of bending, geometric imperfections and local slenderness on structural behaviour. The ultimate moments, moment–curvature relationships and failure modes obtained are discussed. It was found that, overall, postbuckling stability increases and imperfection sensitivity decreases with increasing elliptical hollow section (EHS) aspect ratio. A design method is proposed for Class 4 EHS members that reflects the reduction in resistance due to local buckling with increasing slenderness and extends the range of applicability of existing provisions. A reliability analysis was performed in accordance with EN 1990, indicating that the design methods for EHS in bending, in addition to previous design methods for EHS in compression, are suitable for use in the Eurocode framework with a recommended partial factor of unity.

Journal article

dos Santos GB, Gardner L, 2019, Testing and numerical analysis of stainless steel I-sections under concentrated end-one-flange loading, Journal of Constructional Steel Research, Vol: 157, Pages: 271-281, ISSN: 0143-974X

A comprehensive investigation into the structural behaviour of austenitic stainless steel welded I-section beams under concentrated transverse end-one-flange loading is reported herein. Ten physical experiments are first described. The experimental results are then presented in terms of the full load-web shortening responses, ultimate loads, out-of-plane deformation fields and failure modes. An extensive finite element modelling study accounting for geometric, material and contact non-linearities was also performed. After successful model validation against the test results, a parametric investigation was conducted considering a range of bearing lengths, different distances of the bearing load to the member end and web slenderness values. The combined experimental and numerical data set was used to assess current European and North American design provisions for the resistance of stainless steel welded I-sections to concentrated end-one-flange loading. The results show that the current design formulae generally lead to safe-sided but rather scattered and conservative capacity predictions with considerable scope for the development of improved design formulae.

Journal article

Hadjipantelis N, Kyvelou P, Gardner L, Wadee Met al., Numerical modelling of prestressed composite cold-formed steel flooring systems, Seventh International Conference in Structural Engineering, Mechanics and Computation, Publisher: CRC Press

A novel and highly-efficient prestressed composite flooring system comprising cold-formed steel joists and wood-based floorboards is introduced herein. The prestressing is applied by means of a high-strength steel cable housed within the bottom hollow flange of the steel joist, while the composite action is mobilised by making simple alterations to the currently employed fastening arrangements between the joist and the board. Geometrically and materially nonlinear finite element models with initial geometric imperfections have been developed to simulate the behaviour of the proposed system during the prestressing and vertical loading stages. The structural performance of the prestressed system is compared with that of conventional non-prestressed systems, demonstrating that substantial benefits can be achieved both in terms of load-carrying capacity and serviceability performance. Subsequently, a parametric study is conducted to investigate the effect of the steel section thickness on the ultimate moment capacity and bending stiffness of the system.

Conference paper

Gardner L, Yun X, Fieber A, Macorini Let al., 2019, Steel design by advanced analysis: material modeling and strain limits, Engineering, Vol: 5, Pages: 243-249, ISSN: 2095-8099

Structural analysis of steel frames is typically performed using beam elements. Since these elements are unable to explicitly capture the local buckling behavior of steel cross-sections, traditional steel design specifications use the concept of cross-section classification to determine the extent to which the strength and deformation capacity of a cross-section are affected by local buckling. The use of plastic design methods are restricted to Class 1 cross-sections, which possess sufficient rotation capacity for plastic hinges to develop and a collapse mechanism to form. Local buckling prevents the development of plastic hinges with such rotation capacity for cross-sections of higher classes and, unless computationally demanding shell elements are used, elastic analysis is required. However, this article demonstrates that local buckling can be mimicked effectively in beam elements by incorporating the continuous strength method (CSM) strain limits into the analysis. Furthermore, by performing an advanced analysis that accounts for both geometric and material nonlinearities, no additional design checks are required. The positive influence of the strain hardening observed in stocky cross-sections can also be harnessed, provided a suitably accurate stress–strain relationship is adopted; a quad-linear material model for hot-rolled steels is described for this purpose. The CSM strain limits allow cross-sections of all slenderness to be analyzed in a consistent advanced analysis framework and to benefit from the appropriate level of load redistribution. The proposed approach is applied herein to individual members, continuous beams, and frames, and is shown to bring significant benefits in terms of accuracy and consistency over current steel design specifications.

Journal article

Yang L, Zhao M, Gardner L, Ning K, Wang Jet al., 2019, Member stability of stainless steel welded I-section beam-columns, Journal of Constructional Steel Research, Vol: 155, Pages: 33-45, ISSN: 0143-974X

A comprehensive experimental and numerical study is presented into the behaviour of stainless steel welded I-section beams-columns. Twenty test specimens were fabricated from grade 304 (EN 1.4301) austenitic and grade 2205 (EN 1.4406) duplex stainless steel plates – ten were tested under major axis bending plus compression and ten under minor axis bending plus compression. Material tensile coupon tests and geometric imperfection measurements were also conducted. Numerical models were developed, calibrated against the test results and subsequently employed in parametric studies considering a wider range of specimen geometries. Based on the obtained test and numerical results, the accuracy and reliability of existing design rules given in EN 1993-1-4 and AISC DG 27, as well as recent proposals, were assessed.

Journal article

Afshan S, Theofanous M, Wang J, Gkantou M, Gardner Let al., 2019, Testing, numerical simulation and design of prestressed high strength steel arched trusses, Engineering Structures, Vol: 183, Pages: 510-522, ISSN: 0141-0296

The structural behaviour of prestressed high strength steel arched trusses is studied in this paper through experimentation and numerical modelling. Four 11 m span prestressed arched trusses fabricated from S460 hot finished square hollow section members were loaded vertically to failure. Three of the tested trusses were prestressed to different levels by means of a 7-wire strand cable housed within the bottom chord, while the fourth truss contained no cable and served as a control specimen. Each truss was loaded at five points coinciding with joint locations along its span, and the recorded load-deformation responses at each loading point are presented. Inclusion and prestressing of the cable was shown to delay yielding of the bottom chord and enhance the load carrying capacity of the trusses, which ultimately failed by either in-plane or out-of-plane buckling of the top chord. For the tested trusses, around 40% increases in structural resistance were achieved through the addition of the cable, though the self-weight was increased by only approximately 3%. In parallel with the experimental programme, a finite element model was developed and validated against the test results. Upon successful replication of the experimentally observed structural response of the trusses, parametric studies were conducted to investigate the effect of key parameters such as prestress level, material grade and the top chord cross-section on the overall structural response. Based on both the experimental and numerical results, design recommendations in the form of simple design checks to be performed for such systems are provided.

Journal article

Buchanan C, Gardner L, 2019, Metal 3D printing in construction: a review of methods, research, applications, opportunities and challenges, Engineering Structures, Vol: 180, Pages: 332-348, ISSN: 0141-0296

3D printing, more formally known as additive manufacturing (AM), has the potential to revolutionise the construction industry, with foreseeable benefits including greater structural efficiency, reduction in material consumption and wastage, streamlining and expedition of the design-build process, enhanced customisation, greater architectural freedom and improved accuracy and safety on-site. Unlike traditional manufacturing methods for construction products, metal 3D printing offers ready opportunities to create non-prismatic sections, internal stiffening, openings, functionally graded elements, variable microstructures and mechanical properties through controlled heating and cooling and thermally-induced prestressing. Additive manufacturing offers many opportunities for the construction sector, but there will also be fresh challenges and demands, such as the need for more digitally savvy engineers, greater use of advanced computational analysis and a new way of thinking for the design and verification of structures, with greater emphasis on inspection and load testing. It is envisaged that AM will complement, rather than replace, conventional production processes, with clear potential for hybrid solutions and structural strengthening and repairs. These opportunities and challenges are explored in this paper as part of a wider review of different methods of metal 3D printing, research and early applications of additive manufacturing in the construction industry. Lessons learnt for metal 3D printing in construction from additive manufacturing using other materials and in other industries are also presented.

Journal article

Gardner L, Fieber A, Macorini L, 2019, Formulae for calculating elastic local buckling stresses of full structural cross-sections, Structures, Vol: 17, Pages: 2-20, ISSN: 2352-0124

Formulae for determining the full cross-section elastic local buckling stress of structural steel profiles under a comprehensive range of loading conditions, accounting for the interaction between the individual plate elements, are presented. Element interaction, characterised by the development of rotational restraint along the longitudinal edges of adjoined plates, is shown to occur in cross-sections comprising individual plates with different local buckling stresses, but also in cross-sections where the isolated plates have the same local buckling stress but different local buckling half-wavelengths. The developed expressions account for element interaction through an interaction coefficient ζ that ranges between 0 and 1 and are bound by the theoretical limits of the local buckling stress of the isolated critical plates with simply-supported and fixed boundary conditions along the adjoined edges. A range of standard European and American hot-rolled structural steel profiles, including I-sections, square and rectangular hollow sections, channel sections, tee sections and angle sections, as well as additional welded profiles, are considered. The analytical formulae are calibrated against results derived numerically using the finite strip method. For the range of analysed sections, the elastic local buckling stress is typically predicted to within 5% of the numerical value, whereas when element interaction is ignored and the plates are considered in isolation with simply-supported boundary conditions along the adjoined edges, as is customary in current structural design specifications, the local buckling stress of common structural profiles may be under-estimated by as much as 50%. The derived formulae may be adopted as a convenient alternative to numerical methods in advanced structural design calculations (e.g. using the direct strength method or continuous strength method) and although the focus of the study is on structural steel sections, the functions are

Journal article

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