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

576 results found

Jing Y, Jiang K, Zhao O, Gardner Let al., 2024, Web crippling of stainless steel built-up I-sections under End-Two-Flange loading: tests, simulations and design, Engineering Structures, Vol: 304, ISSN: 0141-0296

This paper presents experimental and numerical investigations into the web crippling behaviour and load-carrying capacity of stainless steel built-up I-section members under End-Two-Flange (ETF) concentrated transverse loading. An experimental programme was conducted on 23 austenitic and duplex stainless steel built-up I-section specimens, with each consisting of two identical press-braked channel sections assembled back-to-back by means of self-tapping screws. Following the experimental programme, a numerical modelling programme was performed, where finite element models were firstly developed to replicate the tests and then used to conduct parametric studies. Based on the test and numerical data, the existing design rules for stainless steel built-up I-section members failing by web crippling, as set out in the current European, American and Australian/New Zealand design standards, were assessed. The comparisons indicated that the European and Australian/New Zealand design standards resulted in overly conservative and rather scattered failure load predictions, while the American design standard led to less scattered failure load predictions, but with many predictions on the unsafe side. Therefore, an improved design approach was proposed and shown to provide substantially improved failure load predictions for stainless steel built-up I-section members under End-Two-Flange loading.

Journal article

Walport F, Zhu Y, Yun X, Gardner Let al., 2024, Experimental and numerical datasets for benchmark tests on high strength steel I-section frames, Data in Brief, Vol: 53, ISSN: 2352-3409

This data article presents experimental and numerical datasets for eight fixed-base, single storey, unbraced high strength steel welded I-section frames subjected to in-plane horizontal and vertical loading. A detailed description of the full-scale frame testing programme is provided in the related research article ‘Benchmark tests on high strength steel frames’. The experimental dataset can be used to steer future research in full-scale structural testing and provide benchmark results that are suitable for the validation of finite element models and the development of system-level design approaches. In addition to the benchmark experimental frame data, all necessary details and data for shell finite element (FE) model validation using geometrically and materially nonlinear analysis (GMNIA) is presented. The general purpose FE software Abaqus was used. The dataset can be used as an illustrative example of GMNIA validation in accordance with EN 1993–1–14 with all relevant data for reproducibility provided.

Journal article

Meng X, Zhi J, Xu F, Gardner Let al., 2024, Novel hybrid sleeve connections between 3D printed and conventional tubular steel elements, Engineering Structures, Vol: 302, ISSN: 0141-0296

Metal additive manufacturing, or metal 3D printing, is an emerging technology within the construction sector that is suitable for hybrid use with conventional manufacturing methods to maximise its benefits. In this context, a novel sleeve connection between metal 3D printed parts and conventionally produced circular hollow section (CHS) members for use in hybrid tubular structures is proposed and experimentally verified in the present study. Hot-finished S355 steel CHS profiles were adopted, while the 3D printed parts were produced by cold metal transfer (CMT)-based wire arc additive manufacturing (WAAM) using ER70S feedstock material. The geometric features of the WAAM specimens were analysed based on 3D laser scan data. Tensile tests on two as-built coupon specimens were carried out to examine the mechanical properties of the printed material. A total of nine hybrid sleeve connection specimens with a variety of bolt layouts were then examined. Initial free rotations in the connections were firstly measured, and their structural performances under axial tension were determined through tensile testing. Digital image correlation (DIC) was employed in both the material and connection tests for full-field measurements of strain and displacement. The examined hybrid sleeve connection specimens exhibited different failure modes (i.e. shear-out and net section fracture in the WAAM parts) and in general, excellent tensile load-carrying and deformation capacities. Compared with the obtained test results, the EC3 resistance predictions were shown to be overly conservative for shear-out failure, but can be optimistic in the cases of net section fracture. Future research is needed to improve the EC3 design rules for hybrid sleeve connections.

Journal article

Kyvelou P, Spinasa A, Gardner L, 2024, Testing and analysis of optimised wire arc additively manufactured steel trusses, Journal of Structural Engineering, Vol: 150, ISSN: 0733-9445

Wire arc additive manufacturing (WAAM) is a method of metal 3D printing that can be used to fabricate large-scale elements of complex form, opening up opportunities to unleash the full potential ofstructural optimisation. To demonstrate this potential, optimised steel trusses have been fabricated bymeans of WAAM and tested to investigate their structural performance. The tested metal 3D printed specimens, which are considered to be the first optimised WAAM structural systems of this kind and scale, comprised members of tubular cross-section with varying diameters and thicknesses. Three different configurations were considered: cantilever, propped cantilever and simply supported, all 2 m in length. Repeat tests enabled the variability in response between specimens to be evaluated; 6 optimised trusses were tested in total. The geometric features of the tested specimens were determined using 3D laser scanning, while digital image correlation was employed to monitor the displacement and strain fields during testing. Full details of the experimental programme are provided; the obtainedresults are analysed and comparisons against equivalent conventional reference designs are made. The results of complementary numerical simulations, undertaken to gain further insight into particular features of the structural response of the examined specimens, are also presented. The structural efficiency of the optimised trusses, as measured by the capacity-to-mass ratio, was found to be up to 95% higher than that of the corresponding reference designs, underlining the benefits in terms of structural efficiency that can be achieved by the combination of WAAM with optimisation methods.

Journal article

Meza F, Baddoo N, Gardner L, 2024, Derivation of stainless steel material factors for European and US design standards, Journal of Constructional Steel Research, Vol: 213, ISSN: 0143-974X

Modern structural design standards are based on the Limit State Design (LSD) philosophy, also known as Load and Resistance Factor Design (LRFD), which involves the use of resistance functions with accompanying safety factors that are based on a statistical evaluation of relevant experimental data, carried out within the framework of a probabilistic reliability theory. This paper presents the derivation of material factors for the yield strength and ultimate tensile strength of austenitic, duplex and ferritic stainless steels, and their associated coefficients of variation (COV) for use in evaluating the safety factors for the European design code EN 1993-1-4 and the American design codes Specification for Structural Stainless Steel Buildings (ANSI/AISC 370–21) and Specification for the Design of Cold-Formed Stainless Steel Structural Members (ASCE/SEI 8–22). Separate material factors and associated COV values are required for the U.S. and European design codes due to the different minimum specified strength values that are given in the respective product standards. The material factors were derived by analysing a large amount of material data provided by different stainless steel producers, which encompassed a wide range of stainless steels and material thicknesses.

Journal article

Yang Z, Meng X, Walport F, Gardner Let al., 2024, Flexural capacity and local buckling half-wavelength of high strength steel tubular beams under moment gradients: an experimental study, Thin Walled Structures, Vol: 195, ISSN: 0263-8231

An experimental investigation into the effect of moment gradients on the flexural behaviour of hot-rolled high strength steel square hollow section (SHS) beams is presented in this paper. In total, 20 beam specimens in steel grades S690 and S770, and with cross-sections spanning from Class 2 to Class 4 based on the Eurocode 3 slenderness limits, were tested under three- and four-point bending. In the three-point bending tests, the beam spans were varied to achieve a range of moment gradients; the influence of different stiffening arrangements at the loading point was also considered. Local geometric imperfections were measured by means of 3D laser-scanning prior to testing and digital image correlation (DIC) was adopted to monitor the displacement and strain fields at critical regions and to assess the local buckling half-wavelengths of the test specimens for which a consistent measurement approach was proposed. The measured local buckling half-wavelengths were compared against the elastic local buckling half-wavelengths calculated using the finite strip method. It was observed that while the measured local buckling half-wavelengths remained approximately constant up to first yield, a significant reduction in half-wavelength was observed with increasing moment due to the non-uniform spread of plasticity. The comparisons also revealed that the local buckling half-wavelengths reduced with both the presence of moment gradients and intermediate stiffeners, with a new parameter proposed to quantify the local moment gradient. It was shown from the tests that the specimens subjected to moment gradients, despite the presence of shear, exhibited higher ultimate moment capacities (up to 10.5% for stiffened specimens and 3.4% for unstiffened specimens) than those subjected to uniform moments. This is attributed to the delay in the local buckling of the critical cross-section of beams under moment gradients due to the restraint provided by the less heavily stressed adjacent cro

Journal article

Wang Z, Hou Y, Huang C, Han Q, Zong L, Chen M-T, Deng K, Gardner Let al., 2024, Experimental study and constitutive modelling of wire arc additively manufactured steel under cyclic loading, Journal of Constructional Steel Research, Vol: 213, ISSN: 0143-974X

Wire arc additive manufacturing (WAAM) is an efficient and cost-effective method of metal 3D printing that is well suited to structural engineering applications. Fundamental test data on the mechanical properties of WAAM materials, especially under cyclic loading, are however lacking. To address this, an experimental study into the cyclic behaviour of WAAM steel plates has been undertaken and is reported herein. Following geometric and quasi-static mechanical characterisation, a total of 40 as-built and machined coupons were tested under different cyclic loading protocols. Key results from the tests, including the full hysteresis curves, are presented and discussed. A cyclic constitutive model allowing for a yield plateau and the degradation of elastic modulus is proposed and validated against the test data. It is shown that the examined WAAM steel exhibited cyclic hardening behaviour dependent on the strain amplitude and strain history. The elastic modulus of the WAAM material was found to decrease in the first few cycles and then remain stable with increasing accumulated plastic strain. Good energy dissipation performance was also observed, indicating the potential of WAAM steels for seismic applications.

Journal article

Kyvelou P, Huang C, Li J, Gardner Let al., 2024, Residual stresses in steel I-sections strengthened by wire arc additive manufacturing, Structures, Vol: 60, ISSN: 2352-0124

Wire arc additive manufacturing (WAAM) is a method of metal 3D printing that can be used to complement traditional steel manufacturing techniques (e.g. hot-rolling and cold-forming), to produce hybrid structural elements of enhanced efficiency. The case of hot-rolled steel I-sections strengthened by the addition of WAAM stiffeners at the flange tips is the focus of the present study. In addition to the improved local buckling resistance and increased flexural rigidity achieved by the addition of the stiffeners, a further source of enhanced load-bearing capacity arises from the inversion, through thermal action, of the adverse residual stress pattern typically associated with hot-rolled I-sections. Thus, in contrast to previous studies, the aim of the present investigation is not to eliminate but to take benefit from residual stresses, by prestressing (pretensioning) specific parts (i.e. the flange tips) of steel sections against subsequent applied compressive loading, thereby delaying yielding and enhancing the load-bearing capacity. The described hypothesis is demonstrated in the present study through the manufacture of a series of hybrid WAAM-strengthened hot-rolled I-sections and the subsequent performance of residual stress measurements. Stiffeners of different dimensions were printed using ER70S-6 welding wire. A detailed description of the adopted experimental technique, which utilised the sectioning method, is provided, while the magnitude and distribution of the measured residual stresses are reported. The addition of WAAM stiffeners at the flange tips of hot-rolled I-sections is shown to lead to the generation of desirable (for structural stability) tensile residual stresses in their vicinity, with peak values reaching the yield strength of the material; self-equilibrating compressive residual stresses are induced in the web and around the web-to-flange junction.

Journal article

Quan C, Walport F, Gardner L, 2024, Equivalent geometric imperfections for the design of steel and stainless steel beam-columns by GMNIA, Journal of Constructional Steel Research, ISSN: 0143-974X

Journal article

Su A, Wang Y, Wang Y, Rasmussen K, Gardner Let al., 2024, Behaviour and design of S960 ultra-high strength steel non-slender welded I-section beam–columns, Engineering Structures, ISSN: 0141-0296

The structural behaviour of S960 ultra-high strength steel (UHSS) welded I-section beam–columns is studied in the present paper. Nonlinear finite element (FE) models were first created and validated against relevant test results collected from the literature; the models were then used to carry out parametric studies to obtain additional FE data over an extensive spectrum of cross-section dimensions, member lengths and loading combinations. Given the lack of existing design rules for S960 UHSS structures, the codified beam–column interaction curves prescribed in the current European (EC3) and American (AISC 360) design specifications for lower strength steel members, were assessed for their applicability to the studied S960 UHSS non-slender welded I-section members, utilizing the generated numerical data. It was found that EC3 yielded conservative failure load predictions for Class 1 and 2 S960 UHSS welded I-section beam–columns buckling about either principal axis; similar results were found for Class3 members under combined compression and strong-axis bending, with the mean FE to predicted failure load ratio being 1.29, the corresponding COV 0.12, but rather conservative strength predictions arose for Class 3 members under combined compression and weak-axisbending, with the mean FE to predicted failure load ratio being 1.50. AISC 360 led to overall accurate and consistent failure load predictions for S960 UHSS non-slender welded I-section beam–columns, but resulted in some unconservative strength predictions for members undercombined compression and minor-axis bending. New design proposals, adopting the format of EC3 design interaction curve, but with more accurate compression and bending end points, interaction factors and buckling reduction factors determined from the continuous strengthmethod (CSM), were proposed and shown to lead to improved design accuracy.

Journal article

Weber B, Meng X, Zhang R, Nitawaki M, Sagawa T, Gardner Let al., 2024, Tensile behaviour of WAAM high strength steel material and members, Materials and Design, Vol: 237, ISSN: 0264-1275

Wire arc additive manufacturing (WAAM), a method of metal 3D printing, has the capacity to create large scale elements suitable for construction applications with a high degree of design freedom and structural efficiency. There is currently however a lack of fundamental experimental data on the material and structural performance of such elements. Towards addressing this limitation, the tensile behaviour of WAAM high strength steel produced using different printing strategies is the focus of the present study. WAAM steel plates and tubular tension members manufactured with different interpass temperatures and toolpaths using ER110S-G welding wire were examined. A total of 60 tensile coupons, consisting of 40 as-built and 20 machined specimens, and 8 as-built circular hollow section (CHS) tension members, were tested. The examined WAAM materials were found to exhibit very little anisotropy, corroborated by a nearly homogeneous crystallographic texture observed by microstructural analysis, while the inherent surface undulations were shown to result in a varying degree of reduction in the material stiffness, strength and ductility at different angles to the print layer orientation. The different printing strategies led to varying surface geometries; combined with different interpass temperatures, they also resulted in different thermal histories and thus different mechanical properties. The tension members showed good structural resistance, but a considerable reduction in ductility compared to the coupon tests, due to the greater geometric variability and manufacturing defects.

Journal article

Walport F, Zhang R, Meng X, Gardner Let al., 2023, The softening effect of welding on the mechanical properties of cold-worked stainless steel, Journal of Constructional Steel Research, Vol: 211, ISSN: 0143-974X

Cold-working can occur during production of flat sheet products and during fabrication of structural cross-sections. In both cases, when the material is cold-worked, plastic deformations result in material strength enhancements. These strength enhancements are particularly significant for materials such as stainless steel, which exhibit rounded stress-strain behaviour and pronounced strain hardening, and for hollow structural sections, where the strength increases arise in both the corner and flat regions of the cross-sections. Numerous studies have shown the importance of utilising this strength enhancement for efficient structural design, and predictive models have been derived for harnessing these enhancements. However, if cold-worked material is welded, some of the enhanced strength can be lost due to partial softening in the heat affected zone (HAZ). The extent of this strength loss is investigated experimentally in the present study. The experimental programme comprised ten tensile coupon tests on 1 mm thick austenitic (Grade 1.4301) stainless steel sheet material, with central welds parallel and transverse to the direction of cold-rolling, as well as twelve full cross-section tensile tests on 80x60 mm and 60x60 mm hollow sections with thicknesses varying between 2 and 4 mm. Digital image correlation was utilised to determine the local constitutive properties of the base metal, the heat affected zone, and the weld metal of each specimen. The hardness and microstructure of the welded samples, along with the widths of the weld and heat affected zone, were also characterised.

Journal article

Guo X, Kyvelou P, Ye J, Gardner Let al., 2023, Experimental investigation of wire arc additively manufactured steel T-stub connections, Journal of Constructional Steel Research, Vol: 211, Pages: 1-22, ISSN: 0143-974X

This paper presents an experimental investigation into the structural response of wire arc additively manufactured (WAAM) steel T-stub connections subjected to tension. A total of eighty T-stub specimens featuring different printing strategies, bolt arrangements and geometric configurations were tested to failure. 3D laser scanning was employed to determine the specimen geometry, while digital image correlation (DIC) was used to monitor the strain and displacement fields during testing. Utilising the DIC data, a new method was introduced for the determination of the failure mode and key points on the load-deformation response of T-stub connections. The structural response of the WAAM T-stubs generally followed the anticipated trends, though the geometric irregularities resulted in a delay in the development of prying action in some specimens and the printing strategy was shown to have a clear influence on the load-carrying capacity. Finally, comparisons between the test results and the capacity predictions yielded by existing design equations were made; overall, reasonable agreement was achieved, but further research is required to establish reliable design rules that are specific to this form of manufacture.

Journal article

Wang F, Young B, Gardner L, 2023, Testing, numerical analysis and design of CFDST cross-sections with square stainless steel outer tubes in bending, Journal of Constructional Steel Research, Vol: 211, Pages: 1-14, ISSN: 0143-974X

The structural performance and design of concrete-filled double skin tubular (CFDST) cross-sections with square stainless steel outer tubes are studied herein. A total of 17 four-point bending tests on CFDST cross-sections with varying concrete grades, together with accompanying material tests, were first conducted. The details of the test rig and procedures, as well as the key experimental results are reported. Following the physical testing, a numerical modelling campaign was carried out. A finite element (FE) model was initially validated against the tests, and then adopted to conduct a parametric study to acquire further FE data, covering a broader spectrum of material strengths and cross-section slendernesses. The obtained test and FE results were used to evaluate the applicability of the general design provisions for concrete-filled carbon steel members in the current European and American design codes. Overall, the examined design codes are shown to provide unduly conservative (less so for the higher concrete grades) and rather scattered moment resistance predictions, though some moment resistances predicted using the European code were on the unsafe side. Modifications to the European design treatment in relation to the assumed stress distribution, to take due account of the partial spread of plasticity in the outer tube, and the effective compressive strength of the concrete infill, to reflect the reduced relative effectiveness of using higher concrete grades, are proposed and shown to improve the consistency of the resistance predictions.

Journal article

Ye J, Guo Q, Lu H, Kyvelou P, Zhao Y, Gardner L, Yi Met al., 2023, Topology optimisation of self-supporting structures based on the multi-axis additive manufacturing technique, Virtual and Physical Prototyping, Vol: 18, ISSN: 1745-2759

Although additive manufacturing (AM) continues to gain widespread adoption, the overhang problem remains a critical issue affecting printing quality. The design of self-supporting structures via topology optimisation approaches has been extensively studied. However, current optimisation research predominantly focuses on 3-axis AM machines, overlooking the more recently developed multi-axis machines. Moreover, the performance sacrifice due to overhang constraints in 3-axis AM can be significant, especially in structures with small volume fractions. To address this, we propose a two-step approach considering overhang constraints for multi-axis AM. This approach begins with a structure optimised using traditional topology optimisation. In the first step, a new optimisation problem determines printing surfaces for the given structure. If the proportion of unprintable elements isn't satisfactory, a second re-optimisation step is carried out to further reduce the unprintable proportion. Several examples demonstrate the effectiveness of the proposed approach. Notably, the significant performance sacrifice associated with the 3-axis AM approach becomes negligible when applying our multi-axis AM-based method.

Journal article

Zong L, Fang W, Huang C, Wang Z, Gardner Let al., 2023, Low cycle fatigue behaviour of wire arc additively manufactured ER70S-6 steel, International Journal of Fatigue, Vol: 176, Pages: 1-11, ISSN: 0142-1123

Wire arc additive manufacturing (WAAM) is a method of 3D printing that is well suited to the cost-sensitive construction industry. Fundamental test data on the mechanical properties of WAAM materials, especially under cyclic loading, are however lacking. To bridge this gap, an experimental study into the low cycle fatigue (LCF) behaviour of WAAM ER70S-6 steel has been conducted and is presented herein. Following quasi-static mechanical and geometric characterisation, a series of as-built and machined coupons was tested in different directions relative to the print layer orientation (θ = 0°, 45° and 90°) under constant amplitude LCF loading, covering a range of strain amplitudes from ±0.2% to ±2.0%. Fractographic analysis of the tested coupons was also performed to assess their failure mechanisms. On the basis of the experimental results, strain-life relationships and cyclic stress-strain curves were derived. The geometric undulations of the as-built coupons resulted in a weakening in the LCF properties, and the weakening effect increased with the loading angle θ and strain amplitude. The cyclic hardening/softening response of the WAAM material varied with the imposed strain amplitudes, while significant non-Masing behaviour was observed.

Journal article

Su A, Wang Y, Rasmussen K, Gardner Let al., 2023, Structural performance and design of S960 ultra-high strength steel non-slender welded I-sections subjected to combined loading, Engineering Structures, Vol: 293, Pages: 1-13, ISSN: 0141-0296

The structural performance of S960 ultra-high strength steel non-slender welded I-sections subjected to combined loading is studied in the present paper. Nonlinear finite element (FE) models were first created and validated with reference to test results collected from the literature, and then used for conducting parametric studies to derive numerical data over an extensive spectrum of cross-sectional geometries, aspect ratios, as well as loading combinations. Given the lack of existing design rules for S960 ultra-high strength steel structures, the suitability of the current provisions for lower strength steels given in the European, Australian and American specifications to S960 ultra-high strength steel was assessed based on the generated numerical data. It was found that (i) the European code and Australian standard can accurately predict the strengths of Class 1 and 2 (i.e. compact) S960 ultra-high strength steel welded I-sections subjected to both strong- and weak-axis combined loading, but results generally in conservative strength predictions for Class 3 I-sections, and (ii) the American specification yields accurate strength predictions for S960 ultra-high strength steel welded I-sections subjected to strong-axis combined loading, but leads to conservative strength predictions for weak-axis combined loading. Finally, the continuous strength method (CSM) was applied to the studied S960 ultra-high strength steel non-slender welded I-sections subjected to combined loading and found to offer significantly improved design consistency and accuracy relative to the current codified provisions. The reliability of the latter design method was also confirmed by means of statistical analyses.

Journal article

Kƶllner A, Gardner L, Wadee MA, 2023, A new approach to the stability design of Ramberg-Osgood material struts, Structures, Vol: 56, Pages: 1-11, ISSN: 2352-0124

An energy formulation employing total potential energy principles is presented to derivea governing equation for strength predictions of struts made from materials followingthe Ramberg–Osgood constitutive law such as stainless steel, cold-formed steel, andaluminium alloys. The formula is generic and applicable to arbitrary cross-sections and allstrut slendernesses for which flexural buckling is critical. Extensive comparisons againstexperimental data on square and rectangular hollow section struts as well as finite elementsimulations demonstrate the accuracy of the developed formula, while the effect of varyingmaterial parameters is examined through comprehensive parametric studies. Owing toits simplicity and its derivation based on mechanical principles, arbitrary configurationsof material parameters and cross-sections can be analysed, making the formula suitablefor use in design practice, representing effectively a non-iterative alternative to the widelyaccepted design load employing the tangent modulus. With the aid of the formula, newcolumn buckling design provisions are developed, which show excellent agreement withexperimental data and meet the reliability requirements specified within the structuralEurocodes.

Journal article

Behzadi-Sofiani B, Wadee MA, Gardner L, 2023, Major-axis buckling of pin-ended stainless steel equal-leg angle section members: FE modelling and design, Eurosteel 2023, Publisher: Ernst und Sohn, Pages: 2625-2630, ISSN: 2509-7075

The behaviour and design of cylindrically-pinned stainless steel equal-leg angle section members under compression and compression combined with strong-axis bending are investigated herein. Numerical models are developed by means of shell finite element modelling formulated within ABAQUS and validated against experimental data. A numerical parametric study is then presented considering both hot-rolled and cold-formed stainless steel equal-leg angle section columns alongside beam-columns with a wide range of cross-section and member geometries. Finally, new design proposals for pin-ended stainless steel equal-leg angle section members under compression and compression plus major-axis bending are developed and verified against the results of physical experiments and numerical simulations. The proposed design rules are shown to offer substantially more accurate and consistent resistance predictions compared to existing codified design rules.

Conference paper

Zhu Y, Yun X, Gardner L, 2023, Cross-sectional behaviour and design of normal and high strength steel welded I-sections under compression and uniaxial bending, Advances in Structural Engineering, Vol: 26, Pages: 2228-2247, ISSN: 1369-4332

A comprehensive numerical investigation into the cross-sectional behaviour and ultimate capacity of non-slender welded I-sections, made of both normal and high strength steels (NSS and HSS), under combined compression and uniaxial bending is presented. Finite element (FE) models were initially established and validated against test results collected from the literature. Subsequently, parametric studies were conducted using the validated FE models to generate extensive numerical data considering different steel grades, cross-section geometries and loading combinations. The obtained numerical data, together with the test results collected from the literature, were utilised to assess the accuracy of the traditional European (EC3) and North American (AISC) design provisions, as well as the Continuous Strength Method (CSM), for NSS and HSS welded I-sections under combined loading. The assessment indicated that the CSM was able to provide more accurate and consistent resistance predictions than the current EC3 and AISC design provisions owing to its ability to capture the spread of plasticity and strain hardening in a systematic, mechanics-based manner. Finally, the reliability levels of the different design methods were statistically evaluated in accordance with EN 1990:2002.

Journal article

Behzadi-Sofiani B, Wadee MA, Gardner L, 2023, Testing, FE modelling and design of pin-ended stainless steel equal-leg angle section columns and beam-columns, Journal of Constructional Steel Research, Vol: 208, Pages: 1-19, ISSN: 0143-974X

The behaviour and design of pin-ended stainless steel equal-leg angle section members under compression and compression plus minor-axis bending are investigated herein. The studied members are cylindrically pinned about the minor axis. An experimental investigation, including material testing, initial geometric imperfection measurements and physical tests on hot-rolled stainless steel equal-leg angle section members is first presented. Numerical models are developed and validated against the new experimental data. A numerical parametric study is then presented considering both hot-rolled and cold-formed stainless steel angle section columns alongside beam–columns with a wide range of slenderness values. Finally, new design proposals for pin-ended stainless steel equal-leg angle section members under compression and combined compression and minor-axis bending are developed and verified against the results of existing physical experiments, as well as the newly-generated test and numerical results. The proposed design rules are shown to offer substantially more accurate and consistent resistance predictions compared to existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor γM1 = 1.1, is verified following the EN 1990 procedure.

Journal article

Zhu Y, Gardner L, Yang H, 2023, Experimental investigation into the transverse impact performance of high-strength circular CFST members, Thin Walled Structures, Vol: 189, Pages: 1-16, ISSN: 0263-8231

Both high-strength steel and high-strength concrete are gaining increasing use in the construction industry. Atthe same time, the benefits of composite construction are also being increasingly recognized and exploited. Inthe present study, high-strength steel and high-strength concrete are considered in combination in high-strengthconcrete-filled high-strength steel tubular (HSCFST) members, with a focus on their behaviour under transverseimpact loading, resistance to which is important for resilient infrastructure. Tests on thirteen HSCFST specimensand six reference CFST specimens under drop weight impact loading are presented. Hot-finished high-strengthS890 steel sections were employed for the outer tubes of the HSCFST specimens, while hot-finished S355 steeltubes were used for the reference CFST specimens. Two core concrete strengths of approximately 60 MPa and100 MPa were considered. The instantaneous impact force and deformation histories of the specimens wererecorded at high frequencies throughout the impact process. A single-degree-of-freedom (SDOF) model wasdeveloped and used to facilitate the analysis of the experimental results. The dynamic moment capacities ofthe test specimens were obtained using the developed SDOF model and a dynamic increase factor (š¯‘…d) wasintroduced to quantify the enhancement in moment capacity relative to the static values. The influence ofthe test parameters on the dynamic increase factors was then analysed. The test results indicated that š¯‘…dispositively correlated with the impact velocity and negatively correlated with the steel grade, steel ratio andspecimen length, while being insensitive to the concrete strength.

Journal article

Meng X, Weber B, Nitawaki M, Gardner Let al., 2023, Optimisation and testing of wire arc additively manufactured steel stub columns, Thin Walled Structures, Vol: 189, Pages: 1-18, ISSN: 0263-8231

Wire arc additive manufacturing (WAAM), a method of directed energy deposition (DED) for metal 3D printing, is capable of producing intricate parts at a relatively high rate and low cost. Despite the great potential of WAAM for applications in construction, knowledge of the performance of WAAM structural elements is still lacking, and the geometric freedom is yet to be fully harnessed. This study is therefore aimed at investigating the local buckling behaviour of carbon steel WAAM elements and exploring the opportunity for improved structural efficiency through optimisation. An optimisation study was initially conducted to derive optimal stiffener layouts for square hollow sections (SHS) under compression. A total of six plain SHS with a broad range of width-to-thickness ratios, along with two SHS strengthened with optimised stiffeners, were manufactured via WAAM and tested. The six benchmark WAAM SHS profiles were examined to allow direct comparisons with conventionally-produced SHS. 3D laser-scanning was carried out to capture the geometric features of the WAAM specimens, and digital image correlation (DIC) was adopted for the full-field measurement of their structural responses during testing. The WAAM SHS stub columns were shown to have comparable load-carrying capacities to conventionally-produced SHS in the stocky range, but exhibited inferior local buckling behaviour in the slender range. Comparisons with the Eurocode 3 (EC3) resistance predictions showed that the existing Class 3 slenderness limit and effective width method specified in EC3 for plated structures may be overly optimistic for WAAM steel elements owing to their typically larger geometric imperfections. Finally, the SHS strengthened with optimised stiffeners were shown to exhibit significantly improved structural efficiency over both the WAAM and conventionally-manufactured plain SHS, bringing disproportionate increases in load-carrying and deformation capacity relative to the increases in mass.

Journal article

Huang C, Zheng Y, Chen T, Ghafoori E, Gardner Let al., 2023, Fatigue crack growth behaviour of wire arc additively manufactured steels, International Journal of Fatigue, Vol: 173, Pages: 1-12, ISSN: 0142-1123

Combining welding technology with robotics, wire arc additive manufacturing (WAAM) is emerging as a viable method of construction. To facilitate its wider application, in particular with structural integrity in mind, an improved understanding of the fatigue properties of WAAM materials is needed. Hence, an experimental investigation into the fatigue crack growth (FCG) behaviour of WAAM plates made of normal- and high-strength steels has been conducted and is reported herein. Following material characterisation, FCG tests on 13 machined compact tension specimens were undertaken, extracted in different directions from WAAM plates built using a parallel deposition strategy. Fractography of the tested specimens was also performed to analyse the mechanisms of crack growth. The material testing demonstrated an approximately proportional relationship between the hardness and ultimate strength. From the FCG tests, Paris’ law constants were derived for the WAAM normal- and high-strength steel specimens, revealing consistently lower material constants m for the latter. Similar FCG behaviour to that of equivalent, conventionally-produced steels, with no significant anisotropy, was found. The observed response was shown to be generally well captured by the FCG laws given in BS 7910, with the recommended curve for unwelded steels providing a close match to the experimental data and the recommended curve for welded steels providing conservative predictions. Additional FCG data on WAAM steels printed using the parallel and oscillatory deposition strategies were collected and analysed. It was shown that the two deposition strategies led to similar crack growth rates for WAAM normal-strength steels, but distinct FCG behavioural trends for WAAM high-strength steels. Finally, the fractographic analysis showed that the WAAM specimens exhibited principally brittle behaviour during FCG but somewhat ductile behaviour before fracture.

Journal article

Vella N, Kyvelou P, Buhagiar S, Gardner Let al., 2023, Innovative shear connectors for composite cold-formed steel-timber structures: an experimental investigation, Engineering Structures, Vol: 287, Pages: 1-18, ISSN: 0141-0296

An experimental investigation into the behaviour of bespoke shear connectors designed to generate composite action in cold-formed steel-timber structures is presented. The response of the shear connectors was assessed through a comprehensive set of push-out tests, where the cold-formed steel thickness and the connector type and material were varied. Previous studies have shown that while the use of ordinary self-drilling screws as shear connectors enables the development of some composite action, their performance was inhibited by timber embedment. Hence, the main feature of the innovative shear connectors was the introduction of a fitting around the screw to mobilise higher timber embedment forces. The best performing shear connectors achieved about double the shear resistance, four times the initial slip modulus ks and seven times the mid-range slip modulus ks,m of ordinary self-drilling screws. An analytical model presented in previous research was extended to describe the response of the innovative shear connectors developed in this study. The model was validated against the push-out test results, and shown to be able to accurately predict the ultimate load, slip at ultimate load, and the two slip moduli ks and ks,m of the innovative connectors, with mean model-to-test ratios of 1.01, 1.15, 1.29 and 1.19 respectively.

Journal article

Huang C, Li L, Pichler N, Ghafoori E, Susmel L, Gardner Let al., 2023, Fatigue testing and analysis of steel plates manufactured by wire-arc directed energy deposition, Additive Manufacturing, Vol: 73, Pages: 1-18, ISSN: 2214-7810

Wire-arc directed energy deposition (DED), also known as wire-arc additive manufacturing (WAAM),is a metal 3D printing technique that is recognised for its high efficiency, cost-effectiveness, flexibilityin build scales and suitability for the construction sector. However, there remains a lack of fundamentaldata on the structural performance of WAAM elements, especially regarding their fatigue behaviour.A comprehensive experimental study into the fatigue behaviour of WAAM steel plates has thereforebeen undertaken and is reported herein. Following geometric, mechanical and microstructuralcharacterisation, a series of WAAM coupons was tested under uniaxial high-cycle fatigue loading. Atotal of 75 fatigue tests on both as-built and machined coupons, covering various stress ranges andstress ratios (R = 0.1, 0.2, 0.3 and 0.4), have been conducted. The local stress concentrations in the asbuilt coupons induced by their surface undulations have also been studied by numerical simulations.The fatigue test results were analysed using constant life diagrams (CLDs) and S-N (stress-life)diagrams, based on both nominal and local stresses. The CLDs revealed that the fatigue strength of theas-built WAAM steel was relatively insensitive to the different stress ratios. The S-N diagrams showedthat the surface undulations resulted in a reduction of about 35% in the fatigue endurance limit for the as-built WAAM material relative to the machined material, and a reduction of about 60% in fatiguelife under the same load level. The as-built and machined WAAM coupons were shown to exhibitsimilar fatigue behaviour to conventional steel butt welds and S355 structural steel plates, respectively.Preliminary nominal stress-based and local stress-based S-N curves were also proposed for the WAAMsteel.

Journal article

Zhang R, Gardner L, Amraei M, Piili Het al., 2023, Mechanical and microstructural testing of additively manufactured stainless steel with laser welded joints, NOLAMP- Nordic Laser Materials Processing Conference

Conference paper

Yun X, Zhu Y, Meng X, Gardner Let al., 2023, Welded steel I-section columns: residual stresses, testing, simulation and design, Engineering Structures, Vol: 282, Pages: 1-19, ISSN: 0141-0296

The flexural buckling behaviour and design of homogeneous and hybrid welded I-section columns, considering a wide range of steel grades, are investigated in the present study. Residual stresses are first examined through the statistical analysis of 71 existing experimental results collected from the literature; on the basis of the findings, a new residual stress model for S235 to S960 steel welded I-sections is proposed. Experiments on a total of five pin-ended homogeneous (S690) and hybrid (S355 web and S690 flanges) welded I-section columns buckling about the major axis are then presented. In parallel with the experimental programme, finite element (FE) models were created and validated against the experimental results obtained from the present study, as well as those collected from the literature. The developed FE models were shown to be capable of accurately replicating the key experimental responses, and were then utilised to carry out extensive parametric studies, through which additional 6000 numerical column buckling data covering a wide range of steel grades, cross-section geometries and member slendernesses were generated. The combined experimental and numerical data were used to evaluate the accuracy of the flexural buckling design rules for welded I-section columns set out in the current European and North American design standards, where shortcomings relating to the consideration of steel grade were identified. A modified Eurocode 3 (EC3) method was devised to reflect the influence of yield strength on the buckling resistances of welded I-section columns more systematically and shown to provide substantially improved resistance predictions in terms of accuracy and consistency; the reliability of the modified approach was statistically verified following the procedure set out in Annex D of EN 1990 and is considered to be suitable for incorporation into future revisions of Eurocode 3.

Journal article

Zhu Y, Yun X, Gardner L, 2023, Numerical modelling and design of normal and high strength steel non-slender welded I-section beam–columns, Thin-Walled Structures, Vol: 186, Pages: 1-14, ISSN: 0263-8231

This paper presents a comprehensive numerical study into the in-plane member stability and design of normal and high strength steel (NSS and HSS) non-slender welded I-section beam–columns subjected to compression plus uniaxial bending. Finite element (FE) models were firstly developed to replicate the structural performance of welded I-section beam–columns made of different steel grades, as observed in existing tests collected from the literature. The validated FE models were then employed in a comprehensive parametric study to generate extensive numerical data covering a wide range of cross-section geometries, steel grades, member slendernesses and loading combinations. The numerically obtained data were utilised to evaluate the accuracy of the current design methods set out in the European Standards EN 1993-1-1:2005 and EN 1993-1-12:2007 as well as the American Specification AISC 360-16 for both NSS and HSS non-slender welded I-section beam–columns. The comparisons revealed that the codified design methods yield varying levels of accuracy in predicting the ultimate capacities of welded I-section beam–columns depending on the steel grade. To address this issue, new design proposals that are compatible with the European beam–column design framework have been developed, featuring more accurate yield strength-dependent interaction curves anchored to improved resistance predictions for members subjected to isolated loading scenarios (i.e. the compression and bending end points of the interaction curves). The new design proposals have been found to provide more accurate and consistent resistance predictions than the existing codified design provisions for NSS and HSS non-slender welded I-section members under compression plus uniaxial bending. The reliability of the new proposals has been confirmed through statistical analyses according to EN 1990: 2002.

Journal article

Zhang Y, Kyvelou P, Wang Y, Li B, Ouyang Y, Gardner Let al., 2023, Experimental investigation and design of slip resistant aluminium alloy-stainless steel connections, Thin Walled Structures, Vol: 186, Pages: 1-15, ISSN: 0263-8231

An experimental investigation into the structural response and slip resistance of aluminium alloy–stainless steel connections fastened by means of high-strength stainless steel swage-locking pins is presented herein. Thirty-seven friction tests on aluminium alloy–stainless steel shear connections featuring different surface treatments and material grades were carried out. Complementary material tests were also performed, while the roughness and hardness of the connected surfaces were measured. The failure modes, load–slip responses, friction coefficients and preloading behaviour of all specimens are fully reported, while the test setup, specimen dimensions, configurations of fasteners, and positions of displacement measurements, are explained. The slip factors corresponding to the different surface treatments are presented, while the key parameters affecting the preload losses of the swage-locking pins and the friction coefficients are described. Finally, the suitability of various surface treatments for aluminium alloy–stainless steel connections is evaluated, while design slip factors, as well as correction coefficients accounting for the influence of different material grades and preload levels on the response of the examined connections, are set out.

Journal article

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