Publications
578 results found
Gardner L, Kyvelou P, Herbert G, et al., 2020, Testing and initial verification of the world's first metal 3D printed bridge, Journal of Constructional Steel Research, Vol: 172, Pages: 1-10, ISSN: 0143-974X
Wire and arc additive manufacturing (WAAM) is a method of metal 3D printing that is suited to the requirements of the construction industry in terms of scale, speed and cost. Using this technology, a 10.5 m span footbridge, the first of its kind, has been printed. The testing, analysis and initial verification of the bridge and its components are described herein. The experiments performed included advanced geometric analysis, material testing, compressive testing of cross-sections and full-scale load testing of the bridge at various stages throughout and post construction. Parallel finite element modelling of the full bridge and its constituent elements has also been performed as part of the verification. Confirmation that the bridge was able to sustain its full serviceability design load enabled to the bridge to be unveiled to the public, with controlled access, for Dutch Design Week 2018. Further testing under ultimate limit state design loading is planned before the bridge is placed in its final location and fully opened to the public. The project highlights the potential for metal 3D printing in structural engineering, as well as the necessary considerations for design.
Arrayago I, Real E, Mirambell E, et al., 2020, The continuous strength method for the design of stainless steel hollow section columns, Thin Walled Structures, Vol: 154, Pages: 1-12, ISSN: 0263-8231
The Continuous Strength Method (CSM) provides accurate resistance predictions for both stocky andslender stainless steel cross-sections; in the case of the former, allowance is made for the beneficialeffects of strain hardening, while for the latter, design is simplified by the avoidance of effective widthcalculations. Although the CSM strain limits can be used in conjunction with advanced analysis for thestability design of members, for hand calculations, the method is currently limited to the determinationof cross-sectional resistance only, i.e. member buckling resistance is not covered. To address thislimitation, extension of the CSM to the design of stainless steel tubular section columns is presentedherein. The proposed approach is based on the traditional Ayrton-Perry formulation, but featuresenhanced CSM cross-section resistances and a generalized imperfection parameter that is a function ofcross-section slenderness. The value of the imperfection parameter increases as the slenderness of thecross-section reduces to compensate for the detrimental effect of plasticity on member stability that isnot directly captured in the elastic/first yield Ayrton-Perry approach. The accuracy of the proposedapproach is assessed against numerical results generated in the current study and existing experimentalresults collected from the literature. The presented comparisons show that the CSM providesconsistently more accurate member buckling resistance predictions than the current EN 1993-1-4 designrules for all stainless steel grades. The reliability of the proposed approach is demonstrated throughstatistical analyses performed in accordance with EN 1990. Finally, the paper presents a frameworkthrough which the proposed approach can be developed for other cross-section types and materials.
Walport F, Gardner L, Nethercot D, 2020, Equivalent bow imperfections for use in design by second order inelastic analysis, Structures, Vol: 26, Pages: 670-685, ISSN: 2352-0124
The stability of compression members is typically assessed through buckling curves, which include the influence of initial geometric imperfections and residual stresses. Alternatively, the capacity may be obtained more directly by carrying out either an elastic or an in elastic second order analysis using equivalent bow imperfections that account for both geometric imperfections and residual stresses. For design by second order elastic analysis, following the recommendations of EN 1993-1-1, the magnitudes of the equivalent bow imperfections can either be back-calculated for a given member to provide the same result as would be obtained from the member buckling curves or can be taken more simply as a fixed proportion of the member length. In both cases, a subsequent M–N (bending + axial) cross-section check is also required, which can be either linear elastic or linear plastic. For design by second order inelastic analysis, also referred to as design by geometrically and materially nonlinear analysis with imperfections (GMNIA)there are currently no suitable recommendations for the magnitudes of equivalent bow imperfections and, as demonstrated herein, it is not generally appropriate to use equivalent bow imperfections developed on the basis of elastic analysis. Equivalent bow imperfections suitable for use in design by second order inelastic analysis are therefore establishedin the present paper.The equivalent bow imperfections are calibrated against benchmark FE results, generated using geometrically and materially nonlinear analysis with geometric imperfections of L/1000(L being the member length)and residual stresses. Based on the resultsobtained, an equivalent bow imperfection amplitude e0= L/150 ( being the traditional imperfection factor set out in EC3), isproposedfor both steel and stainless steel elements and shown to yield accurate results.The reliability of the proposed approach is assessed
Xu F, Chan T-M, Sheehan T, et al., 2020, Prediction of ductile fracture for circular hollow section bracing members under extremely low cycle fatigue (ELCF), Engineering Structures, Vol: 214, ISSN: 0141-0296
The fracture behaviour of concentrically loaded circular hollow section (CHS) bracing members under extremely low cycle fatigue (ELCF) is examined in this paper. Finite element (FE) models capable of predicting fracture initiation and propagation on cyclically loaded braces were developed. A structural steel ductile fracture criterion, together with a damage accumulation rule that can account for the effects of both stress triaxiality and Lode angle, was adopted in the FE models. The FE models were validated against the available test results from different experimental programmes and shown to provide an accurate prediction of both the hysteretic response and the ELCF fracture cracking process. The coupling effects of buckling and fracture on the ELCF performance of braces were assessed through a parametric study. This parametric study examined the influences of the geometry, material and manufacturing process on the local and global deformation, and the ductility of the braces. Predictive equations for the localized strains and member ductility were proposed based on a plastic hinge model. The seismic performance of a chevron braced frame was also evaluated in terms of storey drift angle according to the requirements of the current design code.
Wang F, Young B, Gardner L, 2020, Compressive behaviour and design of CFDST cross-sections with stainless steel outer tubes, Journal of Constructional Steel Research, Vol: 175, ISSN: 0143-974X
A finite element (FE) investigation into the compressive behaviour of concrete-filled double skin tubular (CFDST) cross-sections with lean duplex andferritic stainless steel outer tubesis presented. FE models were initially developed and validated against available test results reported in the literature. Upon successful replication of the ultimatecapacities, load–deformation histories and failure modes exhibited by the tested CFDSTstub columns,a parametric studywas undertaken to investigate the influence of keyvariables, including the local slendernessesof the outer and inner tubes, the concrete strengthand the adopted grade of stainless steel, on the ultimate response of the studied CFDST stub columns. Based on the generated FE data pooland theavailable test results, the applicability of the existing European, Australian and American design provisions for composite carbon steel members to the design of the studied CFDST cross-sectionswas evaluated. All the examined design rules are shown to yield unduly conservative (less so for the higher concrete grades) and rather scattered capacity predictions. Modifications to the design treatment in relation to the effective area of the outer tubes,to take due account of outward-only local buckling,and the effective compressive strength of the infilled concrete,to reflect the reduced relative effectiveness of higher concrete grades,are considered. The modified design rules are shown toimprove the accuracy and consistencyof the design capacity predictions. Finally, statistical analyses were carried out to demonstrate the reliability of the modified design approaches.
Kucukler M, Gardner L, Bu Y, 2020, Flexural-torsional buckling of stainless steel I-section beam-columns: testing, numerical modelling and design, Thin Walled Structures, Vol: 152, Pages: 1-19, ISSN: 0263-8231
The flexural-torsional buckling response and design of stainless steel I-section beam-columns are investigated in this paper. First, a series of laboratory tests on laser-welded stainless steel I-section beam-columns susceptible to flexural-torsional buckling is presented. The results obtained are supplemented by further data generated by means of numerical parametric studies on both conventionally arc-welded and laser-welded stainless steel members covering a wide range of member slenderness and combinations of loading. Existing provisions for the design of welded stainless steel I-section elements against flexural-torsional buckling are then assessed and found to require improvement. Finally, new formulae for the design of stainless steel I-section beam-columns susceptible to flexural-torsional buckling are proposed. The new proposals yield improved accuracy and consistency over existing provisions and their suitability for inclusion in the upcoming version of the European structural stainless steel design code EN 1993-1-4 is confirmed by reliability analysis in accordance with EN 1990.
Kyvelou P, Slack H, Daskalaki Mountanou D, et al., 2020, Mechanical and microstructural testing of wire and arc additively manufactured sheet material, Materials and Design, Vol: 192, ISSN: 0264-1275
Wire and arc additive manufacturing (WAAM) is a method of 3D printing that enables large elements to be built, with reasonable printing times and costs. There are, however, uncertainties relating to the structural performance of WAAM material, including the basic mechanical properties, the degree of anisotropy, the influence of the as-built geometry and the variability in response. Towards addressing this knowledge gap, a comprehensive series of tensile tests on WAAM stainless steel was conducted; the results are presented herein. As-built and machined coupons were tested to investigate the influence of the geometrical irregularity on the stress-strain characteristics, while material anisotropy was explored by testing coupons produced at different angles to the printing orientation. Non-contact measurement techniques were employed to determine the geometric properties and deformation fields of the specimens, while sophisticated analysis methods were used for post processing the test data. The material response revealed a significant degree of anisotropy, explained by the existence of a strong crystallographic texture, uncovered by means of electron backscatter diffraction. Finally, the effective mechanical properties of the as-built material were shown to be strongly dependent on the geometric variability; simple geometric measures were therefore developed to characterise the key aspects of the observed behaviour.
Buchanan C, Zhao O, Real E, et al., 2020, Cold-formed stainless steel CHS beam-columns – testing, simulation and design, Engineering Structures, Vol: 213, Pages: 1-23, ISSN: 0141-0296
The present work was prompted by shortcomings identi ed in existing design provisions for stainless steel circular hollow section (CHS) beam-columns. First, addressing a lack of existing experimental data, a series of ferritic stainless steel CHS beam-column tests was undertaken at the cross-section and member levels. In total, 26 beam-column tests, including two section sizes (a non-slender class 3 and slender class 4 cross-section), two member slenderness values for each cross-section type and a wide range of loading eccentricities were carried out to investigate the interaction between local and global buckling. Followingvalidation of nite element (FE) models, a numerical study was then undertaken to explore the buckling response of stainless steel CHS beam-columns, covering austenitic, duplex and ferritic grades with a wide range of local and global slendernesses and applied loading eccentricities. Over 2000 numerical results were generated and used to assess new designproposals for stainless steel beam-columns, featuring improved compression and bending end points and new interaction factors. The new proposals are more consistent and more accuratein their resistance predictions than the current EN 1993-1-4 (2015) design approach. The reliability of the new proposals has been veri ed by means of statistical analyses accordingto EN 1990 (2005).
Wu K, Wadee MA, Gardner L, 2020, Interactive buckling in prestressed stayed beam-columns, International Journal of Mechanical Sciences, Vol: 174, ISSN: 0020-7403
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. A nonlinear analytical model for prestressed stayed beam-columns with doubly-symmetric and mono-symmetric configurations, based on the Rayleigh–Ritz method, is presented that captures modal interactions for perfect geometries explicitly for the first time. It is demonstrated that the theoretical compressive strength enhancements under certain configurations can only be obtained at the expense of triggering a sequence of destabilizing bifurcations. This can give rise to severely unstable interactive post-buckling behaviour including the so-called ‘mode jumping’ phenomenon. Moreover, for mono-symmetric stayed beam-columns, it is shown that the varying levels of prestress within the stays can lead to different buckling modes which all have their own characteristic post-buckling responses. The analytical model is verified using a nonlinear finite element model formulated within the commercial code ABAQUS and excellent comparisons are obtained.
Fieber A, Gardner L, Macorini L, 2020, Structural steel design using second-order inelastic analysis with strain limits, Journal of Constructional Steel Research, Vol: 168, Pages: 1-19, ISSN: 0143-974X
Steel framed structures are affected, to greater or lesser extent, by (i) geometrical nonlinearity associated with the change in geometry of the structure under load and (ii) material nonlinearity related to the onset and spread of plasticity. In traditional design approaches, the design forces and moments within structural members are usually determined from simplified structural analyses (e.g. first or second-order elastic analysis), after which member design checks are performed to assess the strength and stability of the individual members. The extent to which the strength and deformation capacity of cross-sections is affected by local buckling is typically assessed through the concept of cross-section classification. For example, only compact (Class 1) cross-sections are considered to possess sufficient rotation capacity for plastic hinges to develop and for inelastic analysis methods to be used. This approach results in step-wise capacity predictions and is considered to be overly simplistic. Since the structural analysis of steel framed structures is typically performed using beam finite elements, which are unable to explicitly capture local buckling, a more sophisticated treatment of the available deformation capacity is required if inelastic analysis methods are to be used for all cross-section classes. A novel method of design by advanced inelastic analysis has recently been developed (Gardner et al., 2019a; Fieber et al., 2019a, 2018a, 2018b [[1], [2], [3], [4]]), in which strain limits are employed to represent the effects of local buckling in beam finite element models and thereby control the spread of plasticity and level of force/moment redistribution within a structure. It is thus possible to use a consistent advanced analysis framework to design structures composed of cross-sections of any class. In the present paper, application of the proposed design method to continuous beams and planar frames is illustrated and assessed. Ultimate load capacity p
Vella N, Gardner L, Buhagiar S, 2020, Experimental analysis of cold-formed steel-to-timber connections with inclined screws, Structures, Vol: 24, Pages: 890-904, ISSN: 2352-0124
An experimental investigation into the behaviour of connections between cold-formed steel and timber components, formed with inclined screws, is presented. A series of material tests was firstly carried out to assess the behaviour and limitations of each component of the system under investigation. These were supplemented by three types of interaction test: timber embedment, screw head pull-through and screw thread withdrawal, all aimed at developing a better understanding of the behaviour of the different components of the connection. The slip modulus and peak load-carrying capacity of twelve connection configurations were determined through a series of push-out tests. The results obtained were compared to the typical benchmark arrangement in which the screws were driven in perpendicular to the steel-timber interface. Component variations included the type of timber – particle board and plywood, the thickness of the steel – 1.5 mm and 2.4 mm thick, the inclination of the screws – 0° and 45°, and the presence or not of wings on the screws. The results showed that non-winged screws inclined at 45° gave up to about a 30% increase in peak load-carrying capacity and about a 140% increase in slip modulus when compared to the reference specimen.
Tan Q, Gardner L, Han L-H, et al., 2020, Performance of concrete-filled stainless steel tubular (CFSST) columns after exposure to fire, Thin Walled Structures, Vol: 149, ISSN: 0263-8231
The post-fire performance of concrete-filled stainless steel tubular (CFSST) columns subjected to an entire loading–fire history, including four characteristic phases: (i) ambient temperature loading, (ii) heating, (iii) cooling with constant external loads, and (iv) post-fire loading, is investigated in this paper. Sequentially coupled thermal-stress analyses are performed using ABAQUS to establish the temperature field and structural response of CFSST columns. To improve the precision of the finite element analysis (FEA) models, the influence of moisture on the thermal conductivity and specific heat of the concrete in the heating and cooling phases is considered by using subroutines. Existing fire and post-fire test data on CFSST columns are used to validate the FEA modelling. Comparisons between FEA and test results indicate that the accuracy of the model is acceptable; the FEA model is then extended to simulate CFSST columns subjected to the four characteristic phases. The behaviour of the CFSST columns during the four characteristic phases is explained by analysis of the temperature distribution, load versus axial deformation relations, failure modes and internal force redistribution. The excellent post-fire performance of CFSST columns is examined in comparison with traditional concrete-filled carbon steel tubular (CFST) columns with the same total cross-sectional area. The residual strength index is studied with respect to a series of parametric analyses. It is found that the residual strength of CFSST columns is higher than that of CFST columns after the same fire exposure, and that the diameter of the stainless steel tube, slenderness, heating time ratio and load ratio have a significant influence on the residual strength index.
Yun X, Saari N, Gardner L, 2020, Behaviour and design of eccentrically loaded hot-rolled steel SHS and RHS stub columns at elevated temperatures, Thin-Walled Structures, Vol: 149, Pages: 1-12, ISSN: 0263-8231
The structural fire response of hot-rolled steel square and rectangular hollow sections (SHS and RHS) under combined compression and bending is investigated in this study through finite element (FE) modelling. The developed FE models were firstly validated against available test results on hot-rolled steel SHS/RHS subjected to combined compression and bending at elevated temperatures. Upon validation, an extensive parametric study was then carried out to examine the resistance of hot-rolled steel SHS/RHS under combined loading at elevated temperatures, covering a wide range of cross-section slendernesses, cross-section aspect ratios, combinations of loading and temperatures up to 800 °C. The numerical data, together with the experimental results, were compared with the strength predictions according to the current structural fire design rules in the European Standard EN 1993-1-2 (2002) and American Specification AISC 360–16 (2016) for hot-rolled steel SHS/RHS under combined loading. The comparisons generally indicated significant disparities in the prediction of resistance of hot-rolled steel SHS/RHS under combined loading at elevated temperatures, owing principally to inaccurate predictions of the end points of the design interaction curves. The deformation-based continuous strength method (CSM) has been shown to provide accurate strength predictions for these end points i.e. the resistances of hot-rolled steel SHS/RHS stub columns and beams at elevated temperatures. In this study, proposals are presented to extend the scope of the CSM to the structural fire design of hot-rolled steel SHS/RHS under combined compression and bending. The CSM proposals are shown to offer improved accuracy and reliability over current design methods and are therefore recommended for incorporation into future revisions of international structural fire design codes.
Meng X, Gardner L, 2020, Testing of hot-finished high strength steel SHS and RHS under combined compression and bending, Thin-Walled Structures, Vol: 148, 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.
Wang F, Young B, Gardner L, 2020, CFDST sections with square stainless steel outer tubes under axial compression: experimental investigation, numerical modelling and design, Engineering Structures, Vol: 207, ISSN: 0141-0296
The use of concrete-filled double skin tubular (CFDST) cross-sections for compression members has become increasingly popular in construction. A recently proposed innovative form of CFDST cross-section, ultilising stainless steel for the outer tube, offers the combined advantages of the composite action seen in CFDST member alongside the durability and ductility associated with stainless steel. CFDST sections with stainless steel outer tubes, for which there are currently little experimental data, are the focus of the present study. A comprehensive experimental and numerical investigation into the compressive behaviour of CFDST sections with square stainless steel outer tubes is presented in this paper. A total of 19 specimens was tested under uniform axial compression, and the test observations are fully reported. The ultimate loads, load-displacement curves and failure modes from the tests were used for the validation of finite element (FE) models. Parametric finite element analyses were then performed. The combined set of experimentally and numerically derived data was employed to assess the applicability of the existing European, Australian and 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. 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, as well as a reduction factor η to reflect the reduced relative effectiveness of higher concrete grades. Overall, the comparisons demonstrated that improved accuracy and consistency were achieved when the modified design rules were applied.
Meng X, Gardner L, Sadowski A, et al., 2020, Elasto-plastic behaviour and design of semi-compact circular hollow sections, Thin Walled Structures, Vol: 148, Pages: 1-12, 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
Xing Z, Kucukler M, Gardner L, 2020, Local buckling of stainless steel plates in fire, Thin Walled Structures, Vol: 148, Pages: 1-18, ISSN: 0263-8231
The local buckling behaviour and design of stainless steel plates in fire are investigated in this paper. Finite element models of stainless steel plates able to mimic their response in fire are createdand validated against experimental results from the literature. Parametric studies are then performed and the results are utilised to assess the current design provisions set out in the Europeanstructural steel fire design code EN 1993-1-2; shortcomings in the prediction of the local bucklingresponse of stainless steel plates in fire are revealed. A new effective width based design approachable to reflect the variation in strength and stiffness of stainless steel at different temperature levelsin the determination of the local plate slenderness and thereby the ultimate resistances of stainless steel plates in fire is put forward. The proposed approach is shown to provide significantlyhigher levels of accuracy and reliability relative to the current provisions in EN 1993-1-2 for awide range of plate slendernesses, elevated temperature levels, stainless steel grades and loadingconditions. Incorporation of the proposed design approach into future revisions of EN 1993-1-2 isrecommended
dos Santos GB, Gardner L, 2020, Design recommendations for stainless steel I-sections under concentrated transverse loading, Engineering Structures, Vol: 204, Pages: 1-11, ISSN: 0141-0296
Recent investigations have highlighted the need for improved provisions for determining the resistance of stainless steel I-sections under concentrated transverse loading. Such provisions, which reflect the particular characteristics of the material, have been developed and are described herein. A review of the existing European design formulae for members under concentrated transverse loading is firstly presented. Then a series of parametric studies, based on validated finite element models are described covering I-sections with a range of web slenderness values and different stainless steel grades. On the basis of the numerical results, together with existing experimental data, revised design equations are presented and assessed through reliability analysis performed in accordance with Annex D of EN 1990. The new provisions yield enhanced ultimate load predictions and are expected to be included in the next revision of EN 1993-1-4.
Kucukler M, Xing Z, Gardner L, 2020, Behaviour and design of stainless steel I-section columns in fire, Journal of Constructional Steel Research, Vol: 165, Pages: 1-18, ISSN: 0143-974X
The flexural buckling behaviour and design of stainless steel I-section columns at elevated temperatures are investigated in this paper. Finite element models able to accuratelyreplicate the response of structural stainless steel columns in fire are created and validated.The models are then utilised to carry out extensive numerical parametric studies consideringa broad range of stainless steel grades, cross-section geometries, slendernesses and elevatedtemperature levels. Using the results from the parametric studies, the safety and accuracyof existing design rules provided in the European structural steel fire design code EN 1993-1-2 for stainless steel columns in fire are assessed. It is observed that the existing designrules provide rather scattered and inaccurate ultimate strength predictions for stainless steelcolumns at elevated temperatures. For the purpose of establishing an accurate and practicalmeans of designing stainless steel columns in fire, a new design approach, compatible withexisting design rules in EN 1993-1-2, is proposed. The safety, accuracy and reliability ofthe proposed approach are illustrated for a wide range of cases against the results obtainedthrough nonlinear finite element modelling. The proposed stainless steel column fire designrules are due to be incorporated into the upcoming version of the European steel fire designstandard EN 1993-1-2.
Walport F, Gardner L, Nethercot DA, 2020, A method for the treatment of 2<sup>nd</sup> order effects in plastically-designed steel frames
The susceptibility of steel frames to global second order effects, also referred to as sway effects, 'P-∆' effects and global geometric nonlinearities, is traditionally assessed through the elastic buckling load factor αcr. For elastic analysis, EN 1993-1-1 and other international steel design standards deem second order effects sufficiently small that they may be ignored if the amplification of internal forces and moments is no more than 10% of the original forces and moments, corresponding to a limit of αcr≥10. For plastic analysis, to reflect the influence of the degradation of material stiffness, a stricter limit of 15 is prescribed for second order effects to be neglected. Use of a single limit of 15 for any structural system no matter the degree of influence of plasticity is considered by the authors to be overly simplistic. A parametric investigation to assess the stability of steel frames in the plastic regime is presented herein. Based on the findings, an improved method is proposed for the treatment of global second order effects in plastically-designed steel frames.
Fieber AC, Gardner L, Macorini L, 2020, Advanced analysis with strain limits for the design of steel structures
Structural analysis of steel frames is typically performed using beam finite elements. These elements cannot capture local buckling explicitly and hence limitations on the local strength and rotation capacity of members are required for design purposes. Traditionally, this is achieved by classifying cross-sections and defining class-specific restrictions on the analysis type (i.e. elastic or plastic design) and cross-section design resistance (i.e. plastic, elastic or effective bending capacity). This approach is however considered to be overly simplistic and creates artificial 'steps' in the capacity predictions of structural systems. A more consistent approach is proposed herein, whereby a second order global plastic analysis is performed with strain limits accounting for the effects of local buckling and controlling the deformation capacity of each cross-section. The strain limits are obtained from the Continuous Strength Method. Strains are averaged over a characteristic length to exploit the beneficial effects of moment gradients. The proposed method is applied to members, continuous beams and frames and is shown to be more consistent and user-friendly than current structural steel design methods.
Yun X, Gardner L, 2020, Numerical study of structural steel continuous beams with tubular cross-sections
The structural behaviour and design of hot-rolled and cold-formed steel continuous beams with square and rectangular hollow sections are studied in this paper, with a focus on the beneficial effects of material strain hardening and moment redistribution. Finite element (FE) models were first developed and validated against existing test results, upon which parametric studies were carried out to expand the available structural performance data over a range of cross-section geometries, cross-section slendernesses, steel grades and loading conditions. The experimental results, together with the parametric numerical results, were then used to evaluate the accuracy of the design provisions of EN 1993-1-1 as well as the continuous strength method (CSM) for indeterminate structures, the latter of which is extended in scope in the present study. It was shown that the current provisions of EN 1993-1-1 for the design of continuous beams are rather conservative, while the proposed CSM yields a higher level of accuracy and consistency, due to its rational consideration of both strain hardening at the cross-sectional level and moment redistribution at the global system level.
Wadee MA, Hadjipantelis N, Bazzano JB, et 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 steel truss compression elements significantly. To demonstrate this concept, a system comprising a tubular strut subjected to an external compressive load and a prestressed cable anchored independently of the strut is studied. Energy methods are utilized to define the elastic stability of the perfect and imperfect systems, after which the first yield and rigid-plastic responses are explored. The influence of the key controlling parameters, including the length of the strut, the axial stiffness of the cable and the initial prestressing force, on the elastic stability, the inelastic response and the ultimate strength of the system is demonstrated using analytical and finite element (FE) models. To illustrate the application of the studied structural concept, FE modelling is employed to simulate the structural response of a prestressed hangar roof truss. A nearly two-fold enhancement in the load-carrying capacity of the truss structure is shown to be achieved owing to the addition of the prestressed cable.
Buchanan C, Wan W, Gardner L, 2020, Testing of wire and arc additively manufactured stainless steel material and cross-sections
3D printing, or additive manufacturing (AM), is starting to be explored as a viable manufacturing technique in the construction industry. A 10 m span stainless steel pedestrian bridge is being built using Wire and Arc Additive Manufacturing (WAAM) and, upon completion, will be placed in the centre of Amsterdam, the Netherlands. Material tests and cross-section testing on circular hollow section (CHS) and square hollow section (SHS) columns have been undertaken to support this novel project. The unusual nature of the geometry and material properties has necessitated the use of advanced laser scanning and digital image correlation measurement techniques, along with Archimedes' measurements and silicone casting. The material response has been observed to be anisotropic, with a lower Young's modulus and material strength in certain orientations. The ultimate compressive capacities of the tested cross-sections have been observed to have a larger variation between repeat specimens than typically seen with conventionally produced elements, and the ultimate compressive capacities are generally overpredicted by current design methods.
Gardner L, Buchanan C, Wan W, 2020, Opportunities for metal 3D printing in structural engineering
3D printing, more formally known as additive manufacturing (AM), has the potential to revolutionise the construction industry, with anticipated benefits including greater structural efficiency, reduced 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. 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 are explored in this paper, along with early applications of metal additive manufacturing in the construction industry and other engineering disciplines.
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.
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.
Wang Z, Wang Y, Zhang Y, et 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).
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.
Liu F, Wang Y, Gardner L, et 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.
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