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Sensitivity study on the effects of prestress loss on prestressed stayed columns

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 accurately replicate 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 considering a broad range of stainless steel grades, cross-section geometries, slendernesses and elevated temperature levels. Using the results from the parametric studies, the safety and accuracy of 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 design rules provide rather scattered and inaccurate ultimate strength predictions for stainless steel columns at elevated temperatures. For the purpose of establishing an accurate and practical means of designing stainless steel columns in fire, a new design approach, compatible with existing design rules in EN 1993-1-2, is proposed. The safety, accuracy and reliability of the proposed approach are illustrated for a wide range of cases against the results obtained through nonlinear finite element modelling. The proposed stainless steel column fire design rules are due to be incorporated into the upcoming version of the European steel fire design standard EN 1993-1-2.

Owing to their enhanced load-carrying and serviceability performances, prestressed coldformed steel beams can potentially open up new applications within the construction industry. In the proposed concept, an eccentric prestressing force is applied to cold-formed steel beams by means of a cable that is housed within a bottom hollow flange. During prestressing, tensile stresses are induced within the top region of the beam, thus delaying the occurrence of local instabilities under subsequent vertical loading. Consequently, the moment capacity of the beam is enhanced. Furthermore, owing to the prestressing, a pre-camber is also induced along the member, thus decreasing the overall vertical deflections significantly. Following discussion of the mechanical behaviour of the proposed beams, design recommendations are developed by employing interaction equations alongside the Direct Strength Method. Subsequently, finite element (FE) analysis is employed to investigate the effects of the prestress level and the section slenderness of the steel beam on the benefits obtained from the prestressing process. The parametric FE results are then utilised to assess the design recommendations.

The Continuous Strength Method provides excellent resistance predictions of stocky cross-sections under different loading conditions as it accounts for enhanced material properties, although the method is currently limited to crosssectional resistance. The axial stress in short columns potentially exceeds the yield strength as members are stable enough to allow partial yielding of non-slender cross-sections and the flexural buckling resistance prediction of these short columns is therefore usually underestimated, since equations currently codified in standards do not account for strain hardening effects. This paper presents the extension of the Continuous Strength Method to stainless steel Rectangular and Square Hollow Section columns, allowing strain hardening effects to be incorporated in the resistance prediction of members subjected to compression. The proposed approach is equivalent to the traditional approach for columns codified in different standards based on the Ayrton-Perry formulation but considers a different generalized imperfection parameter that depends on the cross-sectional slenderness and considered material, so strain hardening effects are directly introduced in the formulation. Results show that the proposed Continuous Strength Method approach provides improved predictions of the resistance of Rectangular and Square Hollow Section columns for all stainless steel grades. The reliability of the proposed approach has been demonstrated through the corresponding statistical analyses. Given the purely analytic nature of the proposed approach, it can be used for several materials, such as stainless steel, carbon steel and aluminium, and cross-section types for which the Continuous Strength Method has been defined. Finally, the better estimation of the flexural buckling behaviour of columns will have a direct impact on the accuracy of beam-column checks.

El acero inoxidable se emplea en un amplio abanico de aplicaciones estructurales en ambientes agresivos en los que se requiere garantizar un comportamiento adecuado durante largos períodos de tiempo con niveles mínimos de mantenimiento. Además, el acero inoxidable presenta un aspecto atractivo, es resistente a la vez que ligero, muy dúctil y versátil en términos de manufactura. Este Manual de Diseño proporciona expresiones de dimensionamiento para aceros inoxidables austeníticos, dúplex y ferríticos. Las reglas están en sintonía con las modificaciones propuestas en 2015 sobre el Eurocódigo para el acero inoxidable estructural, EN 1993-1-4. Este Manual de Diseño cubre el cálculo de secciones transversales, elementos, uniones y el cálculo a temperaturas elevadas, así como nuevos métodos de cálculo que consideran los efectos del endurecimiento por deformación, característicos del acero inoxidable. Se proporcionan también guías para la selección de grados, durabilidad y fabricación. Finalmente, el Manual cuenta con quince ejemplos de cálculo que ilustran la aplicación de las reglas de dimensionamiento.

Stainless steel is used for a wide range of structural applications in aggressive environments where reliable performance over long periods with little maintenance is required. In addition, stainless steel has an attractive appearance, is strong yet still light, highly ductile and versatile in terms of manufacturing. This Design Manual gives design rules for austenitic, duplex and ferritic stainless steels. The rules are aligned to the 2015 amendment of the Eurocode for structural stainless steel, EN 1993-1-4. They cover the design of cross-sections, members, connections and design at elevated temperatures as well as new design methods which exploit the beneficial strain hardening characteristics of stainless steel. Guidance on grade selection, durability and fabrication is also provided. Fifteen design examples are included which illustrate the application of the design rules.

Stainless steel is an increasingly popular construction material due to its combination of durability and favourable mechanical properties. Circular hollow sections (CHS) are widely used due to their aesthetic appeal and structural advantages over open cross-sections. It has been observed from the limited existing stainless steel CHS column buckling dataset that the current Eurocode 3 provisions can be unconservative in their capacity predictions. To address both the limited dataset and the unconservative design provisions, a comprehensive stainless steel CHS column buckling experimental programme has been undertaken. The results of 17 austenitic, 9 duplex and 11 ferritic stainless steel CHS column buckling tests and 10 stub column tests are presented. The programme consisted of five different cross-sections (covering class 1 to class 4 sections) and a wide range of member slendernesses. Based on the assembled data, existing proposals are evaluated and initial design recommendations are made for a safe and efficient stainless steel CHS buckling curve.

Stainless steel is gaining wider usage in a range of engineering applications due to its favourable mechanical properties and corrosion resistance, with ferritic grades expected to become more prevalent in construction due to their lower and more stable cost compared with traditional austenitic and duplex grades. Circular hollow sections (CHS) are a popular structural cross-section due to their visual appeal and structural attributes. A series of ferritic stainless steel CHS beam-column tests have been undertaken, addressing a lack of existing experimental data for this cross-section type and metallic material. 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 and a wide range of loading eccentricities have been undertaken to investigate the interaction between local and global buckling. The experimental results have also been used to assess current design guidance along with previous stainless steel CHS continuous strength method proposals. The outcome is a new comprehensive dataset, along with initial design recommendations for CHS ferritic stainless steel beam-columns.

Wire arc additive manufacturing (WAAM) is a metal 3D printing method that enables large-scale structural elements with complex geometry to be built in a relatively efficient and cost-effective manner, offering revolutionary potential to the construction industry. Fundamental experimental data on the material and structural behaviour of WAAM elements are however lacking. Therefore, a comprehensive experimental study into the material properties, the cross-section and the member buckling behaviour of WAAM elements has been conducted and is reported herein. Tensile tests on 137 WAAM steel coupons, covering different steel grades, finishes, thicknesses, extraction directions and locations, and deposition strategies, have been conducted. Microstructural characterisation has also been performed by means of optical microscopy (OM) and electron backscatter diffraction (EBSD). At the cross-section and member levels, four-point bending tests on 14 WAAM stainless steel tubular beams and flexural buckling tests on 18 WAAM stainless steel tubular columns have been undertaken, respectively. Owing to the geometric undulations inherent to the WAAM process, 3D laser scanning and digital image correlation (DIC) were employed in the material, beam and column testing programme to capture the geometric properties and deformation responses of the specimens, respectively. The present research focuses on WAAM elements subjected to predominantly static loading (rather than fatigue loading), with an emphasis on structural stability. The examined WAAM steels exhibited consistent, almost isotropic mechanical properties, a Young’s modulus comparable to conventionally-produced steel plates, marginally lower strength, reflecting the slower cooling conditions than is customary, and good ductility. To describe the full stress-strain response of WAAM steels, material models were proposed and validated against the tensile test results and further experimental data collected from the literature. Following the beam and column tests, the applicability of the current cross-section and column design provisions in EN 1993-1-4 and AISC 370, as well as the continuous strength method (CSM), to WAAM stainless steel elements was assessed by comparing the test results with the strength predictions. The comparisons highlighted the need to allow for the weakening effect of the inherent geometric undulations of WAAM elements, in order to achieve safe-sided strength predictions. Open Access

Cold-formed steel members are widely used in the construction industry due to their versatility, high strength-to-weight ratio and ease of assembly. Stud columns set in tracks, form the main members in cold-formed steel wall systems, which are typically sheathed with plasterboard or oriented strand board (OSB). The interaction between the cold-formed steel members and the sheathing, which can have a significant effect on the ultimate strength and flexural stiffness due to composite action and bracing, is currently not systematically taken into consideration in design. More than 100 material and connection component tests have been performed to study the behaviour of plasterboard and OSB and their interconnection with cold-formed steel through screws acting in shear and tension. The obtained test results, along with a collected dataset totalling more than 400 physical tests, were used to develop analytical models to describe their load-deformation behaviour; these are suitable for use in numerical simulations and advanced design methods. A total of 17 full-scale sheathed wall stud tests were also performed, with varying connector spacing between either plasterboard or OSB sheathing panels and the steel members. Tests were performed under pure compression, pure bending and combined loading. Reducing the spacing of the connectors from 600 mm to 75 mm resulted in up to 30% increase in capacity while also preventing pull-through connector failure for specimens sheathed with plasterboard. Sophisticated finite element models of wall studs were also developed, which, following successful validation against test results, allowed parametric studies to be undertaken, where the influence of the sheathing, connector spacing and section depth was investigated. Finally, a preliminary design guidance has been developed, where the beneficial effects of bracing, composite action and enhanced boundary conditions at the member ends are recognised, thus enabling more efficient design of sheathed cold-formed steel wall systems. Open Access

The stability and design of steel and stainless steel equal-leg angle section members with different loading and support conditions, including fixed-ended angles under axial compression and simply-supported angles under uniaxial bending, are addressed presently. First, elastic buckling behaviour is described; it is shown that torsional and local elastic buckling are similar in fixed-ended equal-leg angle section columns and equal-leg angles under minor-axis bending. In addition, it is shown that equal-leg angle section beams under minor-axis bending are susceptible to lateral-torsional buckling and Brazier flattening. An experimental investigation, comprising material testing, measurements of initial geometric imperfection and physical member tests, including 5 tests on fixed-ended hot-rolled stainless steel equal-leg angle section columns, 7 tests on hot-rolled steel and a further 7 tests on hot-rolled stainless steel equal-leg angle section beams, is then presented. The test results, in combination with existing experimental data on steel and stainless steel equal-leg angle section members collected from the literature, are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Validation is performed by means of comparisons between test and numerical results, considering ultimate loads and failure modes, all of which are shown to be generally in good agreement. A numerical parametric study is then conducted considering both hot-rolled and cold-formed steel and stainless steel equal-leg angle section members with a wide range of cross-section geometries and member lengths. The behaviour and load-carrying capacity of the members is shown to be dependent on, not only the member slenderness, but also the ratio of the torsional-flexural to minor-axis flexural elastic buckling load and the local to lateral-torsional elastic buckling moment for columns and beams, respectively. Finally, on the basis of the above observations and data new design equations, suitable for incorporation into future revisions of Eurocode 3, for both steel and stainless steel equal-leg angle section members, are developed. It is demonstrated that the new proposals offer substantially improved accuracy and consistency in strength predictions compared to the existing codified design rules. Open Access

The stability design of steel structures by second order inelastic analysis with strain limits is addressed in this thesis. In the proposed design approach, a geometrically and materially nonlinear analysis with imperfections (GMNIA) of members and frames is carried out and the ultimate strength of the structure is signified by reaching either the strain limit defined according to the Continuous Strength Method (simulating cross-section failure) or the peak load factor, whichever occurs first. The method has been established for the in-plane design of prismatic structural components. In the present thesis, its scope is extended to the in-plane design of members with three-flanged cross-sections (representing the haunch and apex regions of portal frames) and web-tapered members, subjected to combined loading conditions ranging from pure compression to pure uniform and non-uniform bending. Explicit formulae, which have been developed previously to predict the elastic local buckling stress of single I-sections, are first extended to cover the case of three-flanged cross-sections, subjected to combined loading conditions ranging from pure compression to pure major axis bending. The predictions are shown to be more accurate than the calculations performed on an element-by-element basis – the way in which the local buckling of cross-sections is typically treated in current structural steel design specifications. Attention then turns to the development of the proposed design approach for scenarios in which out-of-plane stability effects, with a focus on lateral-torsional buckling, govern. First, equivalent imperfections for the out-of-stability design of steel and stainless steel members by second order inelastic analysis are established. Then, the existing strain limits for in-plane design are extended to allow for the influence of shear, torsion and warping, to enable their general applicability to three-dimensional buckling problems. It is shown that the proposed design method provides more accurate and consistent ultimate strength predictions than traditional design methods and also streamlines the design process by eliminating the need for cross-section classification and individual member design checks. Open Access

Slender struts supported with prestressed cable-stays in conjunction with periodically-spaced crossarms distributed longitudinally, offer an attractive, innovative and pragmatic solution to reducing structural self-weight while maintaining buckling strength. The use of the crossarm--stay system generally results in a higher load-carrying capacity. Previous research has been primarily conducted for vertical stayed members, but there is a potential demand for very long struts to be used in the horizontal plane. Hence, the current thesis investigates the buckling and post-buckling behaviour of stayed beam-columns where the axial load and the self-weight act orthogonally to each other. Mechanical modelling is employed to determine the zones of behaviour in the stayed beam-column. Two of the existing three zones for vertical stayed columns are each sub-divided into two further sub-zones owing to self-weight. It is found that there is considerable post-buckling reserve especially at relatively higher prestressing levels, and the arrangement where the upper crossarm is shorter than the lower crossarm has a beneficial effect in some cases. A nonlinear analytical model for prestressed stayed beam-columns with doubly-symmetric and mono-symmetric configurations, based on the Rayleigh--Ritz method, is subsequently presented. It captures modal interactions for perfect geometries explicitly. It is also found that the varying prestressing levels and crossarm length ratios can lead to different buckling modes. From the obtained insight, finite element models are then employed to investigate the post-buckling behaviour including pre-cambering, imperfection and prestressing sensitivities. The actual optimum prestressing force is determined considering ultimate behaviour and efficiency indicators. It is found that mono-symmetric cases are more sensitive to the level of pre-cambering than doubly-symmetric cases, and for cases with upward pre-cambering, mono-symmetric arrangements perform better than their doubly-symmetric counterparts. Moreover, recommended upper and lower limits on prestressing levels are presented. Finally, a set of design equations is proposed based on the findings, such that stayed beam-columns may be used safely in practice. Open Access

High strength steel tubular elements combine the merits of both high strength steel and tubular profiles and are being increasingly used in the construction industry. However, more widespread use is inhibited by current structural design provisions, which are limited in scope and hindered in their development by a scarcity of experimental and numerical data. The primary aim of this study is therefore to expand the test and numerical data pool for high strength steel tubular elements and to develop more efficient yet safe structural design rules for use in practice. Four standard tubular profiles – circular, elliptical, square and rectangular hollow sections, are considered in this study. An experimental investigation into the cross-sectional behaviour of hot-rolled and cold-formed high strength steel hollow sections was conducted, including eleven stub column tests, twelve beam tests and 45 short beam-column tests; 3D laser-scanning and digital image correlation were employed in these experiments. Finite element (FE) models were established to replicate the test results and to generate supplementary numerical data. Shortcomings in the existing codified design rules were highlighted through comparisons of design predictions with the obtained test and FE data, and suitable amendments to the Eurocode 3 (EC3) design provisions were proposed accordingly. The member buckling behaviour of high strength tubular elements was subsequently examined through an experimental study, which consisted of 24 column buckling tests and twenty long beam-column tests, and a parallel numerical simulation programme. Modified EC3 design rules, featuring a yield strength-dependent imperfection factor and re-calibrated buckling and interaction curves, were developed and shown to yield greatly improved design predictions over the current approaches in terms of accuracy and consistency. In addition to the improvements within the EC3 design framework, a novel approach incorporating the generalised slenderness and reference resistances was proposed for the structural design of steel tubular sections to further enhance the ease of use and design efficiency. The benefits were clearly shown through comparisons of predicted resistances with the available test and FE results. Open Access

Recent years have witnessed substantial research into the behaviour and design of stainless steel structures, enabling the development and expansion of, among others, the European structural stainless steel design standard EN 1993-1-4. However, despite considerable progress in the establishment of room temperature design guidance, design rules for elevated temperatures are far less developed. The fire design provisions set out in EN 1993-1-2 for stainless steel structural components are largely based on those originally developed for carbon steel, as well as the room temperature stainless steel design rules given in EN 1993-1-4; as a result, there is significant scope for improvement. To this end, a comprehensive study has been carried out into the structural behaviour of stainless steel I-sections in fire. At the cross-section level, finite element models of stainless steel plates and I-sections, capable of mimicking their response in fire, were first created and validated against experimental results from the literature; the validated models were then employed to generate extensive structural performance data. A new cross-section classification approach and new effective width equations that are able to reflect the variation in strength and stiffness of stainless steel at different temperature levels were put forward. At the member level, eight column fire tests and six beam-column fire tests were conducted, the results of which were replicated numerically. Extensive parametric studies were then performed on stainless steel I-section columns and beam-columns at elevated temperatures. For the purpose of establishing an accurate and practical means of designing stainless steel columns and beam-columns in fire, new design rules were also proposed. The fire design provisions proposed in this research are due to be incorporated into the upcoming version of EN 1993-1-2. Open Access

Stainless steel is used widely across a range of industries due to its favourable structural properties, excellent corrosion resistance and fire resistance. Stainless steel structural design standards have generally been developed based on assumed analogies with carbon steel design. Consequently, resulting design solutions can be inaccurate and overly conservative due to the rounded nature of the stress–strain curve not being suitably reflected. The nonlinear material stress–strain response has a direct influence on the structural behaviour of stainless steel and it is fundamental that a design methodology reflects this. With an emphasis on frame level design, a series of proposals are made in this thesis to enable the safe and efficient assessment of stainless steel structures. A number of the proposals are also applicable to carbon steel design. The issue of overall frame stability in the inelastic regime is first addressed. An approach to allow for the premature loss of stiffness due to material nonlinearity is developed and a modified elastic buckling load factor method is proposed to account for the increased global second order effects. The modified critical load factor takes account of the reduced stiffness of the frame due to plastification and can be applied to both carbon steel and stainless steel frames. The opportunities offered through design by advanced (second order inelastic) analysis are then explored. This approach is particularly suited to stainless steel because of the design complexities that arise from the rounded material stress–strain response. The key components required for the design of stainless steel structures by advanced analysis are developed herein. The new method employs computationally efficient beam elements yet safely captures cross-section failure by utilising strain limits defined through the continuous strength method (CSM). The CSM strain limits simulate the effects of local buckling and control the extent to which plasticity, moment redistribution and strain hardening can be exploited. The practical application of the proposed method is facilitated through the development of equivalent bow imperfections derived specifically for design by second order inelastic analysis. The proposed method is applied to both individual members and systems and is shown to provide consistent and significant benefits over current design methodologies. Open Access

Stainless steel is gaining increasing use in structural engineering applications due to its corrosion resistance, low maintenance costs, high recyclability, aesthetic appeal, excellent fire resistance and favourable structural properties. The stress-strain behaviour of stainless steels differs fundamentally from that of carbon steel: stainless steel has a more rounded response with no clear yield point and significant strain hardening. This has a profound effect on the structural behaviour of stainless steel elements. The aim of this work is to investigate the behaviour of stainless steel I-sections under concentrated transverse loading and to develop design rules that reflect the particular characteristics of the material. Concentrated transverse loading is a load case where a force acts perpendicular to the flange of a girder over a relatively small area, causing local failure of the web beneath the load and flange bending. The current design code for structural stainless steel elements, namely Eurocode 3: Part 1.4, adopts the same design expressions for stainless steel as for carbon steel for such loading conditions. A comprehensive experimental and numerical investigation has therefore been conducted to evaluate the existing provisions and propose new design rules. A total of 34 member tests and over 500 finite element simulations have been performed covering three types of concentrated transverse loading – internal one-flange, internal two-flange and end one-flange loading, three stainless steel grades – austenitic, duplex and ferritic and a range of the key influential parameters. The results showed that the existing design recommendations are conservative and that there is considerable scope for the development of more economical design guidance. The new design equations offer 10% - 20% improvements in capacity predictions over the current design formulae. An alternative design approach, based on numerically generated reference loads, namely the elastic buckling and plastic collapse load under concentrated loading, in conjunction with strength curves, has also been proposed. This required the development of a consistent method for the numerical determination of plastic collapse loads, which is known to be challenging for the complex failure modes associated with localised loading. The reliability of both proposed design approaches have been verified by means of statistical analyses in accordance with EN 1990. Open Access

Steel 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 of steel structures, these effects are accounted for partially during the analysis and partially through subsequent cross-section and member checks. Design by advanced analysis avoids some of the issues encountered by traditional design methods (e.g. calculating effective length factors, choice of appropriate type of analysis) and provides a more accurate visualisation of the failure mechanisms, as most of the limit states governing the behaviour of a structure are directly captured in the analysis. The structural analysis of steel frames is typically performed using beam finite elements, which are usually not able to capture local buckling explicitly. Instead, in traditional steel design the assessment of local buckling and rotation capacity is made through the concept of cross-section classification, which places class-specific restrictions on the analysis type (i.e. plastic or elastic) and defines the cross-section resistance based on idealised stress distributions (e.g. the plastic, elastic or effective moment capacity in bending). This approach is however considered to be overly simplistic and creates artificial ‘steps’ in the capacity predictions of structural members. A more consistent approach is proposed herein, whereby a geometrically and materially nonlinear analysis with imperfections (GMNIA) of the structure is performed using beam finite elements, with strain limits employed to mimic the effects of local buckling. The strain limits are obtained from the Continuous Strength Method and effectively control the spread of plasticity, capture the effects of local moment gradients and, ultimately, define the structural resistance in bending dominated cases. Conversely, the structural capacity of stability governed systems is defined by the peak load factor of the advanced analysis. The practical application of the proposed design method is facilitated through the development of explicit functions to predict the elastic local buckling stress and half-wavelength of full cross-sections (i.e. including the effects of element interaction) subjected to pure compression, pure bending and combined compression plus bending. The presented functions have been derived from the results of finite strip analyses and are based on the concept that the full cross-section response lies between the lower and upper bound limits of the critical isolated plates with simply-supported and fixed boundary conditions along the adjoined edges. The developed method of design by advanced analysis is applied to individual columns, beams, beam-columns, continuous beams and planar frames. Capacity predictions are compared to the results obtained from benchmark shell finite element models and conventional steel design. It is found that the proposed method predicts safe-sided ultimate capacities that are consistently more accurate than current design methods, particularly for structures benefiting from strain hardening and for structures composed of non-compact cross-sections that previously were not able to benefit from inelastic redistribution. Open Access

At both room temperature and elevated temperatures, the cross-sectional load-carrying capacity of structural steel members is limited by the effects of local buckling. The strength and stiffness of steel also reduce with temperature and the stress-strain relationship becomes increasingly nonlinear. Current structural fire design codes utilise the concept of cross-section classification and the effective width method in line with the corresponding steel design rules at ambient temperature for the design of steel sections at elevated temperatures; this approach artificially separates structural steel cross-sections into discrete behavioural classes and does not reflect the inherent continuous relationship between the cross-section resistance and its local slenderness. Moreover, the utilization of two different design yield strengths results in further discontinuities in the predicted fire resistances at the boundary between non-slender and slender cross-sections. Existing steel design rules at ambient temperature are also based on an assumed elastic, perfectly plastic stress-strain response, which does not well represent the actual nonlinear stress-strain response of steels at elevated temperatures. A series of experiments to investigate the structural response and failure of cross-sections at room temperature is presented. A deformation-based design approach known as the continuous strength method (CSM), which provides an alternative treatment to the conventional concept of cross-section classification and enables a rational exploitation of strain hardening, is then developed for the design of steel cross-sections at elevated temperatures. Predictions of resistance using the CSM for compression and bending, as well as new proposals for combined loading, have been compared with experimental and numerical results and those obtained using EN 1993-1-2 and AISC 360-16; the CSM predictions are shown to offer higher levels of accuracy and reliability than the current design methods. Open Access

A novel concept, whereby prestressing techniques are utilised to enhance the load-carrying capacity and serviceability performance of cold-formed steel beams, is proposed. The prestressing is applied by means of a high-strength steel cable located within the bottom hollow flange of the cold-formed steel beam at an eccentric location with respect to its strong geometric axis. Since the initial stresses generated by the prestressing are opposite in sign to those induced during the subsequent imposed vertical loading stage, the development of instabilities is delayed and thus the capacity of the cold-formed steel beam is enhanced. Owing to the pre-camber that is induced along the member during the prestressing stage and the contribution of the cable to the system bending stiffness, the overall vertical deflections of the member are reduced significantly. Prestressed cold-formed steel beams can provide highly-efficient structural solutions. Owing to their enhanced structural performance, for a given span and imposed vertical load, a smaller cross-section is required and thus a more lightweight solution is achievable. Potentially, the proposed beams can open up new applications for cold-formed steel in construction, particularly in cases where increased load-carrying capacities, longer spans and reduced deflections are desired. The current thesis presents the conceptual development of prestressed cold-formed steel beams. The mechanical behaviour of the proposed structural system is explored using analytical and numerical models with the origin of the obtained structural benefits being demonstrated. Finite element modelling is employed to simulate the behaviour of the proposed beams during the prestressing and imposed vertical loading stages and parametric studies are conducted to investigate the influence of the key controlling parameters, such as the prestress level, cable size, section slenderness and centroid location. Finally, design recommendations and failure criteria, which define the permissible design zone for the prestressed system, are developed with their implementation being demonstrated through practical a worked example. Open Access

The material modelling and design of hot-rolled and cold-formed structural steel cross-sections are addressed in this thesis. Regarding the material modelling, constitutive equations that accurately represent the stress-strain response of hot-rolled and cold-formed steels have been developed and calibrated based on a large dataset of coupon test results collected from the global literature. The models are suitable for use in advanced numerical simulations and parametric studies. For the design of structural steel elements, a deformation based design approach named the Continuous Strength Method (CSM), which provides an alternative treatment to the conventional concept of cross-section classification adopted in current design specifications and enables a rational exploitation of strain hardening, has been extended to cover both hot-rolled and cold-formed steels. The CSM design proposals were underpinned by existing test results collected from the literature in conjunction with additional cross-section tests on stocky hot-rolled I-sections conducted in the present study and numerical results obtained from validated finite element models. The design of cross-sections in compression, bending and combined loading has been addressed. The new design approach has been shown to yield a higher level of accuracy and consistency compared with the current codified design methods, and the reliability of the proposed design approach has been confirmed by performing statistical analyses. This study promotes the CSM as a universal design approach that now covers the design of stainless steel, aluminium alloy and structural carbon steel cross-sections. Recommendations for future developments to the CSM has also been made. Open Access

The topic of this thesis is the investigation of the structural behaviour of cold-formed steel flooring and purlin systems, taking into consideration the beneficial effect of interaction between structural components. Experiments have been conducted on flooring systems comprising cold-formed steel joists and wood-based particle boards, considering the typical screw fixings employed in current practice as well as alternative means of shear connection. The experimental findings showed that mobilisation of composite action within this type of system, through enhancement, beyond that currently used, of the employed shear connection, is feasible, bringing corresponding increases in capacity and stiffness. In order for the influence of the key parameters to be further examined, a finite element model simulating the examined systems has been developed, validated and employed for parametric studies. Analyses confirmed the experimental findings, showing that significant benefits in terms of capacity and stiffness can be achieved, especially for systems comprising thinner steel sections. Based on the obtained experimental and numerical results, a full design method, following the fundamental concepts of current design codes for composite structures, has been devised, providing accurate predictions of moment capacity and flexural stiffness. Finally, a numerical investigation has been performed on continuous two-span roof systems comprising cold-formed steel purlins, accounting for their interaction with the corrugated sheeting. The study showed that moment redistribution is possible within these systems, but usually accompanied by a reduction of the moment capacity of the central support. A previously devised method for the design of continuous purlin systems, making direct use of cross-section capacities at key locations, together with a factor to allow for the fall-off in moment at the central support, has been assessed and advanced. Open Access

The topic of this thesis is the testing and design of conventionally formed and additive manufactured stainless steel hollow structural sections. Although design codes currently exist for stainless steel hollow structural elements, the provisions are based on limited structural data and therefore require further evaluation for their suitability, and are not intended to apply to additive manufactured elements. Focussing on conventionally formed circular hollow sections (CHS), the existing design provisions have been carefully reappraised based on a dataset of tests and finite element results generated in this study and existing tests collected from the literature. In total, 37 concentrically loaded column tests, 26 beam-column tests and 10 stub column tests have been undertaken on austenitic, duplex and ferritic stainless steel CHS. The experimental data has been supplemented with over 2400 finite element case studies. The reappraisal highlighted that there is additional capacity to be sought at the cross-section level for pure compression, bending and combined loading, and at the member level for beam-columns, but the current CHS flexural buckling provisions were found to be unconservative for certain global slenderness values. Based on these observations, revised design rules have been proposed. Additive manufactured sections, not currently covered by structural design standards, have also been investigated. An experimental programme consisting of 28 tensile coupon tests, 14 compressive coupon tests and 5 square hollow section (SHS) stub column tests has been undertaken. The initial results indicate general applicability of existing design standards to these new novel sections. Open Access

Concrete filled elliptical hollow sections (CFEHS) are a relatively new addition to the range of composite cross-sections available to structural engineers. The European design code EN 1994-1-1 (BSI, 2004) provides design rules for composite cross-sections, including concrete filled circular hollow sections (CFCHS) and concrete filled rectangular sections (CFRHS), but CFEHS are not considered in the present design code. In order to contribute to the development of design provisions for CFEHS, a comprehensive experimental and numerical study of their column and beam-column behaviour has been carried out. The testing programme covered a range of member lengths, reinforcement ratios and loading eccentricities and consisted of 27 column and beam-column member tests, 70 concrete cylinder compression tests, 3 reinforcing bar tensile tests and 2 steel tube tensile coupon tests. Numerical models were developed using the nonlinear finite element package Abaqus and validated against the experimental results. Using the validated models, detailed numerical parametric studies of CFEHS members have been conducted addressing three different scenarios: (1) members under axial compression, (2) members under combined axial compression and uniaxial bending and (3) members under combined axial compression and biaxial bending. Based on the combined test and numerical data set, along with previous experimental results reported in the literature, new design rules for CFEHS are proposed. It is shown that the current provisions of EN 1994-1-1 (BSI, 2004) for the design of CFCHS and CFRHS are appropriate for the design of members of elliptical cross-section, using either buckling curve b or c, depending on the level of steel reinforcement for members under axial compression. Cross-section axial load-moment (N-M) interaction curves are generally employed for the design of composite members under combined loading. A numerical approach, developed in Matlab, was used to the generate the N-M interaction diagram for CFEHS, which was found to offer a suitable design basis to be used in conjunction with calculated axial forces and second-order moments. Finally, an assessment of the reliability of the design proposals for CFEHS columns and beam-columns in accordance with Annex D of EN 1990 (BSI, 2002) was carried out. Open Access

Stainless steel structures have been increasingly used in engineering applications because of its corrosion resistance, aesthetic appeal, favourable structural properties as well as the development of stainless steel guidance. While the structural response of cold-formed stainless steel sections has been extensively studied in the literature, welded sections have received less attention to date. A recent addition to the range of structural stainless steel products is that of laser-welded sections. Owing to the high precision and low heat input of the fabrication process, the resulting sections have smaller heat affected zones, lower thermal distortions and lower residual stresses than would typically arise from traditional welding processes. There currently exists very limited experimental data on laser-welded stainless steel members owing to their recent introduction to the construction industry and their design is not covered by current design standards. To address the lack of test data and to investigate their structural response, a comprehensive experimental programme has been carried out covering complementary residual stress measurements, 9 stub column tests, 20 bending tests and 24 combined loading tests at the cross-sectional level, as well as 22 flexural buckling tests and 18 beam-columns at the member level. The derived test results were then employed in the numerical studies where the numerical models were first validated against the test results and then used to conduct a series of parametric studies to generate further data over a wider range of cross-sectional slendernesses, member slendernesses and loading combinations. The test and numerical results were used to assess the current design provisions and it was found that the design codes yield conservative predictions due to the neglect of strain hardening. Revised design rules have been proposed, offering less scattered, more accurate predictions for conventionally welded members and, for the first time, design provisions for laser-welded members. Open Access

Dissertação de Mestrado Integrado em Engenharia Civil apresentada à Faculdade de Ciências e Tecnologia O aço enformado a frio tem vindo a assumir cada vez mais importância no mundo da construção civil, fato esse devido às inúmeras vantagens que pode oferecer, garantindo um bom desempenho estrutural. A crescente popularidade conseguida nas últimas décadas é devida às propriedades únicas características do aço enformado a frio como, a sua leveza, a sua alta resistência e rigidez, a sua não combustibilidade, assim como a redução tanto no tempo da sua montagem como nos custos associados.A falta de estudos e investigações em relação a elementos de aço enformado a frio é avassaladora, especialmente em colunas em situação de incêndio, e o conhecimento adquirido até à data é ainda escasso sendo necessário mais investigações nesta matéria. A ausência de estudos torna-se, mais evidente se os efeitos da restrição axial e rotacional forem considerados na investigação, fator que simula uma situação real em que a coluna é parte integrante de uma estrutura.A presente dissertação, foi elaborada como parte de um projeto de pesquisa nacional denominado FireColdSteel – Análise experimental e numérica de elementos estruturais de aço enformado a frio sujeito a incêndio – PTDC/EMC/116859/2010 (FCT) e realça a necessidade do desenvolvimento de metodologias especificas para este tipo de elementos. A norma atualmente disponível está presente na EN1993-1-2:2005 (2005) e foi especificamente desenvolvida para aços laminados a quente, e também é aplicável a elementos de aço enformado a frio com secção transversal de classe 4.Os resultados obtidos experimentalmente foram posteriormente usados na validação dum modelo de elementos finitos desenvolvido, usando o software Abaqus (2014), para reproduzir o comportamento das colunas observado nos ensaios experimentais. O modelo numérico foi validado de encontro com os dados recolhidos na análise experimental, com o objetivo de expandir as simulações a um maior número de casos para lá dos limites impostos nesta dissertação.Os testes realizados mostraram que as colunas apresentaram baixa resistência ao fogo. Durante os ensaios experimentais foi observado que para colunas compostas o colapso foi controlado pela restrição axial imposta, já para as simples o colapso parece ter sido controlado pelo aumento das temperaturas levando a uma deterioração das propriedades mecânicas do material. The cold-formed steel has become more popular over the years in construction industry, mostly due to its innumerous advantages that this type of steel can offer, ensuring both structural behaviour. The increase popularity achieved over the last decades it is due to a number of unique properties characteristic of this steel such as, its lightness in weight, high strength and stiffness, non-combustibility, and a reduction in both erection time and construction costs.The lack of studies and researches regarding cold-formed steel members is overwhelming, specially in terms of columns in fire situation, and the knowledge achieved up to date is still scarce requiring a better understanding on this type of columns. The absence of studies regarding cold-formed steel columns is even clearer if the effects of axial and rotational restraint are considered within the research, which simulates a real-life situation when the column is an integral part of the structure.The present dissertation was conducted as part of a National Research Project entitled FireColdSteel – Experimental and Numerical Analyses of Cold-Formed Steel Members Under Fire Conditions – PTDC/EMC/116859/2010 (FCT) and enhances the need to develop specific guidelines for cold-formed steel members. The current available design standards are present in EN 1993-1-2:2005 (2005) specifically designed for hot-rolled steel members that are also applicable to cold-formed steel members with class 4 cross-section.The experimental results obtained were posteriorly used as input in order to validate the finite element models developed, using the finite element analysis Abaqus (2014), to reproduce the behaviour of the cold-formed steel specimens observed in the experimental fire tests. The finite element model was validated against the retrieved experimental data, and it is intended to expand the simulations into a larger number of elements outside of the bounds of this dissertation.The tests conducted showed that cold-formed steel columns present low fire resistance. During the experimental tests, it was observed that for built-up columns the restraint to thermal elongation seemed to control the failure, whereas for single columns the failure was due to the temperature increase leading to the degradation of the mechanical properties.

High strength steels (HSS), which are generally considered to be those with yield strengths over 460 MPa, are being increasingly utilised in construction, particularly in high rise structural applications and where long and column-free spans are an important design requirement. In place of ordinary carbon steels, the use of HSS can enable structural elements with smaller cross-sections, resulting in significant material savings. However, compared to normal strength steels, the structural use of HSS is still quite rare. The European design code EN 1993-1-12 provides design rules for HSS up to S700, but was conceived as a simple extension of the rules in EN 1993-1-1 for normal strength steels. In order to contribute to the existing limited HSS data pool and to verify and develop the current Eurocode 3 design rules, a comprehensive experimental programme on hot-finished S460 and S690 square and rectangular hollow sections has been carried out. The testing programme covered different structural aspects at the material, cross-section and member levels and consisted of 40 tensile coupon tests, 11 compressive coupon tests, 11 stub column tests, 11 full section tensile tests, 22 in-plane bending tests, 12 eccentrically loaded stub column tests, 30 long column tests, as well as measurements of geometrical imperfections and residual stresses. Numerical models, validated against the test results, were also developed to examine the cross-section and member behaviour, and subsequently employed in a comprehensive parametric study in order to generate further data. Based on the combined test and numerical data set, as well as experimental results reported in the literature, the current HSS design rules in Eurocode 3, including the slenderness limits for cross-section classification, effective width equation, N-M interaction curves and column buckling curves, were assessed by means of reliability analyses in accordance with Annex D of EN 1990. To realise the potential of HSS in long span structures, a novel structural form was also examined, namely an HSS truss with prestressing cables housed within the tubular bottom chord. A total of 4 prestressed trusses, made of S460 square hollow sections with different prestress levels, were tested under static downward loading. The truss test results showed the enhanced structural efficiency brought about by the addition of prestressing cables and by the application of prestress. Additionally, 12 tensile and 10 compressive member tests with cables, representing the bottom chord of the truss under gravity and uplift loading, respectively, were carried out to investigate the behaviour of individual prestressed cable-in-tube members. Analytical models and numerical models were also established to compare with the test behaviour and to contribute to the development of design rules for prestressed cable-in-tube systems. Open Access

Stainless steel has been gaining increasing use in a variety of engineering applications due to its unique combination of mechanical properties, durability and aesthetics. Significant progress in the development of structural design guidance has been made in recent years. However, an area that has remained relatively unexplored is that of combined loading. Testing and analysis of structural stainless steel elements subjected to combined axial load and bending is therefore the subject of this thesis. A comprehensive experimental programme was firstly carried out. At the cross-sectional level, a total of 15 stub column tests, 10 four-point bending beam tests, and 58 combined loading tests were conducted on different cross-section profiles (square, rectangular and circular hollow sections) from different material grades (austenitic, duplex and ferritic stainless steels). At the member level, 48 beam-columns were tested under both equal and unequal end moments to investigate the buckling behaviour of beam-column members subjected to constant bending and various moment gradients, respectively. The test results were then employed in a parallel finite element (FE) study. The FE models were first validated against the test results and then used to conduct a series of parametric studies to extend the available results over a wider range of cross-section sizes, member slendernesses, loading eccentricities, and moment gradients. The experimentally and numerically derived data were used to study the structural performance of stainless steel elements subjected to combined axial load and bending moment, to assess the accuracy of current codified design provisions, and to establish new, more accurate, design rules. Following the comparisons between the test and FE results and the existing design provisions, it was found that all existing design codes lead to unduly conservative strength predictions for stainless steel cross-sections under combined loading, mainly due to the neglect of strain hardening. For stainless steel beam column members, there was a high degree of scatter in the strength predictions, with both conservative and unsafe results. This was attributed mainly to inaccurate predictions of the column buckling and bending end points of the design interaction curves and inaccurate interaction factors. Improved design expressions were sought through extension of the deformation-based continuous strength method (CSM) to the case of combined loading at both cross-section and member levels. Comparisons of the predicted capacities with over 6000 test and FE results indicated that the CSM proposals yield accurate and consistent strength predictions for both stainless steel cross-sections and members under combined loading. The reliability of the proposals has been confirmed by means of statistical analyses, demonstrating their suitability for incorporation into future revisions of international design codes for stainless steel structures. Open Access

Stiffness reduction offers a practical means of considering the detrimental influence of imperfections and the spread of plasticity on the strength and stability of steel structures. In this thesis, a stiffness reduction approach for structural steel design is presented. The proposed method is carried out by reducing the stiffness of steel members through developed stiffness reduction functions and performing Linear Buckling Analysis (LBA-SR) and Geometrically Nonlinear Analysis (GNA-SR). Since the deleterious influence of geometrical imperfections, residual stresses and the spread of plasticity is fully taken into account through the developed stiffness reduction functions, the proposed design approach does not require the use of member design equations, but only requires cross-section checks, thus leading to practical design. Finite element models of steel members and frames are created and validated using experimental results from the literature. Geometrically and Materially Nonlinear Analyses (GMNIA) of the validated finite element models are used to verify the proposed stiffness reduction method in all considered cases. The proposed stiffness reduction method is initially developed for the flexural buckling assessment of columns and the in-plane design of beam-columns, where a stiffness reduction function is derived using the European column buckling curves, providing the same strength predictions as determined through these curves. The proposed method is then extended to the lateral-torsional buckling assessment of steel beams and the flexural-torsional buckling assessment of steel beam-columns. Having established its validity for individual members, the proposed method is applied to steel frames. Both non-redundant and redundant benchmark frames from the literature are considered. It is observed that the proposed stiffness reduction method provides a reliable and accurate design approach. Open Access

Aluminium alloys are nonlinear metallic materials with continuous stress-strain curves that are not well represented by the simplified elastic, perfectly plastic material model used in most existing design specifications. The aims of this study are to develop a more efficient design method for aluminium alloy structures by rationally exploiting strain hardening. The key components of this study include laboratory testing, numerical modelling and development of design guidance for aluminium alloy structures. As part of the present study, the experimental programme included tests on 11 stub columns, 40 simply supported beams, 46 continuous beams and corresponding tensile coupon tests. Numerical investigations of aluminium alloy simply supported beams and continuous beams were also conducted. The validated finite element models were used for extensive parametric studies, generating 96 results for beams under three-point bending, 96 under four-point bending and 210 for continuous beams. The experiments and numerical simulations have shown the following key features of the inelastic behaviour of aluminium alloy structural elements: (1) the significance of strain hardening, indicated by the ultimate stress over the yield stress, could be up to 50%; (2) non-slender section capacities could be generally up to 40% higher than the yield limits in compression, and 50% greater than the plastic moments in bending; (3) the experimental and numerical ultimate loads of continuous beams on non-slender sections go beyond the calculated loads corresponding to the occurrence of the first hinge by more than 10%. Previous experimental data on aluminium alloy stub columns and simply supported beams were also collected. These collected test data were used together with the newly generated experimental and numerical results obtained from this study, totalling about 900 data, to assess the design predictions of the American, Australian/New Zealand and European specifications. On average, the existing design methods under-estimated the capacity of aluminium alloy stub columns by around 15%~22%, simply supported beams by around 18%~40% and continuous beams by around 27% ~ 50%. Existing section classification limits in Eurocode 9 (2007) were also assessed, and while they were found to be safe, some improved limits were proposed. The combined experimental and numerical results were used to develop and calibrate a new design method, termed the continuous strength method (CSM). Two key components of the CSM – a base curve and a bi-linear material model for aluminium alloys have been proposed in this study. Global plastic analysis allowing for moment redistribution has also been adopted in the CSM. Unlike current practices, the CSM has the merits of adopting the continuous treatment for the cross-section deformation response, rationally exploiting the available capacity beyond the yield limit and reasonably allowing for redistributing the internal forces. The capacity predictions of aluminium alloy structural members have been improved by more than 30% using the CSM. Reliability analyses have also been performed to assess the reliability level of different design methods according to the American Institute of Steel Construction (2010) and European Standard EN1990 (2002) approaches. The CSM has been shown to be safe, efficient and consistent for aluminium alloy structural members. Open Access

The behaviour and design of prestressed steel structures, with an emphasis on trussed arches, are examined in this thesis. For long-span structural systems, where self-weight becomes an increasingly dominant component of the design loading, significant material savings can be achieved through the use of high tensile strength steel cables in conjunction with conventional steelwork. Further benefits can be achieved by prestressing the cables. In the system currently being investigated, the prestressed cables, which are housed within the bottom chord of tubular arched trusses, apply a compressive force to the chord members, which is opposite in nature to the resultant forces arising from the externally applied gravity loads. The stability of the trussed elements under prestress and the load--deformation response of the prestressed elements to the subsequent application of tensile loading are examined analytically, numerically and experimentally, with good correlation achieved between the three approaches. The benefits of prestressing, in terms of increased member strength and stiffness, are demonstrated, and optimal prestress levels are investigated. In instances of load reversal (e.g. due to wind uplift) in trusses without horizontal end anchorage that would allow catenary forces to develop, the presence of prestress can become detrimental. To examine this, a total of eight pin-ended cable-in-tube systems, featuring both non-grouted and grouted members, were tested in compression. Increasing initial prestress levels was found to reduce the capacity of the system in compression, but initial prestress was shown to be less detrimental than externally applied compressive loading of the same magnitude, due to the absence of second order bending moments. Finite element models were developed and, following accurate replication of test results, were used to generate parametric results for a range of member slendernesses and prestress levels. The test and FE results were compared against capacity predictions based on a proposed modified Perry-Robertson design method. Consistent, accurate and generally safe-side predictions were achieved. Following the examination of behaviour of individual prestressed elements within the truss, a series of analytical and numerical models of the full arched truss system were developed to investigate its global structural behaviour. Parametric studies revealed that the horizontal end boundary conditions, prestress level, truss depth and diagonal member arrangements were the key parameters influencing the stiffness, load bearing capacity and failure mode of the structure. Open Access

The external envelope of steel framed industrial buildings normally involves the use of purlins and rails spanning between the main hot-rolled frames to support the roofing/cladding. These purlins are typically light-gauge cold-formed steel members of complex shape for which the thinness of the material means that local instabilities will significantly influence their structural behaviour. In this thesis, the finite element (FE) method (ABAQUS) has been used to develop numerical analyses to study the buckling behaviour and degree of moment redistribution in continuous and sleeved cold-formed steel 2-span purlin systems. Five types of nonlinear FE analyses have been validated against reported physical tests: (i) continuous 2-span beams subjected to uniformly distributed load (UDL), (ii) single span beams subjected to a moment gradient, (iii) single span beams subjected to pure bending (iv) sleeved 2-span beams subjected to a UDL and (v) single span sleeved sections subjected to a moment gradient. The FE analyses were used to generate a large portfolio of new results for gravity and uplift loading for continuous and sleeved 2-span arrangements covering a wide range of cross-sections by varying the flange and web dimensions and material thickness. The effects of local and distortional buckling and limited rotational capacities for single span FE models were investigated. The 2-span FE results formed the basis for a simple modification to conventional plastic design that recognises the possibility of a reduction in moment with increasing rotation in the interior support region. The assumption of full moment redistribution for gravity loading was found to be only valid for stocky sections but not for slender sections. For uplift loading in addition to the potential reductions in moment at the interior support, limitations in the span moment due to lateral torsional buckling (LTB) for slender members were also accounted for. Based on the FE results, an α- reduction framework was established to predict the collapse load for continuous and sleeved 2-span systems. It was assumed that the cross-section or LTB resistance was achieved in the span while a reduced cross-section resistance allowing for the post-peak fall in capacity was achieved at the interior support. The accuracy of the proposed design method was compared against elastic and full plastic design cases by considering their ultimate load carrying capacities. Whereas the elastic design method provides overly-conservative results and plastic design overestimates the capacity of slender sections, the proposed design method gave accurate predictions of the failure load with minimal scatter for all cases. The developed α-reduction framework provides a foundation for allowing the use of other purlin sections and interior support connections by inserting alternative cross-sectional moment capacity inputs obtained from several sources such as physical testing, hand calculations from design codes and FE analyses. Open Access

The current practice of ultimate limit state design for steel structures involves an elastic--perfectly plastic material model and the classification of cross-sections into discrete behavioural classes. This leads to a design philosophy which is simple, but generally over-conservative. The Continuous Strength Method is a strain based design approach which allows for the beneficial influence of strain hardening. At the core of the method is a base curve which relates the deformation capacity of a cross-section to its cross-section slenderness. Deformation capacity is defined through a strain ratio, which is the ratio of the maximum strain that a cross-section can endure to its yield strain. The formulation for the base curve was derived by means of stub column and bending tests collected from the literature. Knowing the limiting strain and assuming plane sections remain plane, the resistance of cross-sections to combinations of axial load and bending moments can be calculated by integrating the stresses arising from a suitable strain hardening material model over the area of the cross-section. Analytical and design expressions have been developed, and the resistance predictions for open and closed cross-section shapes have been compared with existing collated test data, and shown to give additional capacity over current design approaches, with a reduction in scatter and a more consistent method. Beyond the analysis of the cross-section, the method has been extended to the global instability of pin-ended columns by utilising moment--curvature--thrust curves. The curves were paired with an assumed buckled displacement shape to find applicable equilibrium configurations, and to extract the peak axial loads for producing buckling curves. The column buckling curves showed two distinct regions based on the global slenderness of the column. Firstly a region of global-dominated failure, where the columns failed by a loss of overall flexural rigidity, and secondly a local-dominated failure region, where the mid-height cross-sections failed by local buckling. The local cross-section failure mode allowed for axial loads greater than the cross-section yield loads. The column buckling curves were found to be dependent on the initial out-of-straightness, the cross-section geometry and the material yield stress. An experimental program provided insight into the cross-section resistance of hot-rolled rectangular hollow sections (RHS). The experiments included 32 material tensile coupon tests, eight stub column tests and four simply supported beam tests, and exhibited little strain hardening. Overall, a series of developments to a strain based approach for steel structures has been presented, and areas for future developments have also been highlighted. Open Access

This thesis examines the behaviour of cold-formed stainless steel tubular structural members, with an emphasis on ferritic stainless steels. Owing to the high comparative expense of stainless steel relative to traditional carbon steel, this study aims to identify and develop means of utilising the material more efficiently. A comprehensive material test programme was carried out as part of an extensive study into the prediction of strength enhancements in cold-formed structural sections that arise during production. Material tests on a total of 51 flat coupons and 28 corner coupons, extracted from a total of 18 cross-sections formed from a wide range of materials, were performed. A new, simple and universal predictive model for harnessing the cold-formed induced strength enhancements was developed which offers, on average, 19% and 36% strength enhancements for the cross-section flat faces and corner regions, respectively, relative to the strength of the unformed material. Ferritic stainless steels, having no or very low nickel content, offer a more viable alternative for structural applications to the more commonly used austenitic stainless steels, reducing both the level and variability of the initial material cost. There is currently limited information available on the structural performance of this type of stainless steel. Therefore, to overcome this limitation, a series of material, cross-section and member tests have been performed on two ferritic grades - EN 1.4003 and EN 1.4509. The experimental results were used to assess the applicability of the current codified design provisions to ferritic stainless steel structural components. Moreover, the elevated temperature performance of ferritic stainless steels, covering the material response and the flexural buckling behaviour, was investigated through analysis of experimental and numerical results, leading to proposals for suitable design recommendations. Finally, simplifications and refinements to the recently developed continuous strength method (CSM) were made. Comparison of the predicted capacities with over 140 collected test results on stainless steel stub columns and cross-sections in bending shows that the CSM offers improved accuracy and reduced scatter relative to the current design methods. The reliability of the approach has been demonstrated by statistical analyses, enabling its use in structural design standards. Open Access

Many of the principal concepts that underpin current metallic structural steel design codes, notably Eurocode 3, were developed on the basis of elastic, perfectly plastic material behaviour, essentially ignoring strain-hardening; such material behaviour lends itself to the concept of discrete cross-section classification. A newly proposed, deformation based approach to structural steel design represents an alternative treatment to cross-section classification that is based upon a continuous relationship between cross-section slenderness and deformation capacity, as well as a rational exploitation of strain- hardening. This method is referred to herein as the Continuous Strength Method. The aim of this research is to develop preliminary guidance for the use of the Continuous Strength Method at the member level, focusing on the behaviour of simply supported and continuous beams. Particular attention will be given to determining the maximum laterally unsupported lengths prior to which the full capacity predictions of the Continuous Strength Method can be achieved, as well as the performance of lateral bracing elements in various structural configurations. Through a programme of experiments, numerical modelling and parametric studies, the implications of allowing for strain-hardening in the design of laterally restrained steel beams is investigated with particular emphasis on the performance of the bracing elements. A total of fourteen tests on simply supported beams and six tests on continuous beams were performed considering two basic scenarios: discrete rigid restraints and discrete elastic restraints of varying stiffness. In all tests, bending resistances in excess of the plastic moment capacity were observed, but generally it was concluded that closer restraint spacing than specified in current design codes to achieve the cross-section capacity may be required to harness significant benefit from strain-hardening and to develop the full CSM bending resistance. The forces generated in the restraints were within current code requirements although some modifications were suggested. Furthermore, the spacings of the restraints were also considered and a new limiting slenderness and transition curve for the CSM was proposed. The results from the experiments were supplemented by parametric studies conducted using analytical and numerical models developed as part of this thesis, as well as through the use of proprietary software packages. A parallel experimental investigation into the material modelling assumptions of the Continuous Strength Method was also conducted, employing an innovative full cross-section tensile test to capture average cross-section material properties. The results from the investigation validated the modelling assumptions of the Continuous Strength Method and improved the accuracy of the predictive capacity equations. Open Access

The current work analyses the lateral stability of imperfect discretely-braced steel beams using variational methods. To facilitate the analysis, Rayleigh-Ritz approximations are used to model the lateral deflection and the angle of twist. The applicability of the methods is initially demonstrated for the cases of unrestrained and continuously restrained beams by comparison with both analytical and numerical solutions of the governing differential equations of the respective systems. The method is then applied in full to the case of a discretely-braced beam. Initially, it is assumed that the degrees of freedom (DOFs) can be represented by single harmonics; this is then compared to the more accurate representation of the DOFs as full Fourier series. After carrying out a linear eigenvalue analysis of the system, it is found that the beam can buckle into two separate classes of modes: a finite number of modes, equal to the number of restraints provided, which involve displacement of the restraint nodes and interaction between distinct sets of harmonics, and an infinite number of single harmonic internodal buckling modes where the nodes remain undeflected. Expressions are derived for the elastic critical moment of the beam, the forces induced in the restraints and the threshold stiffness, i.e. the minimum stiffness required to enforce the first internodal buckling mode, whereupon the beam attains its maximum achievable critical moment. The analytical results for the critical moment of the beam are validated by the finite element program LTBeam, while the results for the deflected shape of the beam are validated by the numerical continuation software Auto-07p, with very close agreement between the analytical and numerical results. Design formulae, from which practical design rules can be developed, are given for the critical moment, restraint force and threshold stiffness. The design rules return values close to those predicted from theory. When compared against equivalent design rules developed based on analogies with column behaviour, it is found that the column rules are generally overly conservative for restraints attached close to the compression ange and considerably unsafe for restraints attached close to the shear centre.

The load-carrying capacity of slender columns is limited by global instability. However, through the addition of strategically placed cross-arms and external prestressed cables, buckling displacements can be inhibited and the load-carrying capacity considerably enhanced. Such systems, known as prestressed stayed columns offer efficient and lightweight structural solutions. Previous research on prestressed stayed columns has been largely analytical and numerical studies. To the knowledge of the author, experimental investigations into the antisymmetric and interactive modes of buckling in stayed columns have not been attempted hitherto. In addition, although some previous studies have attempted to investigate the optimum prestressing configuration of stayed columns for design purposes, generic design guidance for this type of structural component has been lacking. Therefore, the primary aim of the current study is to conduct experimental and numerical investigations into the possible buckling and post-buckling behaviour and to develop an efficient design method for these structural components. A full scale experimental investigation has been conducted with a total of 18 test specimens to demonstrate the critical modes of buckling (symmetric and antisymmetric) with interactive post-buckling. This has also investigated the imperfection sensitivity of the stayed columns. Nonlinear finite element (FE) modelling was conducted in parallel with the experiments. These models were utilized, after successful validation against the experimental results, to investigate the sensitivity of the stayed system to the variation of key parameters. Subsequently, an efficient design method for prestressed stayed columns has been developed, including design charts and equations relating the resistance of the stayed column system to the level of the initial prestress in the cables for varying cross-arm lengths and global imperfection levels. Structural reliability analysis, using the procedures in Annex D of EN 1990, was conducted to evaluate the design safety factor. Worked examples of the proposed design method are also presented to demonstrate the developed procedure for diiferent key cases. It is shown that a straightforward yet rational design method has been formulated.

Despite growing interest in the use of stainless steel in the construction industry and the development of a number of national and regional design codes, stainless steel is often regarded as only suitable for specialised applications. This is attributed largely to the high initial material cost associated with the most commonly adopted austenitic grades of stainless steel, as well as some conservatism embedded in current stainless steel guidance. A recently developed grade, known as lean duplex stainless steel (EN 1.4162), possesses higher strength than the common austenitic grades and has a lower cost, along with good corrosion resistance and adequate weldability and fracture toughness. The structural performance of lean duplex stainless steel remains relatively unexplored to date with only a few studies having been performed. The main aim of this study is to examine the structural behaviour of lean duplex stainless steel welded I-sections, and to assess the applicability of the current European stainless steel design guidance. As part of this research, a total of fifty two material tests, four stub column tests, eight 3-point and 4-point bending tests, eight continuous beam tests and nine shear buckling tests were carried out. The experimental programme was complemented by a parallel numerical investigation, in which finite element models were initially validated against the test results and subsequently used for parametric studies. These test and numerical results were used in conjunction with existing test data on stainless steel welded I-sections to characterise the basic material properties, assess the codified slenderness limits for cross-section classification, investigate the applicability of plastic design to indeterminate stainless steel structures, and establish new shear resistance design equations for stainless steel plate girders. Based on the findings, it was concluded that the present European design provisions can be safely applied to lean duplex but are rather conservative in some areas. To rectify this, modifications have been proposed for cross-section classification, plastic design and shear resistance calculations. These proposals, together with additional developments to the strain based continuous strength method of design, are suitable for incorporation into future revisions of Eurocode 3.

Current structural steel design codes, such as EN 1993-1-1, were developed on the basis of a bi-linear (elastic, perfectly-plastic) material model, which lends itself to the idea of cross-section classification. This step-wise design concept is a useful, but somewhat artificial simplification of the true behaviour of structural steel since the relationship between the resistance of a structural cross-section and its slenderness is, in reality, continuous. The aim of this study is therefore to develop a more efficient structural steel design method recognising this relationship and rationally exploiting strain-hardening, whilst maintaining, where possible, consistency with current design approaches. As part of the present study, laboratory tests were carried out on cold-formed and hot-rolled steel hollow sections. A total of 6 simple beams and 12 continuous beams (with two configurations) and corresponding material coupon tests were conducted. These experimental results were added to existing collected test data to develop and calibrate a new structural steel design method. The test results indicated that capacities beyond the yield load in compression and the plastic moment capacity in bending could be achieved due to strain-hardening. The new design approach, termed the continuous strength method (CSM), enables this extra capacity to be harnessed. The developed deformation based steel design method employs a continuous ‘base curve’ to provide a relationship between cross-section slenderness and deformation capacity in conjunction with a strain-hardening material model. The material model is elastic, linear-hardening and has been calibrated on the basis of collected stress-strain data from a range of structural sections. The CSM has been developed for both statically determinate and indeterminate structures utilising both experimental data and that generated through sophisticated numerical modelling. Comparisons between test results and predictions according to EN 1993-1-1 and the proposed method were made. The results revealed that the CSM provides a more accurate prediction of test response and enhanced structural capacity over current design methods.

Concentrically braced frames are a common form of earthquake resistant structure. Performance of the structure is largely dependent on the ability of the key dissipative components, in this case the diagonal bracing members, to undergo significant inelastic deformations. Whilst many earlier studies have examined the hysteretic response of bracing members, comparatively less attention has been given to the ultimate behaviour and failure conditions. There are also significant uncertainties in existing models for predicting the ductility capacity of braces owing to their semi-empirical nature as well as the scatter of test results. This research project examines the cyclic behaviour of tubular braces made of a familiar structural material, carbon steel, and an increasing popular alternative structural material, stainless steel, which is known for its high tensile ductility. As part of the current study, laboratory tests were performed on hot-rolled carbon steel, cold-formed carbon steel and cold-formed austenitic stainless steel hollow section members and materials coupon cut from them. A total of 24 tensile coupon tests, 62 cyclic material tests and 16 cyclic member tests were conducted. Strain-life relationships of the materials under low and extremely low cycle fatigue regimes were established from the results of the cyclic material tests. These data were also used for calibrating material cyclic hardening models, which were incorporated in numerical models of hollow section members. These models, verified against the results of the cyclic member tests from this study and other research programmes, were employed in parametric studies to investigate the influence of member geometry and material properties on the behaviour of the bracing members. Although the three materials exhibit similar strain-life relationships, cold-formed stainless steel members perform better in terms of displacement ductility and energy dissipation, which is due to the cyclic hardening and higher post-yield stiffness of the stainless steel material. Implications of these findings on the design of earthquake resistant concentrically braced frames are discussed and design guidance for stainless steel bracing members is proposed.

The initial material cost of structural stainless steel is about four times that of structural carbon steel, due largely to the expense of the alloying elements and the relatively low volume of production. Given broadly similar structural performance, additional areas of benefit need to be identified and exploited in order to establish stainless steel as a viable alternative material for construction. In addition to the familiar benefits of corrosion resistance, low maintenance, high residual value and aesthetics, one such area is fire resistance. Material properties and their response to elevated temperatures form an essential part of structural fire design. The mechanical and thermal properties of stainless steel differ from those of carbon steel due to variation in chemical composition between the materials. A comparison of these properties for austenitic stainless steel with those for structural carbon steel is presented in this thesis, and implications of the differences explored. A total of 23 column buckling tests, 6 stub column tests, 5 simply supported beams, 1 continuous beam and 14 temperature development tests have previously been conducted on stainless steel sections in fire. These tests have been replicated numerically using the non-linear finite element package ABAQUS. Following accurate replication of the tests, a series of parametric studies were performed to expand the range of available data. Based on comparisons between all available test data and the current design rules in Eurocode 3: Part 1.2, together with the findings of the numerical study, a number of revisions to the code have been proposed. They include revised values for the heat transfer coefficient and emissivity, revised buckling curve, consistent strain limits and a new approach to the treatment of cross- section classification and local buckling. These revisions have led to a more accurate determination of temperature development in structural stainless steel, and provide more efficient and more consistent treatment of buckling of stainless steel structures in fire. Open Access