<p> A research programme is presented for assessing the capability of Hollow Structural Sections in Plastic Design. This investigation attempts to relate the flange slenderness and yield stress to the rotation capacity of Hollow Structural Sections subjected to both constant moment regions and to moment gradients. </p> <p> An experimental programme was performed on 31 different cross sections to evaluate the moment-curvature relationship which is of fundamental importance in Plastic Methods. The occurrence of local buckling for some sections in the compression flange and the consequent reduction in moment resistance is the critical factor which separates members into compact and non compact categories. </p> <p> The moment-curvature relations from tests are compared with analytical predictions. The plastic hinge rotations delivered by the present test sections are compared with the maximum practical requirements for plastically designed continuous beams. Theoretical elastic and inelastic buckling solutions of plate elements are also presented to relate to possible local buckling of the flats of square and rectangular hollow structural sections. </p> <p> Plate ratios of compression flanges are then selected for use in plastic design of hollow structural sections. Such a separation permits segregation into compact and non compact categories and can be used in working stress or elastic design methods. </p> / Thesis / Master of Engineering (ME)

This paper presents the behaviour and design of axially loaded concrete filled stainless steel circular and square hollow sections. The experimental investigation was conducted using different concrete cube strengths varied from 30 to 100 MPa. The column strengths and load-axial shortening curves were evaluated. The study is limited to cross-section capacity and has not been validated at member level. Comparisons of the tests results together with other available results from the literature have been made with existing design methods for composite carbon steel sections ¿ Eurocode 4 and ACI. It was found that existing design guidance for carbon steel may generally be safely applied to concrete filled stainless steel tubes, though it tends to be over-conservative. A continuous strength method is proposed and it is found to provide the most accurate and consistent prediction of the axial capacity of the composite concrete filled stainless steel hollow sections due largely to the more precise assessment of the contribution of the stainless steel tube to the composite resistance.

No que se refere as estruturas, apesar de os esforços resistentes de cálculo (força axial de compressão e momento fletor) em situação de incêndio estarem previstos nas prescrições normativas da ABNT, Eurocode, AISC e outros documentos pertinentes ao tipo de análise, o efeito da instabilidade local no esforço resistente das seções dos perfis \"I\" de aço soldados e laminados ainda não se encontra bem delineado em função da elevação de temperatura, sendo que as normas técnicas acabam superestimando o referido efeito, conduzindo a um dimensionamento antieconômico. Devido às vantagens associadas ao pequeno peso próprio e à eficiência de perfis \"I\" de aço, esbeltos, esses elementos são amplamente utilizados na construção civil. Entretanto, as regras atuais de dimensionamento em situação de incêndio das normas técnicas são imprecisas, podendo inviabilizar a utilização de perfis esbeltos para essa finalidade. A presente pesquisa teve como objetivo o estudo aprofundado do efeito da instabilidade local em perfis \"I\" de aço no dimensionamento em situação de incêndio. Foi realizada modelagem computacional via SAFIR, programa de computador de análise termestrutural, com base no Método dos Elementos Finitos (MEF), prospecção de resultados e comparação dos esforços resistentes advindos da aplicação das prescrições normativas àqueles obtidos via MEF. Para perfis de catálogos comerciais ou normatizados, os resultados obtidos permitem identificar que as normas técnicas ABNT NBR 14.323:2013 e Eurocode 3 Parte 1-2 são conservadoras para pilares na faixa de temperatura entre 600,00 ºC e 800,00 ºC e vigas na faixa de temperatura entre 400,00 ºC e 800,00 ºC, subestimando os resultados. Ademais, foi possível verificar que as normas técnicas não são conservadoras em geral, sendo que para algumas circunstâncias os resultados são satisfatórios ou até mesmo contra a segurança. Neste trabalho, mostrou-se a necessidade de aprimoramentos em termos de precisão nas normas técnicas vigentes no que se refere à instabilidade local em situação de incêndio. / Regarding structures, although fire resistance efforts (axial compression and bending moment) are predicted according to the ABNT, Eurocode, AISC and other documents related to that kind of analysis, the local buckling effect on resistance of steel \"I\" welded and hot rolled cross sections in function of temperature requires further studies, being that the technical references overestimate its value leading to a non-economical design. Considering slender \"I\"-shaped profiles advantages linked to its lightweight and effectiveness, those materials are widely used in civil construction. However, actual design rules in fire situation are imprecise, being able to make unfeasible the usage of slender cross sections. The present research aimed a comprehensive study of the local buckling effect on steel \"I\" cross sections in fire situation. It has been conducted computational modeling through SAFIR, specific software of thermostructural analysis, based on the Finite Element Method (FEM), prospection of results and comparison of the resistance efforts obtained from the application of the normative prescriptions and the ones obtained through FEM. For commercial and standardized catalog profiles, obtained results allow to identify that ABNT NBR 14.323:2013 and Eurocode 3 Part 1-2 are conservative for temperature range between 600,00 ºC and 800,00 ºC for columns and for temperature range between 400,00 ºC to 800,00 ºC for beams, underestimating the results. In addition, it was possible to verify that the standards are not conservative at all, being that for some circumnstances results are satisfactory or even against safety. In the present work, it was exhibited the need of enhancements in terms of precision on current standards regarding local buckling at elevated temperatures.

Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis. This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled: Compression Stability of High Strength Steel Sections with Low Strain-Hardening. For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes. A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement to the experimental results. Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models. As expected for the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities. The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian & New Zealand Standard for Cold-Formed Steel Structures AS&NZS 4600 and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel. Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.

Thin-walled steel sections made from high strength thin cold-reduced G550 steel to Australian Standard AS 1397-1993 under compression are investigated experimentally and theoretically in this thesis. This thesis describes three series of compression tests performed on box-section stub columns, box-section long columns and lipped channel section columns cold-formed from high strength steel plates in 0.42 mm or 0.60 mm thickness with nominal yield stress of 550 MPa. The tests presented in this thesis formed part of an Australian Research Council research project entitled: Compression Stability of High Strength Steel Sections with Low Strain-Hardening. For the fix-ended stub column tests, a total of 94 lipped-square and hexagonal section stub columns were tested to study the influence of low strain hardening of G550 steel on the compressive section capacities of the column members. For the pin-ended long column tests, a total of 28 box-section columns were tested to study the stability of members with sections which undergo local instability at loads significantly less than the ultimate loads. For the fix-ended lipped channel section columns, a total of 21 stub and long columns were tested to study the failure resulting from local and distortional buckling with interaction between the modes. A numerical simulation on the three series of tests using the commercial finite element computer program ABAQUS is also presented as part of this thesis. The post-buckling behaviour of thin-walled compression members is investigated. The effect of changing variables, such as geometric imperfections and end boundary conditions is also investigated. The ABAQUS analysis gives accurate simulations of the tests and is in good agreement to the experimental results. Theoretical studies using finite strip methods are presented in this thesis to investigate the buckling behaviour of cold-formed members in compression. The theoretical studies provide valuable information on the local and distortional buckling stresses for use in the interaction buckling studies. The finite strip models used are the semi-analytical and spline models. As expected for the stub columns tests, the greatest effect of low strain hardening was for the stockier sections where material properties play an important role. For the more slender sections where elastic local buckling and post-local buckling are more important, the effect of low strain hardening does not appear to be as significant. The pin-ended and fix-ended long column tests show that interaction, which is between local and overall buckling in the box sections, and between local and distortional buckling in the open channel sections, has a significant effect on their member capacities. The results of the successful column tests and ABAQUS simulation have been compared with the design procedures in the Australian & New Zealand Standard for Cold-Formed Steel Structures AS&NZS 4600 and the North American Specification for Cold-Formed Steel Structural Members prepared by the American Iron and Steel Institute. The stub column tests show that the current design rules give too conservative predictions on the compressive section capacities of the column members; whereas the long column tests show that the current column design rules are unconservative if used in their current form for G550 steel. Three design proposals are presented in this thesis to account for the effects of high strength thin steels on the section and member capacities.

No description available.

Cold-formed steel roof truss systems that use complex stiffener patterns in existing hat shape members for both top and bottom chord elements are a growing trend in the North American steel framing industry. When designing cold-formed steel sections, a structural engineer typically tries to improve the local buckling behavior of the cold-formed steel elements. The complex hat shape has proved to limit the negative influence of local buckling, however, distortional buckling can be the controlling mode of failure in the design of chord members with intermediate unbraced lengths. The chord member may be subjected to both bending and compression because of the continuity of the top and bottom chords. These members are not typically braced between panel points in a truss. Current 2001 North American Specifications (NAS 2001) do not provide an explicit check for distortional buckling. This dissertation focuses on the behavior of complex hat shape members commonly used for both the top and bottom chord elements of a cold-formed steel truss. The results of flexural tests of complex hat shape members are described. In addition, stub column tests of nested C-sections used as web members and full scale cold-formed steel roof truss tests are reported. Numerical analyses using finite strip and finite element procedures were developed for the complex hat shape chord member in bending to compare with experimental results. Both elastic buckling and inelastic postbuckling finite element analyses were performed. A parametric study was also conducted to investigate the factors that affect the ultimate strength behavior of a particular complex hat shape. The experimental results and numerical analyses confirmed that modifications to the 2001 North American Specification are necessary to better predict the flexural strength of complex hat shape members, especially those members subjected to distortional buckling. Either finite strip or finite element analysis can be used to better predict the flexural strength of complex hat shape members. Better understanding of the flexural behavior of these complex hat shapes is necessary to obtain efficient, safe design of a truss system. The results of these analyses will be presented in the dissertation. / Ph. D.

The importance of fire safety design has been realised due to the ever increasing loss of properties and lives caused by structural failures during fires. In recognition of the importance of fire safety design, extensive research has been undertaken in the field of fire safety of buildings and structures especially over the last couple of decades. In the same period, the development of fire safety engineering principles has brought significant reduction to the cost of fire protection. However the past fire research on steel structures has been limited to heavier, hot-rolled structural steel members and thus the structural behaviour of light gauge cold-formed steel members under fire conditions is not well understood. Since cold-formed steel structures have been commonly used for numerous applications and their use has increased rapidly in the last decade, the fire safety of cold-formed steel structural members has become an important issue. The current design standards for steel structures have simply included a list of reduction factors for the yield strength and elasticity modulus of hot-rolled steels without any detailed design procedures. It is not known whether these reduction factors are applicable to the commonly used thin, high strength steels in Australia. Further, the local buckling effects dominate the structural behaviour of light gauge cold-formed steel members. Therefore an extensive research program was undertaken at the Queensland University of Technology to investigate the local buckling behaviour of light gauge cold-formed steel compression members under simulated fire conditions. The first phase of this research program included 189 tensile coupon tests including three steel grades and six thicknesses to obtain the accurate yield strength and elasticity modulus values at elevated temperatures because the deterioration of the mechanical properties is the major parameter in the structural design under fire conditions. The results obtained from the tensile tests were used to predict the ultimate strength of cold-formed steel members. An appropriate stress-strain model was also developed by considering the inelastic mechanical characteristics. The second phase of this research was based on a series of more than 120 laboratory experiments and corresponding numerical analyses on cold-formed steel compression members to investigate the local bucking behaviour of the unstiffened flange elements, stiffened web elements and stiffened web and flange elements at elevated temperatures up to 800°C. The conventional effective design rules were first simply modified considering the reduced mechanical properties obtained from the tensile coupon tests and their adequacy was studied using the experimental and numerical results. It was found that the simply modified effective width design rules were adequate for low strength steel members and yet was not adequate for high strength cold-formed steel members due to the severe reduction of the ultimate strength in the post buckling strength range and the severe reduction ratio of the elasticity modulus to the yield strength at elevated temperatures. Due to the inadequacy of the current design rules, the theoretical, semi-empirical and empirical effective width design rules were developed to accurately predict the ultimate strength of cold-formed steel compression members subject to local buckling effects at elevated temperatures. The accuracy of these new design methods was verified by comparing their predictions with a variety of experimental and numerical results. This thesis presents the details of extensive experimental and numerical studies undertaken in this research program and the results including comparison with simply modified effective width design rules. It also describes the advanced finite element models of cold-formed steel compression members developed in this research including the appropriate mechanical properties, initial imperfections, residual stresses and other significant factors. Finally, it presents the details of the new design methods proposed for the cold-formed steel compression members subject to local buckling effects at elevated temperatures.

A rather complete picture of automatic plastic-hinge analysis onto steel frames under static loads is made in the present thesis. One/two/three-linear behaviours of mild steel are considered. The frames are submitted to fixed or repeated load. The geometric nonlinearity is taken into account. The beam-to-column joints of structures could be rigid or semi-rigid. The compact or slender cross-sections are examined. The investigation is carried out using direct or step-by-step methods. Both analysis and optimization methodologies are applied. From the fundamental theory to the computer program aspect are presented. Various benchmarks in open literatures are tested demonstrating the efficiency of the implementation.

L’objectif de ce travail de thèse est de développer une approche analytique permettant de définir la charge ultime d’une poutre cellulaire à ouvertures sinusoïdales. En effet, l’évolution des techniques de production a permis le développement d’une nouvelle forme de poutre cellulaire munie d’ouvertures sinusoïdales : la poutre AngelinaTM. Cette nouvelle forme d’ouverture implique de nouveaux modes de ruine. De ce fait, en vue de développer un modèle de calcul analytique adapté à cette nouvelle forme d’ouverture, une campagne d’essais expérimentaux a été menée sur des poutres cellulaires à ouvertures sinusoïdales à grande échelle (≈10m). Au travers de ces études expérimentales sur trois configurations de poutre, nous avons montré que le principal mode de ruine est lié à la flexion Vierendeel. En effet, la flexion locale des membrures de l’ouverture la plus sollicitée engendre soit la formation de 4 rotules plastiques aux 4 coins de l’ouverture, soit l’instabilité locale des parois d’âme comprimées. Un modèle aux éléments finis a été développé sur le logiciel SAFIR afin d’analyser les différents modes de ruines observés. Ce modèle a été validé sur la base des résultats expérimentaux et nous a permis d’identifier deux points particuliers : d’une part l’existence d’un maintien rotationnel entre le montant intermédiaire et la paroi d’ouverture et d’autre part, la ruine de l’ouverture ne se produit qu’au travers d’un mécanisme combinant les ruines des différents quarts d’ouverture. Une seconde campagne d’études expérimentales et numériques a ensuite été menée sur des parties isolées, extraites des poutres préalablement testées, afin d’étudier de manière locale le comportement à la flexion des quarts d’ouverture. Ces études ont servi à valider un second modèle aux éléments finis, développé sur le logiciel Cast3m. Celui-ci nous a permis, au travers d’une étude paramétrique, de quantifier le maintien rotationnel apporté par le montant intermédiaire sur la paroi d’âme d’ouverture adjacente. Cette étude a confirmé l’importance de la rigidité apportée par le montant intermédiaire aux parois d’âme adjacente. Ainsi, cet apport de rigidité doit être pris en compte dans l’approche analytique pour définir de manière réaliste la résistance au voilement local des différentes parties d’une ouverture sinusoïdale. Finalement, cette thèse a abouti au développement d’un nouveau modèle analytique de calcul de la résistance ultime des parois d’une ouverture sinusoïdale. Du fait des éventuelles instabilités locales, le modèle analytique s’est appuyé sur des éléments théoriques de stabilité des plaques. De plus, une étude numérique détaillée du mécanisme de ruine d’une ouverture isolée nous a permis de justifier une approche cinématique de ruine de l’ouverture sinusoïdale. Cette approche combine les résistances ultimes des différents quarts d’ouverture. Le modèle analytique proposé permet de considérer à la fois la résistance ultime de chaque partie de l’ouverture et leurs modes de ruine. Une étude comparative avec des résultats numériques a montré que ce modèle est fiable et représentatif de la réalité pour caractériser l’état limite ultime des poutres cellulaires à ouvertures sinusoïdales. / The aim of this thesis is to develop an analytical approach so as to define the ultimate failure load of cellular beams with sinusoidal openings. Indeed, the evolution of the conception techniques led to the development of a new opening shape for cellular beams: the sinusoidal openings (AngelinaTM beam). This new opening shape involves new failure modes at ultimate limit state and cannot be calculated with existing methods. Thus, experimental tests have been carried out on full scale cellular beams with sinusoidal openings (≈10m span). It has been shown, through those experimental studies led on three beam configurations, that the main failure mode is linked to the Vierendeel mechanism. At ultimate limit state, the local bending of the members of the most stressed opening leads to either the formation of four plastic hinges or the local buckling of the compressed opening web panels. A finite element model has been developed on the software SAFIR in order to better understand the different observed failure modes. This model has been validated on the basis of the experimental results and allowed identifying two main points: firstly, the existence of a rotational restraint between the intermediate web-post and the opening web panel and secondly, the failure of the sinusoidal opening corresponds to a mechanism combining the failure of each opening quarters. A second experimental and numerical test campaign has been conducted on isolated parts of the previous tested beams, in order to study locally the behavior of the opening quarters under local bending. Those studies allowed validating a finite element model developed on Cast3m software. This model has been used for a parametrical study which allowed quantifying the rotational restraint supplied by the intermediate web-post on the adjacent opening panel. This study confirmed the importance of the stiffness supplied by the intermediate web-post to the opening panels. Thus, it has to be taken into account to get a representative analytical failure load of each opening quarter. Finally, this thesis led to the development of a new analytical model for the calculation of the ultimate strength of the different opening parts. Due to the possible local buckling, the model is based on theoretical elements on the stability of plates. Furthermore, a detailed numerical study of the mechanism of failure of an isolated opening allowed justifying a cinematic approach for the failure of the opening. This approach combines the ultimate strength of each opening parts. The proposed analytical model permits to consider each opening quarter failure mode and strength. A comparative study with finite element results has showed that this model is reliable and representative of reality for defining the ultimate limit state of cellular beams with sinusoidal openings.