Ce sujet est d’actualité suite à une évolution rapide des types de conception de structures élancées utilisées dans les installations provisoires. C’est seulement depuis une vingtaine d’années que ces structures sont préfabriquées en cadres multidirectionnels (de sections tubulaires en acier ou en aluminium). Ces structures sont légères et leur stabilité réside seulement dans les raideurs internes au niveau des files de montants et au niveau horizontal par les planchers ainsi que dans les liaisons avec l’ouvrage. Ce travail concerne l’étude des instabilités (flambement-déversement) en tenant compte de différents types d’imperfections. De nouvelles courbes de flambement ainsi que les facteurs d’imperfection associés sont proposés dans cette thèse. Ces courbes sont obtenues en imposant une déformée initiale représentant les défauts géométriques et mécaniques (contraintes résiduelles). Les résultats expérimentaux confrontés avec les prévisions théoriques de l’Eurocode 3 montrent que les valeurs des imperfections figurant dans la littérature sont extrêmement exagérées. Les valeurs préconisées dans ce travail pourraient présenter un certain intérêt pour une modification éventuelle des courbes européennes de flambement pour ce type de structure. En ce qui concerne l’instabilité latérale, une méthodologie originale d’essais en vraie grandeur de poutres à treillis formés d’éléments tubulaires a été également mise au point. Le système de chargement à « roues libres » développé permet de libérer le point d’application de l’effort dès le début de l’instabilité. Cette technique conduit à des mesures plus précises du moment critique de déversement. / This subject becomes topical following a rapid evolution of design procedures for slender structures used widely in the temporary installations. Only through the last twenty years that these structures are prefabricated of multidirectional frames (steel or aluminium tubular sections). These structures are lightweight and their stability lies only in the internal stiffness at rows of posts and horizontally by the planking as well as the links with the building. This work concerns the study of instabilities (buckling - lateral torsional buckling) taking into account different types of imperfections. New buckling curves and the associated imperfection factors are proposed in this thesis. These curves are obtained by imposing an initial deformed representing the geometrical and mechanical defects (residual stress). The experimental results were confronted with theoretical predictions of Eurocode 3 which show that the values of the imperfections in the literature are extremely exaggerated. The values advocated in this work could be of interest for a possible adjustment of the European buckling curves for this type of structure. Regarding the lateral instability, an original methodology in real scale tests of trusses consist of tubular elements was also developed. The developed loading system of "free wheels" allows releasing the point of application of the force from the beginning of instability. This technique leads to more accurate measurements of the critical lateral torsional buckling moment.

This thesis deals with the problems of reliability of steel members sensitive to imperfections. Formulas for nonlinear analysis of compressed and bended members are derived. For compressed elements, there is described the issue of buckling, for bended elements it is lateral-torsional buckling. Then a statistical analysis of reliability of these problems is performed and also comparison with the approach described in Eurocode 3.

Built-up timber beams consist of individual lumber laminations connected together using mechanical fasteners such as nails, bolts and screws. Lateral torsional buckling (LTB) is an important failure mode that needs to be considered in deep beams with long spans and insufficient lateral supports. Due to the mechanical connectors, built-up beams are expected to have a lower moment capacity compared with solid beams with similar dimensions. The behaviour of built-up beams is greatly affected by the stiffness of the fasteners joining the individual laminations and determining the level of partial composite action attained in the beam. The current research aims to investigate the buckling behaviour of timber built-up beams. This is done by initially investigating the important parameters that play a role in the behaviour through an extensive sensitivity analysis. The focus of the analysis is on the contribution of the connections, since the buckling behaviour of individual solid timber beam element has been relatively well-established. Input parameters for the connection properties are obtained from joint level experimental tests. Finally, recommendations for specific fastener patterns and accompanying reduction factors on the buckling capacity relative to equivalent solid sections are developed and proposed.

Horisontella laster som uppstår från till exempel vindlast, snedställning och initialkrokighet kan ge upphov till vippning och måste därför hanteras. En vanlig metod som tillämpas för att hantera vippning är stagning, men för att kunna staga ett konstruktionselement är det viktigt att ha kännedom om hur elementet beter sig under vippning. Denna studie har behandlat stagningens betydelse för fackverk som utsatts för initialkrokighet. Med hjälp av ett FEM-program har krokiga modeller ritats upp och analyserats i syfte att ge en djupare förståelse för hur stagning av fackverk är betydande. För modellerna har flera olika typer av tvärsnittshöjder, upplagsförhållanden (fritt respektive kontinuerligt upplag) och randvillkor för diagonalerna prövats. Fackverk är ett konstruktionselement som i många avseenden verkar likt en balk, men hur det beter sig under vippning är ett hittills inte fullt utrett område. Vid en jämförelse mellan fackverksmodell och en anpassad balkmodell med samma förutsättningar påvisades att skillnaden mellan konstruktionselementen låg i underramens upplag. Studien har visat hur fackverkets överram kan stagas av ett takfackverk i horisontalplanet, men för att fackverket ska kunna uppnå global stabilitet är det kritiskt att underramen stagas via överramen från ovanliggande åsar. Detta realiseras först då diagonalerna kan tillgodogöra sig vridinspänningen från åsarna, vilket innebär att en fast inspänning mellan diagonaler och överram måste utformas på ett tekniskt korrekt sätt. Resultaten har påvisat att ett allmänt förhållande mellan stagkraft och axialkraft existerar, och att detta har kunnat jämföras mot rekommendationen från Eurokoderna EC3. Resultaten har också visat att en rekommendation för förhållande mellan lokala moment i knutpunkter och axialkraft kan ges, vilket är av betydelse vid knutpunktsutformning. / Horizontal loads caused by, for example, wind loads, initial sway imperfections and initial bow imperfections, can result in lateral torsional buckling and must therefore be managed. A common method used to handle lateral torsional buckling is lateral and torsional restraints, but in order to restrain a constructional element it’s important to have knowledge of how the element behaves during lateral torsional buckling. In this study the importance of the restrain for truss elements affected by initial bow imperfections has been considered. The truss element is a constructional element that in many ways acts like a beam, but how it behaves during lateral torsional buckling is a non-fully investigated area. With the help of a FEM-software initial bow imperfect models have therefore been drawn up and analysed to provide deeper understanding for how and when lateral and torsional restraints are of importance. The models have been trialled with different section heights, support conditions (simply and continuously supported) and boundary conditions for the diagonals. When comparing a truss model to a beam model with the same conditions, it was demonstrated how the difference between the elements were due to the support conditions of the lower chord. The study has demonstrated how the upper chord of the truss element can be restrained by a horizontal truss construction, but for the truss element to reach global stability it’s critical that the lower chord is restrained via the upper chord by overlying ridges. This cannot be realised until the diagonals can utilize the torsional connection from the ridges, which means that a fixed connection between diagonals and upper chord of truss element must be executed in a technically correct manner. The results have shown that a general relationship between restraint force and axial force exists, and that it can be compared with the recommendation from the Eurocode EC3. The results have also shown that a recommendation for the relationship between local moment appearing in the truss connections and axial force can be provided, which is helpful to the design process of the connections.

AASHTO LRFD requires the use of longitudinal stiffeners in plate girder webs when the web slenderness D/tw is greater than 150. This practice is intended to limit the lateral flexing of the web plate during construction and at service conditions. AASHTO accounts for an increase in the web bend buckling resistance due to the presence of a longitudinal stiffener. However, when the theoretical bend buckling capacity of the stiffened web is exceeded under strength load conditions, the Specifications do not consider any contribution from the longitudinal stiffener to the girder resistance. That is, the AASHTO LRFD web bend buckling strength reduction factor Rb applied in these cases is based on an idealization of the web neglecting the longitudinal stiffener. This deficiency can have significant impact on girder resistance in regions of negative flexure. This research is aimed at evaluating the improvements that may be achieved by fully considering the contribution of web longitudinal stiffeners to the girder flexural resistance. Based on refined FE test simulations, this research establishes that minimum size longitudinal stiffeners, per current AASHTO LRFD requirements, contribute significantly to the post buckling flexural resistance of plate girders, and can bring as much as a 60% increase in the flexural strength of the girder. A simple cross-section Rb model is proposed that can be used to calculate the girder flexural resistance at the yield limit state. This model is developed based on test simulations of straight homogenous girders subjected to pure bending, and is tested extensively and validated for hybrid girders and other limit states. It is found that there is a substantial deviation between the AISC/AASHTO LTB resistance equations and common FE test simulations. Research is conducted to determine the appropriate parameters to use in FE test simulations. Recommended parameters are identified that provide a best fit to the mean of experimental data. Based on FE simulations on unstiffened girders using these recommended parameters, a modified LTB resistance equation is proposed. This equation, used in conjunction with the proposed Rb model also provides an improved handling of combined web buckling and LTB of longitudinally stiffened plate girders. It is observed that the noncompact web slenderness limit in the Specifications, which is an approximation based on nearly rigid edge conditions for the buckling of the web plate in flexure is optimistic for certain cross-sections with narrow flanges. This research establishes that the degree of restraint at the edges of the web depend largely on the relative areas of the adjoining flanges and the area of the web. An improved equation for the noncompact web slenderness limit is proposed which leads to a better understanding and representation of the behavior of these types of members.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Lateral-torsional buckling (LTB) is an important failure mode that needs to be taken into account during the design of steel beams. The fundamental equation for determining the elastic critical moment of a beam was derived with the assumption that the beam is subjected to a uniform bending moment distribution. Loads on steel structures generate a great variety of bending moment distributions. The effect of the bending moment distribution is taken into account by a parameter known as the equivalent moment factor. The procedure outlined in the South African National Standard for limit-states design of hot-rolled steel work, SANS 10162-1:2011, for determining the equivalent moment factor was originally developed for a bending moment that is uniformly or linearly distributed, however it is currently used for all bending moment distributions. A Finite Element (FE) model was developed in this investigation for determining the equivalent moment factor. The numerical model included residual stresses and initial geometric imperfections commonly found in hot-rolled steel beams. To validate the assumptions made during the development of the FE model an in-depth experimental investigation was conducted on simply supported beams. Three different load configurations were considered in the experimental study in order to simulate various bending moment distributions. A comparison of the equivalent moment factor between the numerical results and the results obtained from various steel specifications, including SANS 10162-1:2011, was carried out in an attempt to quantify the positive and negative attributes of the various methods employed by steel design specifications. The experimental investigation concluded that the FE model is able to successfully represent a simply supported beam with realistic characteristics that include residual stresses and imperfections. The comparative study illustrated that for a bending moment distribution with a constant moment gradient, SANS 10162-1:2011 provides excellent results. However, for the other distributions considered in this investigation highly conservative results were obtained for the equivalent moment factor. The relevance of these findings were made clear by considering three design cases found in steel structures. The resistance moment of the beams in each of these cases was calculated according to each of the steel specifications. It was found that the use of a highly conservative procedure for determining the equivalent moment factor can lead to the uneconomical design of a structure. / AFRIKAANSE OPSOMMING: Laterale-torsie knik is ’n belangrike falings modus wat in ag geneem moet word tydens die ontwerp van staal balke. Die fundamentele vergelyking vir die bepaling van die elastiese kritieke moment van ’n balk is afgelei met die aanname dat die balk onderworpe is aan ’n eenvormige buigmoment verdeling. Belastings op staalstrukture genereer ’n groot verskeidenheid van buigmoment verdelings. Die effek van hierdie buigmoment verdelings word in ag geneem deur ’n parameter wat bekend staan as die ekwivalente moment faktor. Die prosedure uiteengesit in die Suid-Afrikaanse Nasionale Standaard vir die ontwerp van warm-gewalste staalwerk, SANS 10162-1:2011, vir die bepaling van hierdie faktor is oorspronklik ontwikkel vir ’n buigmoment wat uniform of linieêr verdeel is oor die lengte van die balk, maar dit word tans gebruik vir alle buigmoment verdelings. ’n Eindige Element (FE) model is ontwikkel in hierdie ondersoek vir die bepaling van die ekwivalente moment faktor. Die numeriese model sluit die residuele spannings en aanvanklike geometriese imperfeksies wat in die algemeen teenwoordig is in warm-gewalste profiele in. Die aannames wat gemaak is tydens die ontwikkeling van die FE model is bevestig met ’n in diepte eksperimentele ondersoek oor die gedrag van eenvoudig opgelegde balke. Drie verskillende las konfigurasies is oorweeg in die eksperimentele studie om verskeie buigmoment verspreidings na te boots. ’n Vergelyking van die ekwivalente moment faktor tussen die numeriese resultate en die resultate verkry van verskeie staal spesifikasies, insluitend SANS 10162-1:2011, is uitgevoer in ’n poging om die positiewe en negatiewe eienskappe van die verskillende metodes wat gebruik word in verskillende staal ontwerp spesifikasies, te kwantifiseer. Die eksperimentele ondersoek het tot die gevolgtrekking gelei dat die FE model in staat is om ’n eenvoudige opgelegte balk te verteenwoordig, met realistiese eienskappe wat residuele spannings en imperfekies insluit. Die vergelykende studie toon dat SANS 10162-1:2011 uitstekende resultate bied vir ’n buigmoment verdeling met ’n konstante moment gradiënt. Dit was egter gevind dat vir ander verdelings wat in hierdie ondersoek oorweeg is, SANS 10162-1:2011 hoogs konserwatiewe resultate bied. Die toepaslikheid van hierdie bevindinge is duidelik gemaak deur drie ontwerp gevalle wat algemeen in staalstrukture gevind word te bestudeer. Die weerstandsmoment is in elk van die gevalle bereken volgens elke staal spesifikasies. Daar is gevind dat die gebruik van ’n hoogs konserwatiewe prosedure vir die bepaling van die ekwivalente moment faktor kan lei tot die ontwerp van ’n onekonomiese struktuur.

The current design procedure for steel frame structures is a two-step process including an elastic analysis to determine design actions and a separate member capacity check. This design procedure is unable to trace the full range of load-deflection response and hence the failure modes of the frame structures can not be accurately predicted. In recent years, the development of advanced analysis methods has aimed at solving this problem by combining the analysis and design tasks into one step. Application of the new advanced analysis methods permits a comprehensive assessment of the actual failure modes and ultimate strengths of structural steel systems in practical design situations. One of the advanced analysis methods, the refined plastic hinge method, has shown great potential to become a practical design tool. However, at present, it is only suitable for a special class of steel frame structures that is not subject to lateral torsional buckling effects. The refined plastic hinge analysis can directly account for three types of frame failures, gradual formation of plastic hinges, column buckling and local buckling. However, this precludes most of the steel frame structures whose behaviour is governed by lateral torsional buckling. Therefore, the aim of this research is to develop a practical advanced analysis method suitable for general steel frame structures including the effects of lateral-torsional buckling. Lateral torsional buckling is a complex three dimensional instability phenomenon. Unlike the in-plane buckling of beam-columns, a closed form analytical solution is not available for lateral torsional buckling. The member capacity equations used in design specifications are derived mainly from testing of simply supported beams. Further, there has been very limited research into the behaviour and design of steel frame structures subject to lateral torsional buckling failures. Therefore in order to incorporate lateral torsional buckling effects into an advanced analysis method, a detailed study must be carried out including inelastic beam buckling failures. This thesis contains a detailed description of research on extending the scope of advanced analysis by developing methods that include the effects of lateral torsional buckling in a nonlinear analysis formulation. It has two components. Firstly, distributed plasticity models were developed using the state-of-the-art finite element analysis programs for a range of simply supported beams and rigid frame structures to investigate and fully understand their lateral torsional buckling behavioural characteristics. Nonlinear analyses were conducted to study the load-deflection response of these structures under lateral torsional buckling influences. It was found that the behaviour of simply supported beams and members in rigid frame structures is significantly different. In real frame structures, the connection details are a decisive factor in terms of ultimate frame capacities. Accounting for the connection rigidities in a simplified advanced analysis method is very difficult, but is most critical. Generally, the finite element analysis results of simply supported beams agree very well with the predictions of the current Australian steel structures design code AS4100, but the capacities of rigid frame structures can be significantly higher compared with Australian code predictions. The second part of the thesis concerns the development of a two dimensional refined plastic hinge analysis which is capable of considering lateral torsional buckling effects. The formulation of the new method is based on the observations from the distributed plasticity analyses of both simply supported beams and rigid frame structures. The lateral torsional buckling effects are taken into account implicitly using a flexural stiffness reduction factor in the stiffness matrix formulation based on the member capacities specified by AS4100. Due to the lack of suitable alternatives, concepts of moment modification and effective length factors are still used for determining the member capacities. The effects of connection rigidities and restraints from adjacent members are handled by using appropriate effective length factors in the analysis. Compared with the benchmark solutions for simply supported beams, the new refined plastic hinge analysis is very accurate. For rigid frame structures, the new method is generally more conservative than the finite element models. The accuracy of the new method relies on the user's judgement of beam segment restraints. Overall, the design capacities in the new method are superior to those in the current design procedure, especially for frame structures with less slender members. The new refined plastic hinge analysis is now able to capture four types of failure modes, plastic hinge formation, column buckling, local buckling and lateral torsional buckling. With the inclusion of lateral torsional buckling mode as proposed in this thesis, advanced analysis is one step closer to being used for general design practice.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Restrições de espaço e altura são frequentemente impostas às edificações residenciais, comerciais, industriais, depósitos e galpões com um ou diversos pavimentos em função de aspectos de regulamentos regionais, técnicos, econômicos ou ainda de natureza estética. A fim de proporcionar a passagem de tubulações e dutos de grande diâmetro sob vigas de aço, grandes alturas são normalmente requeridas, demandando por vezes, magnitudes de altura inviáveis entre pavimentos de edificações. Diversas soluções estruturais podem ser utilizadas para equacionar tais obstáculos, onde dentre outras, pode-se citar as vigas com inércia variável, stub-girders, treliças mistas, vigas misuladas e vigas com uma ou múltiplas aberturas na alma com geometrias variadas. No que tange às vigas casteladas, solução estrutural pautada neste estudo, a estabilidade é sempre um motivo de preocupação tipicamente durante a construção quando os contraventamentos laterais ainda não estão instalados. De qualquer forma, o comprimento destravado em geral alcançado pelos vãos destas vigas, são longos o suficiente para que a instabilidade ocorra. Todavia, o acréscimo substancial da resistência à flexão de tais membros devido ao aumento da altura oriundo de seu processo fabril em relação ao perfil matriz, aliada a economia de material e utilidade fim de serviço, garante a atratividade no aproveitamento destas, para grandes vãos junto aos projetistas. Não obstante, este aumento proporcional no comprimento dos vãos faz com que a instabilidade lateral ganhe importância especial. Neste contexto, o presente trabalho tem por objetivo desenvolver um modelo numérico que permita a realização de uma avaliação paramétrica a partir da calibração do modelo com resultados experimentais, efetuar a análise do comportamento de vigas casteladas e verificar seus mecanismos de falha, considerando comportamento elasto-plástico, além das não-linearidades geométricas. Também é objetivo deste trabalho, avaliar, quantificar e determinar a influência das diferenças geométricas características das vigas casteladas em relação às vigas maciças com as mesmas dimensões, analisando e descrevendo o comportamento estrutural destas vigas de aço para diversos comprimentos de vãos. A metodologia empregada para tal estudo baseou-se em uma análise paramétrica com o auxílio do método numérico dos elementos finitos. / Restrictions of space and height are often imposed to residential, commercial, industrial, warehouses and sheds with one, or several floors, due to aspects like regional regulations, technical, economic or aesthetic nature. In order to provide the passage of pipes and ducts with large diameter under steel girders, that normally requires great heights, sometimes leads to floors building highs with an unviable magnitudes. Several structural solutions can be used to overcome these obstacles, where the most popular are the beams with variable inertia (tapered and haunched beams), stub-girders, trusses, and beams with one or more web openings with various geometries. When castellated beams, structural solution studied and discussed in this dissertation, are considered stability issues at the construction stage becomes to attract the structural engineer attention since the lateral bracing are not yet installed. The unbraced span lengths generally reached by these beams are often long enough to cause instability effects. However, the substantial enhancement in the flexural strength of such members, due to the extra height coming from the manufacturing process in relation with the original profile, combined with the economy of material and utility services, ensures to the designers the attractiveness of this solution, especially when applied to large spans. On the other hand, this proportional increase in span lengths enhances the significance of improving the beam lateral instability resistance. The present investigation aims to develop a numerical model, that calibrated with experimental results, enables the development of a parametric analysis. This analysis aimed to determine the structural behavior of the castellated beams and their associated failure mechanisms, considering an elastic-plastic behavior as well the geometric non-linearities. The investigation was also able to evaluate, quantify and determine the influence of geometry variables like span lengths and cross sections dimensions. The methodology used for this study was developed based on a parametric analysis centred on the finite element method.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Restrições de espaço e altura são frequentemente impostas às edificações residenciais, comerciais, industriais, depósitos e galpões com um ou diversos pavimentos em função de aspectos de regulamentos regionais, técnicos, econômicos ou ainda de natureza estética. A fim de proporcionar a passagem de tubulações e dutos de grande diâmetro sob vigas de aço, grandes alturas são normalmente requeridas, demandando por vezes, magnitudes de altura inviáveis entre pavimentos de edificações. Diversas soluções estruturais podem ser utilizadas para equacionar tais obstáculos, onde dentre outras, pode-se citar as vigas com inércia variável, stub-girders, treliças mistas, vigas misuladas e vigas com uma ou múltiplas aberturas na alma com geometrias variadas. No que tange às vigas casteladas, solução estrutural pautada neste estudo, a estabilidade é sempre um motivo de preocupação tipicamente durante a construção quando os contraventamentos laterais ainda não estão instalados. De qualquer forma, o comprimento destravado em geral alcançado pelos vãos destas vigas, são longos o suficiente para que a instabilidade ocorra. Todavia, o acréscimo substancial da resistência à flexão de tais membros devido ao aumento da altura oriundo de seu processo fabril em relação ao perfil matriz, aliada a economia de material e utilidade fim de serviço, garante a atratividade no aproveitamento destas, para grandes vãos junto aos projetistas. Não obstante, este aumento proporcional no comprimento dos vãos faz com que a instabilidade lateral ganhe importância especial. Neste contexto, o presente trabalho tem por objetivo desenvolver um modelo numérico que permita a realização de uma avaliação paramétrica a partir da calibração do modelo com resultados experimentais, efetuar a análise do comportamento de vigas casteladas e verificar seus mecanismos de falha, considerando comportamento elasto-plástico, além das não-linearidades geométricas. Também é objetivo deste trabalho, avaliar, quantificar e determinar a influência das diferenças geométricas características das vigas casteladas em relação às vigas maciças com as mesmas dimensões, analisando e descrevendo o comportamento estrutural destas vigas de aço para diversos comprimentos de vãos. A metodologia empregada para tal estudo baseou-se em uma análise paramétrica com o auxílio do método numérico dos elementos finitos. / Restrictions of space and height are often imposed to residential, commercial, industrial, warehouses and sheds with one, or several floors, due to aspects like regional regulations, technical, economic or aesthetic nature. In order to provide the passage of pipes and ducts with large diameter under steel girders, that normally requires great heights, sometimes leads to floors building highs with an unviable magnitudes. Several structural solutions can be used to overcome these obstacles, where the most popular are the beams with variable inertia (tapered and haunched beams), stub-girders, trusses, and beams with one or more web openings with various geometries. When castellated beams, structural solution studied and discussed in this dissertation, are considered stability issues at the construction stage becomes to attract the structural engineer attention since the lateral bracing are not yet installed. The unbraced span lengths generally reached by these beams are often long enough to cause instability effects. However, the substantial enhancement in the flexural strength of such members, due to the extra height coming from the manufacturing process in relation with the original profile, combined with the economy of material and utility services, ensures to the designers the attractiveness of this solution, especially when applied to large spans. On the other hand, this proportional increase in span lengths enhances the significance of improving the beam lateral instability resistance. The present investigation aims to develop a numerical model, that calibrated with experimental results, enables the development of a parametric analysis. This analysis aimed to determine the structural behavior of the castellated beams and their associated failure mechanisms, considering an elastic-plastic behavior as well the geometric non-linearities. The investigation was also able to evaluate, quantify and determine the influence of geometry variables like span lengths and cross sections dimensions. The methodology used for this study was developed based on a parametric analysis centred on the finite element method.

<div>The design of bridges prior to 1994 was carried out by either the Load Factor Design (LFD) or the Allowable Stress Design (ASD) methodologies. Load rating of these bridges was primarily conducted by Load Factor Rating (LFR). In 1994, the American Association of State Highway and Transportation Officials (AASHTO) developed and encouraged the use of a probabilistic-based method titled Load and Resistance Factor Design (LRFD) for carrying out bridge design. A new methodology consistent with LRFD was also developed and adopted for conducting load rating. Thus, a new Load and Resistance Factor Rating (LRFR) was adopted by AASHTO in 2001 for load rating. Today, the bridges that were designed by the old LFD methodology are rated by both LFR and LRFR. Continued development suggests that load rating in future will be based only on LRFR, therefore LRFR is the recommended method for carrying out load rating of bridges even if they were designed by LFD. </div><div><br></div><div>The Indiana Department of Transportation (INDOT) came across some LFD designed bridges which were adequate by LFR methodology, i.e., produced a rating factor of more than 1.0, but inadequate for LRFR. The load ratings were carried out using AASHTOWare Bridge Rating (BrR) software. These bridges belonged to five different limit states: lateral torsional buckling, changes in cross-section along the member length, tight stringer spacings, girder end shear and moment over continuous piers. </div><div><br></div><div>This research study explores the inherent differences between LFR and LRFR to justify the inconsistencies in the rating values. To find an explanation for these discrepancies, load ratings of these bridges were carried out extensively on AASHTOWare BrR. To verify the results produced by BrR, a separate analysis was also conducted using Mathcad and structural analysis results from SAP2000 for comparison purposes. Finally, the study also recommends some modifications in the BrR software that can be adopted for each of the above-mentioned limit states to resolve inconsistencies found between LFR and LRFR rating values. </div><div><br></div>