Based on the definition given in the AASHTO LRFD Bridge Design Specifications, twin steel box-girder bridges are classified as bridges with fracture critical members (FCMs), in which a failure of a tension member is expected to lead to a collapse of the bridge. However, a number of such bridges with either a partial or full-depth crack in one girder have been reported and are still providing service without collapsing. The main objective of this research project is to understand the behavior of twin steel box-girder bridges and to develop methods for evaluating their redundancy level in the event of the fracture of one tension member. The research project included an experimental investigation on a small-scale steel twin box-girder bridge, field testing of a full-scale twin box-girder, analysis of existing research and design data, and an extensive amount of numerical analyses carried out on calibrated 3-D nonlinear finite element models. The results from this study provide in-depth understanding of twin steel box-girder bridge behavior before and after a fracture in the tension member occurs. In addition to the experimentally verified finite element method, the report also proposes simplified methods for evaluating the load-carrying capacity of twin steel box-girder bridges under vii concentrated loads and provides a list of important factors that could control the reserve capacity of the damaged bridge. The main conclusion of this research is that the redundancy exists in twin steel boxgirder bridges in an event that a fracture of a tension member(s) takes place. This research project also provides a comprehensive roadmap for assessing the redundancy of twin steel box-girder bridges in which the elements of the roadmap are identified, and solutions for several of the steps are provided. The development of solutions for remaining steps of the roadmap is proposed for a future research.

[EN] Bridges, as an important component of infrastructure, are expected to meet all the requirements for a modern society. Traditionally, the primary aim in bridge design has been to achieve the lowest cost while guaranteeing the structural efficiency. However, concerns regarding building a more sustainable future have change the priorities of society. Ecological and durable structures are increasingly demanded. Under these premises, heuristic optimization methods provide an effective alternative to structural designs based on experience. The emergence of new materials, structural designs and sustainable criteria motivate the need to create a methodology for the automatic and accurate design of a real post-tensioned concrete bridge that considers all these aspects. For the first time, this thesis studies the efficient design of post-tensioned concrete box-girder road bridges from a sustainable point of view. This research integrates environmental, safety and durability criteria into the optimum design of the bridge. The methodology proposed provides multiple trade-off solutions that hardly increase the cost and achieve improved safety and durability. Likewise, this approach quantifies the sustainable criteria in economic terms, and evaluates the effect of these criteria on the best values of the variables. In this context, a multi-objective optimization is formulated to provide multiple trade-off and high-performing solutions that balance economic, ecologic and societal goals. An optimization design program selects the best geometry, concrete type, reinforcement and post-tensioning steel that meet the objectives selected. A three-span continuous box-girder road bridge located in a coastal region is selected for a case study. This approach provides vital knowledge about this type of bridge in the sustainable context. The life-cycle perspective has been included through a lifetime performance evaluation that models the bridge deterioration process due to chloride-induced corrosion. The economic, environmental and societal impacts of maintenance actions required to extend the service life are examined. Therefore, the proposed goals for an efficient design have been switch from initial stage to life-cycle consideration. Faced with the large computational time of multi-objective optimization and finite-element analysis, artificial neural networks (ANNs) are integrated in the proposed methodology. ANNs are trained to predict the structural response based on the design variables, without the need to analyze the bridge response. The multi-objective optimization problem results in a set of trade-off solutions characterized by the presence of conflicting objectives. The final selection of preferred solutions is simplified by a decision-making technique. A rational technique converts a verbal pairwise comparison between criteria with a degree of uncertainty into numerical values that guarantee the consistency of judgments. This thesis gives a guide for the sustainable design of concrete structures. The use of the proposed approach leads to designs with lower life-cycle cost and emissions compared to general design approaches. Both bridge safety and durability can be improved with a little cost increment by choosing the correct design variables. In addition, this methodology is applicable to any type of structure and material. / [ES] Los puentes, como parte importante de una infraestructura, se espera que reúnan todos los requisitos de una sociedad moderna. Tradicionalmente, el objetivo principal en el diseño de puentes ha sido lograr el menor coste mientras se garantiza la eficiencia estructural. Sin embargo, la preocupación por construir un futuro más sostenible ha provocado un cambio en las prioridades de la sociedad. Estructuras más ecológicas y duraderas son cada vez más demandadas. Bajo estas premisas, los métodos de optimización heurística proporcionan una alternativa eficaz a los diseños estructurales basados en la experiencia. La aparición de nuevos materiales, diseños estructurales y criterios sostenibles motivan la necesidad de crear una metodología para el diseño automático y preciso de un puente real de hormigón postesado que considere todos estos aspectos. Por primera vez, esta tesis estudia el diseño eficiente de puentes de hormigón postesado con sección en cajón desde un punto de vista sostenible. Esta investigación integra criterios ambientales, de seguridad estructural y durabilidad en el diseño óptimo del puente. La metodología propuesta proporciona múltiples soluciones que apenas encarecen el coste y mejoran la seguridad y durabilidad. Al mismo tiempo, se cuantifica el enfoque sostenible en términos económicos, y se evalúa el efecto que tienen dichos criterios en el valor óptimo de las variables. En este contexto, se formula una optimización multiobjetivo que proporciona soluciones eficientes y de compromiso entre los criterios económicos, ecológicos y sociales. Un programa de optimización del diseño selecciona la mejor combinación de geometría, tipo de hormigón, armadura y postesado que cumpla con los objetivos seleccionados. Se ha escogido como caso de estudio un puente continuo en cajón de tres vanos situado en la costa. Este método proporciona un mayor conocimiento sobre esta tipología de puentes desde un punto de vista sostenible. Se ha estudiado el ciclo de vida a través de la evaluación del deterioro estructural del puente debido al ataque por cloruros. Se examina el impacto económico, ambiental y social que produce el mantenimiento necesario para extender la vida útil del puente. Por lo tanto, los objetivos propuestos para un diseño eficiente han sido trasladados desde la etapa inicial hasta la consideración del ciclo de vida. Para solucionar el problema del elevado tiempo de cálculo debido a la optimización multiobjetivo y el análisis por elementos finitos, se han integrado redes neuronales en la metodología propuesta. Las redes neuronales son entrenadas para predecir la respuesta estructural a partir de las variables de diseño, sin la necesidad de analizar el puente. El problema de optimización multiobjetivo se traduce en un conjunto de soluciones de compromiso que representan objetivos contrapuestos. La selección final de las soluciones preferidas se simplifica mediante una técnica de toma de decisiones. Una técnica estructurada convierte los juicios basados en comparaciones por pares de elementos con un grado de incertidumbre en valores numéricos que garantizan la consistencia de dichos juicios. Esta tesis proporciona una guía que extiende y mejora las recomendaciones sobre el diseño de estructuras de hormigón dentro del contexto de desarrollo sostenible. El uso de la metodología propuesta lleva a diseños con menor coste y emisiones del ciclo de vida, comparado con diseños que siguen metodologías generales. Los resultados demuestran que mediante una correcta elección del valor de las variables se puede mejorar la seguridad y durabilidad del puente con un pequeño incremento del coste. Además, esta metodología es aplicable a cualquier tipo de estructura y material. / [CAT] Els ponts, com a part important d'una infraestructura, s'espera que reunisquen tots els requisits d'una societat moderna. Tradicionalment, l'objectiu principal en el disseny de ponts ha sigut aconseguir el menor cost mentres es garantix l'eficiència estructural. No obstant això, la preocupació per construir un futur més sostenible ha provocat un canvi en les prioritats de la societat. Estructures més ecològiques i durables són cada vegada més demandades. Davall estes premisses, els mètodes d'optimització heurística proporcionen una alternativa eficaç als dissenys estructurals basats en l'experiència. L'aparició de nous materials, dissenys estructurals i criteris sostenibles motiven la necessitat de crear una metodologia per al disseny automàtic i precís d'un pont real de formigó posttesat que considere tots estos aspectos. Per primera vegada, esta tesi estudia el disseny eficient de ponts de formigó posttesat amb secció en calaix des d'un punt de vista sostenible. Esta investigació integra criteris ambientals, de seguretat estructural i durabilitat en el disseny òptim del pont. La metodologia proposada proporciona múltiples solucions que a penes encarixen el cost i milloren la seguretat i durabilitat. Al mateix temps, es quantifica l'enfocament sostenible en termes econòmics, i s'avalua l'efecte que tenen els dits criteris en el valor òptim de les variables. En este context, es formula una optimització multiobjetivo que proporciona solucions eficients i de compromís entre els criteris econòmics, ecològics i socials. Un programa d'optimització del disseny selecciona la millor geometria, tipus de formigó, armadura i posttesat que complisquen amb els objectius seleccionats. S'ha triat com a cas d'estudi un pont continu en calaix de tres vans situat en la costa. Este mètode proporciona un major coneixement sobre esta tipologia de ponts des d'un punt de vista sostenible. S'ha estudiat el cicle de vida a través de l'avaluació del deteriorament estructural del pont a causa de l'atac per clorurs. S'examina l'impacte econòmic, ambiental i social que produïx el manteniment necessari per a estendre la vida útil del pont. Per tant, els objectius proposats per a un disseny eficient han sigut traslladats des de l'etapa inicial fins a la consideració del cicle de vida. Per a solucionar el problema de l'elevat temps de càlcul degut a l'optimització multiobjetivo i l'anàlisi per elements finits, s'han integrat xarxes neuronals en la metodologia proposada. Les xarxes neuronals són entrenades per a predir la resposta estructural a partir de les variables de disseny, sense la necessitat d'analitzar el pont. El problema d'optimització multiobjetivo es traduïx en un conjunt de solucions de compromís que representen objectius contraposats. La selecció final de les solucions preferides se simplifica per mitjà d'una tècnica de presa de decisions. Una tècnica estructurada convertix els juís basats en comparacions per parells d'elements amb un grau d'incertesa en valors numèrics que garantixen la consistència dels dits juís. Esta tesi proporciona una guia que estén i millora les recomanacions sobre el disseny d'estructures de formigó dins del context de desenrotllament sostenible. L'ús de la metodologia proposada porta a dissenys amb menor cost i emissions del cicle de vida, comparat amb dissenys que seguixen metodologies generals. Els resultats demostren que per mitjà d'una correcta elecció del valor de les variables es pot millorar la seguretat i durabilitat del pont amb un xicotet increment del cost. A més, esta metodologia és aplicable a qualsevol tipus d'estructura i material. / García Segura, T. (2016). Efficient design of post-tensioned concrete box-girder road bridges based on sustainable multi-objective criteria [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/73147 / TESIS

Live load distribution factors are used to determine the live-load moment for bridge girder design when a two dimensional analysis is conducted. A simple, analysis of bridge superstructures are considered to determine live-load factors that can be used to analyze different types of bridges. The distribution of the live load factors distributes the effect of loads transversely across the width of the bridge superstructure by proportioning the design lanes to individual girders through the distribution factors. This research study consists of the determination of live load distribution factors (LLDFs) in both interior and exterior girders for horizontally curved concrete box girder bridges that have central angles, with one span exceeding 34 degrees. This study has been done based on real geometry of bridges designed by a company for different locations. The goal of using real geometry is to achieve more realistic, accurate, and practical results. Also, in this study, 3-D modeling analyses for different span lengths (80, 90, 100, 115, 120, and 140 ft) have been first conducted for straight bridges, and then the results compared with AASHTO LRFD, 2012 equations. The point of starting with straight bridges analyses is to get an indication and conception about the LLDF obtained from AASHTO LRFD formulas, 2012 to those obtained from finite element analyses for this type of bridge (Concrete Box Girder). After that, the analyses have been done for curved bridges having central angles with one span exceeding 34 degrees. Theses analyses conducted for various span lengths that had already been used for straight bridges (80, 90, 100, 115, 120, and 140 ft) with different central angles (5º, 38º, 45º, 50º, 55º, and 60º). The results of modeling and analyses for straight bridges indicate that the current AASHTO LRFD formulas for box-girder bridges provide a conservative estimate of the design bending moment. For curved bridges, it was observed from a refined analysis that the distribution factor increases as the central angle increases and the current AASHTO LRFD formula is applicable until a central angle of 38º which is a little out of the LRFD`s limits.

In order to more accurately quantify the behavior and degradation of bridges throughout their service life, the Federal Highway Administration lunched the Long-Term Bridge Performance Program. As part of this program an I-girder, integral abutment bridge near Perry, Utah and a two span, box-girder bridge south of Sacramento, California were instrumented with foil strain gauges, velocity transducers, vibrating wire strain gauges, thermocouples, and tiltmeters. In this research study, data from the thermocouples was used to calculate average bridge temperature and compare it to the recommended design criteria in accordance to the 2010 LRFD Bridge Design Specifications of the American Association of State Highway and Transportation Officials (AASHTO). The design maximum average bridge temperature defined in the 2010 LRFD Bridge Design Specifications was exceeded for both bridges. The accuracy of the 1991 Kuppa Method and the 1976 Black and Emerson Method to estimate the average bridge temperature based on ambient temperature was studied and a new method that was found to be more accurate was proposed. Long-term predictions of average bridge temperature for both bridges were calculated. Temperature gradients were measured and compared to the 2010 AASHTO LRFD Bridge Design Specifications and the 1978 Priestley Method. Calculated flexural stresses as a function of maximum positive and negative temperature gradients were found to exceed the service limit state established in the 2010 AASHTO LRFD Bridge Design Specifications in the case of the California bridge.

As part of the Long-Term Bridge Performance (LTBP) program, a flagship research program funded by the Federal Highway Administration in response to the aging bridge network, the Lambert Road Bridge near Elk Grove California was selected as the California Pilot bridge set to undergo non-destructive testing and monitoring. The purpose of the program is to obtain a database of scientific quality data concerning the health and maintenance procedures currently in use across the nation. FHWA program managers along with members of the Utah State University LTBP research team selected the bridge with the assistance of the National Bridge index and site visits. Dynamic modal analysis and long-term health monitoring are two of the test procedures that the test bridge will undergo. Dynamic modal analysis is performed by introducing a known vibration into the system and recording the response. The dynamic properties are extracted in this manner, which allows any changes in the structure to be tracked over time as the dynamic properties change. The long-term health monitoring of the bridge will include an array of sensors designed to capture the real-time structural response of the bridge under normal operating conditions at key locations. An array of 1-Hz Velocity Transducers was used to record the bridge response to the introduced vibrations. The data collected over 4 days of testing was analyzed using the "peak picking method" to locate the resonant frequencies, mode shapes, and damping ratios of the structure. In this thesis the dynamic testing results and the finite element model were compared and correlated both visually and with a modal assurance criterion. The long-term health monitoring is also discussed in this thesis. The types and reason for each sensor are presented and the installation procedure is explained and documented.

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When designing a bridge it is of high importance that the geometry for the cross section is optimized for the structure. This is partly due to the influence of the amount of material needed and its impact on the budget and environment. The influence of choosing the right amount of each material lies in the unit-price of the different material, where they can differ significantly. The Swedish Transport Administration, Trafikverket, has ordered the construction of Stockholm Bypass which is one of Swedens largest infrastructure project and is valued to 27.6 billion SEK according to the price index of the year 2009. The infrastructure project is divided into multiple projects where one of them is assigned to Implenia and Veidekke through a joint venture (Joint venture Hjulsta, JVH) and is valued to nearly 800 MSEK. The reference bridge that is used in the analysis of the master’s thesis is a part of the project. The aim of this masters thesis was to analyze and compare the two construction methods, mov- able scaffolding system (MSS) and incremental launching for the reference bridge with respect to amount post-tensioning and slenderness. Furthermore, an economical comparison between the two construction methods was carried out based on the obtained results. The analysis of the MSS was carried out by modeling the reference bridge structure in the finite element software SOFiSTiK AG. The bridge was modeled with different cross section height, i.e. different slenderness where the optimal amount of post-tension tendons could be determined by iteration until stress conditions from the Eurocode were fulfilled. For the incremental launching method, a numerical analysis was performed. The optimal amount of required post-tensioning was evaluated in the construction stages and final stages with different construction heights i.e. different values of slenderness. A cost analysis was also performed where the aim was to analyze how the total cost of the construc- tion of the bridge would be influenced by the different slenderness of the bridge as a comparison for the two construction methods. This was done by dividing the costs into fixed costs and variable costs. The results showed that the structural rigidity had a large influence on the required amount of prestressing steel for both construction methods. In other words, the smaller the cross section the more prestressing steel was required. Incremental launching proved to require a much greater amount of (PT) tendons compared to the MSS although the identical cross sections and properties for both methods, except for the PT. The prestressing for incremental launching is generally by centrical prestressing during the construction stages. A intersection point was obtained in the cost analysis for the construction methods. The incremental launching was the cheaper solution for slenderness smaller than the intersection point at slenderness between 17 and 18. The MSS was cheaper than the incremental launching for slenderness larger than the intersection point. / Vid dimensionering av tvärsektioner i broar är det av stor vikt att optimera geometrin avseende materialåtgång då mängden material har stor på verkan på ett projekts budget samt miljö. Eftersom konstruktioner ofta består av olika byggnadsmaterial gäller det vid optimering att välja byggnadsmaterialen genom optimerad proportionalitet. Förbifart Stockholm, beställt av Trafikverket, är ett av Sveriges största infrastrukturprojekt och värderas till 27,6 miljarder kronor enligt 2009 års prisnivå. Infrastrukturprojektet är uppdelat i flera mindre entreprenader eller så kallade etapper. Den entreprenad som omfattar trafikplats Hjulsta Södra har blivit tilldelat till Implenia och Veidekke genom ett konsortium (Jointventure Hjulsta, JVH) och värderas till cirka 800 miljoner kronor. Den förspända betongbro som byggs i trafikplats Hjulsta ligger till grund för analysen i detta examensarbete och har använts som referens under vår studie. Syftet med examensarbete var att analysera och jämföra två de två olika produktionsmetoderna, Movable scaffolding system (MSS) och etappvis lansering med hänsyn till erforderlig mängd förspänningskablar och slankhet. Vidare, baserat på erhållna resultat, utfördes en ekonomisk analys och jämförelse mellan produktionsmetoderna. Analysen av MSS utfördes genom att modellera brokonstruktionen i mjukvaruprogrammet SOFiSTiK AG som bygger på finita elementmetoder. Konstruktionen modellerades för olika slankheter, där slankheten definieras som kvoten mellan maximala spannlängden och brons tvärsnittshöjd. Spannlängden hölls konstant medan tvärsnittshöjden varierade för att erhålla olika slankheter. Den optimala slankheten bestämdes genom iterering av mängd förspänningskablar tills spänningsvillkoren var uppfyllda enligt Eurocode. För analysen av etappvis lansering utfördes en numerisk analys vars den optimala mängden förspänningskablar utvärderades i byggskedet (construction stages) samt i slutskedet (final stage). Analysen utfördes på samma sätt för de olika slankheterna. Slutligen genomfördes en konstandsanalys för de olika metoderna. Syftet var att jämföra hur den totala kostnaden för uppförandet av brokonstruktionen skiljde sig för de olika slankheterna. Jämförelsen genomfördes genom att dela upp de olika kostnaderna i fasta kostnader samt rörliga kostnader. Resultaten från analysen visade att den erforderliga mängd förspänningskablar som behövs i en förspänd betongbro är beroende av den strukturella styvheten i tvärsektionen. En högre slankhet, alltså lägre tvärsnittshöjd, ger lägre styvhet och därav mer erforderlig förspänningskablar. Etappvis lansering visade sig vara den metod som krävde mer mängd förspänningskablar. I resultaten för kostnadsanalysen uppmättes en skärningspunkt, för en slankhet mellan 17-18, mellan de två olika metoderna. För förspända betongbroar med slankhet lägre än skärningsupunkten vid 17-18 är etappvis lansering det billigare alternativet. För slankheter högre än 17-18 är MSS det mer ekonomiskt lönsamma alternativet.