Multi-beam bridges made from precast concrete box girders are one of the most common bridge types used in the United States. One problem that affects these bridges is the development of longitudinal or reflective cracks on the road surface because of failure of the shear keys. Some states have attempted to correct this problem by redesigning the shear key or adding post-tensioning, but the problem persists in many new bridges. The purpose of this study is to investigate why these shear key failures are occurring. This project studies two types of box girder designs, the common Precast/Prestressed Concrete Institute (PCI) box girder bridges and the Texas Department of Tranportation (TxDOT) box girder bridge. In the past, reflective cracking has occurred in bridges of both types. The analysis procedure involves finite element analyses of bridge models with realistic support and loading conditions, and comparing the PCI and TxDOT bridges. The results indicate that both PCI and TxDOT box girder have sufficient strength to resist cracking from vehicular loads, but uneven temperature changes and shrinkage strains cause high tensile stresses in the shear key regions and lead to reflective cracking. The analyses showed the highest stresses were often times near the supports, rather than at midspan. Past studies have proposed using larger composite deck slabs, transverse posttensioning, or full-depth shear keys to prevent shear key failure. Composite slabs were the most effective way to reduce high stresses in shear keys, and were effective for all loading cases considered. Post-tensioning and full-depth keys also showed a reduction in shear key stresses, but were less effective.

Post-tensioned segmental concrete box girders have been in use in the United States since the early 1970s. This unique bridge system uses post-tensioning to connect many smaller concrete bridge segments into very efficient long span bridges. However, because of the slender components, localized transverse bending becomes more critical when compared to more conventional bridge types. Bridge owners are finding that ratings for standard loads and permit trucks are often controlled by the transverse behavior of the girders near concentrated wheel loads. The popular analysis methods used today range from two dimensional frame models to three dimensional finite element models of the entire bridge. Currently, engineers must make sound engineering judgments on limited available information, while balancing safety and economy. To quantify and understand longitudinal and transverse behavior, the results from three live load tests of single cell segmental concrete box girder bridges are presented. Each bridge was instrumented with longitudinal and transverse strain sensors on at least two cross sections as well as rotation and deflection sensors, when possible. Two dimensional transverse frame models and three dimensional shell models were compared to the test results for each subject bridge. The two dimensional frame analyses using the common bottom web pin and roller boundary conditions provide mean absolute percent error in excess of 250%. Conversely, the newly introduced boundary conditions using pin supports at the top and bottom of each web was shown to reduce mean absolute percent error to 82%, which is on the same order of magnitude as longitudinal beamline analysis. The three dimensional shell models were insensitive to several changes including mesh fineness, number of spans modeled, and support conditions. Using uniform surface loading, the transverse modeling procedure was shown to provide significantly more accurate results than the common two dimensional frame models. A faster and more convenient analysis method using a program generated, structure specific, influence surface was also outlined. This method produced similar results when compared to the uniform surface loading method, while allowing additional automation for easier load application. / Ph. D.

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Concrete bridges in Texas have developed large cracks in bent caps and pretensioned trapezoidal bridge girders. The bridges show premature concrete deterioration due to alkali-silica reaction (ASR) and delayed ettringite formation (DEF). There is concern that deterioration due to ASR/DEF may lead to a loss of structural capacity. However, there are no quantitative guidelines to relate the level of concrete deterioration due to ASR/DEF to structural performance. Using such guidelines, the need for rehabilitation of beams with ASR/DEF cracking can be assessed. The goal of this research was to determine the shear capacity of pretensioned trapezoidal box girder specimens exhibiting varying degrees of ASR and/or DEF cracking and to use the shear testing results to evaluate the severity of the problem that may exist in Texas bridge structures. To achieve this goal, beams that were severely deteriorated due to ASR/DEF over a period of more than ten years were transported to the University of Texas for testing to failure. Both severely deteriorated and uncracked beams were tested in shear. The test results were used to evaluate the shear performance of trapezoidal box beams affected by ASR/DEF. In addition, three different types of forensic analyses were conducted on the beams to understand the nature of the ASR/DEF cracks and severity of the deterioration. After testing, it is found that the shear capacity of the test specimens was not significantly reduced even with heavy ASR/DEF cracking. Assessment using current US design provisions for bridges or buildings (ACI 318-08 and AASHTO LRFD 2008) and the proposed provision from an earlier project (TxDOT Project 5253) yielded conservative estimates of strength. Results from forensic analyses provided a qualitative indication of ASR/DEF damage but did not correlate with the observed levels of ASR/DEF deterioration. / text

A great deal of the design of bridges today is carried out using finite element models and analyses. These models are usually generalized versions of the actual bridge, as these kinds of models often is able to produce results similar to reality. In this thesis, a study regarding how the considerations of some of the details of a bridge affect the results using a FE analysis is performed. The analyses are focused on the change of transverse load distribution in the bridge slab. The thesis also addresses the use of a parametric design approach in FE modeling and analyses. The models are created using Python code with the possibility of choosing which properties that will be active or not before each analysis. The study is focused on the modeling of composite bridges with parallel box-girders. The bridge Kollektivtrafikbron, which is a new bridge currently in design as part of the Hisingsbridge project in Gothenburg, is used as a case study. The study has shown that a great deal of simplifications is reasonable to carry out in the modeling of a composite bridge, as long as the designer is aware of how these simplifications affect the results obtained. For the type of bridge investigated, the study has shown that the presence of transverse stiffening beams in the model as well as how the material properties of the edge beams are specified have a large impact on the transverse bending moment. The study has also shown that the use of shell or beam elements in the modeling of the main beams have a large impact on the shear force in the slab. Furthermore, the choice of element types and how constraints are specified have been shown to entail greater differences in the results than the detailing level of the model. These aspects should therefore be thoroughly considered in all modeling work. Additional aspects treated in the study is the impact of radius, the recess in the slab for the track as well as the size of the surface specified when applying traffic loads. The parametric design approach implemented in this study have provided the possibility of carrying out more analyses and investigating more details than what would have been possible if each model had been created manually. The greatest benefit of using a parametric design approach have been the possibility of carrying out analyzes automatically without monitoring or adjusting, which has made it possible to utilize the time provided for the study in a highly effective way. The design approach has been shown to be most effective when using simplified FE models, as these can be generated automatically fairly quickly. Therefore, it could be very beneficial to implement this kind of design approach in the early stages of design. / Idag utförs ofta dimensioneringen av broar med hjälp av finita elementmodeller och analyser. Dessa modeller utgörs i regel av generaliserade versioner av den verkliga bron då en kraftigt förenklad beräkningsmodell antas generera resultat som är väldigt nära verkligheten. I detta examensarbete undersöks hur vissa utvalda detaljer i brons utformning påverkar de resultat en finita elementanalys ger utifrån hur de beaktas vid modellering. Analyserna är inriktade på förändring av lastfördelning i tvärled hos brons farbana. Vidare undersöks möjligheten att använda ett parametriserat arbetssätt vid modellering och analys. Modellerna som analyseras formuleras i kod med möjlighet att välja vilka egenskaper som ska vara aktiva inför varje analys. Arbetet är koncentrerat på modellering av samverkansbroar med parallella lådbalkar som huvudbalkar. Som fallstudie används Kollektivtrafikbron. En ny plankrökt bro med denna utformning under pågående projektering inom projektet med den nya Hisingsbron i Göteborg. Arbetet har visat att det finns många förenklingar som är rimliga att genomföra vid modellering, så länge det finns en medvetenhet kring hur dessa påverkar resultaten. För den undersökta typen av bro har arbetet visat att förekomsten av tvärbalkar i modellen samt hur materialegenskaperna definieras för kantbalkar har stor inverkan på tvärledsmomentet. Utöver detta har arbetet visat att tvärkraften påverkas kraftigt utifrån om huvudbalkarna modelleras med skalelement respektive balkelement. Genomgående resultat är att modellering av huvudbalkarna med balkelement medför mycket högre max värden för tvärkraft. Vidare har arbetet visat att val av elementtyp och definiering av kopplingar kan ge större differenser än detaljrikedom och att dessa delar bör beaktas med stor noggrannhet vid all modellering. Ytterligare aspekter som behandlats är inverkan av radie, rälursparingar i farbanan samt definiering av lastytans storlek vid applicering av trafiklast. Det parametriserade arbetssättet som implementerats i detta arbete har gjort det möjligt att utföra fler analyser och undersöka fler faktorers inverkan än vad som hade varit möjligt om varje modell hade byggts upp manuellt. Den största vinsten med detta arbetssätt har varit att tiden som avsatts för arbetet kunnat utnyttjas mycket effektivt då analyser har kunnat utföras automatiskt utan krav på övervakning eller manuella korrigeringar. Mest effektivt har arbetssättet visats vara med modeller med lägre detaljrikedom då dessa förhållandevis snabbt kan genereras automatiserat. Detta kan framförallt vara väldigt effektivt att implementera i inledande projekteringsskeden.

As vigas celulares ocupam um lugar de destaque crescente na construção de pontes em concreto protendido. Dentro desta opção, a tendência moderna é de se construírem pontes unicelulares cada vez mais largas, o que traz como conseqüência um aumento considerável da flexão transversal em seus elementos, especialmente em suas almas, submetendo-as a uma combinação de cisalhamento com flexão transversal que pode atingir valores importantes. Este trabalho tem por finalidade apresentar um novo critério de dimensionamento das almas das vigas de seção celular, incluindo o caso do estado limite último de fadiga. Este critério foi idealizado a partir de uma análise crítica dos modelos vigentes, os quais são analisados e comparados por meio de gráficos de interação relacionando força cortante com flexão transversal, que permitem a escolha da melhor opção para as situações de projeto. Desenvolveu-se uma investigação experimental, a fim de verificar a validade do critério de dimensionamento desenvolvido. Foram analisados os seguintes modos de colapso: esmagamento das bielas comprimidas de concreto, alongamento plástico excessivo dos estribos e ruptura dos estribos por fadiga. Os resultados experimentais mostraram uma boa aproximação do Critério de Dimensionamento Proposto e revelaram novidades nos ensaios de fadiga: a ruptura dos estribos por fadiga se deu por etapas, um estribo de cada vez, num processo gradual. A ruptura por fadiga ocorreu sistematicamente próximo à ligação da alma com a mesa inferior e não no dobramento dos estribos. / Box-girders have received a growing attention in the field of prestressed concrete bridges. The modern trend is to build wider unicellular bridges, which leads to a considerable increase in the transverse bending moment acting mainly in their webs. These are subjected to a combination of shear force and transverse bending moments, which may reach important values. The purpose of this thesis is to introduce a new design approach of box-girder webs, including the Ultimate Limit State due to fatigue. This design approach is derived from a critical analysis of the current criteria. The different criteria for the design of box-girder webs are analyzed and compared by means of shear-bending moment interaction diagrams as an attempt to identify the more realistic one. An experimental investigation has been undertaken with the purpose of verifying the validity of the new developed approach. The following failure modes have been considered: crushing of the compressed struts, excessive plastic deformation of the stirrups and rupture of the stirrups due to fatigue. The experimental results have shown good agreement with those predicted by the proposed approach. Furthermore, the tests have revealed new aspects of the fatigue behavior: the rupture of the stirrups due to fatigue occurred in stages, one at a time in a gradual manner. In all cases the failure took place near the top face of the lower flange and not at the corner of the stirrups.

As vigas celulares ocupam um lugar de destaque crescente na construção de pontes em concreto protendido. Dentro desta opção, a tendência moderna é de se construírem pontes unicelulares cada vez mais largas, o que traz como conseqüência um aumento considerável da flexão transversal em seus elementos, especialmente em suas almas, submetendo-as a uma combinação de cisalhamento com flexão transversal que pode atingir valores importantes. Este trabalho tem por finalidade apresentar um novo critério de dimensionamento das almas das vigas de seção celular, incluindo o caso do estado limite último de fadiga. Este critério foi idealizado a partir de uma análise crítica dos modelos vigentes, os quais são analisados e comparados por meio de gráficos de interação relacionando força cortante com flexão transversal, que permitem a escolha da melhor opção para as situações de projeto. Desenvolveu-se uma investigação experimental, a fim de verificar a validade do critério de dimensionamento desenvolvido. Foram analisados os seguintes modos de colapso: esmagamento das bielas comprimidas de concreto, alongamento plástico excessivo dos estribos e ruptura dos estribos por fadiga. Os resultados experimentais mostraram uma boa aproximação do Critério de Dimensionamento Proposto e revelaram novidades nos ensaios de fadiga: a ruptura dos estribos por fadiga se deu por etapas, um estribo de cada vez, num processo gradual. A ruptura por fadiga ocorreu sistematicamente próximo à ligação da alma com a mesa inferior e não no dobramento dos estribos. / Box-girders have received a growing attention in the field of prestressed concrete bridges. The modern trend is to build wider unicellular bridges, which leads to a considerable increase in the transverse bending moment acting mainly in their webs. These are subjected to a combination of shear force and transverse bending moments, which may reach important values. The purpose of this thesis is to introduce a new design approach of box-girder webs, including the Ultimate Limit State due to fatigue. This design approach is derived from a critical analysis of the current criteria. The different criteria for the design of box-girder webs are analyzed and compared by means of shear-bending moment interaction diagrams as an attempt to identify the more realistic one. An experimental investigation has been undertaken with the purpose of verifying the validity of the new developed approach. The following failure modes have been considered: crushing of the compressed struts, excessive plastic deformation of the stirrups and rupture of the stirrups due to fatigue. The experimental results have shown good agreement with those predicted by the proposed approach. Furthermore, the tests have revealed new aspects of the fatigue behavior: the rupture of the stirrups due to fatigue occurred in stages, one at a time in a gradual manner. In all cases the failure took place near the top face of the lower flange and not at the corner of the stirrups.