| Summary: |
This project focuses on continuous bridge decks constructed with precast beams (each beam having a length similar to the span length) transversally connected by a thin slab.
One particular type of continuous precast bridge decks, which has been criticized by the technical community, will be analyzed in this project: those constructed with precast girders monolithically connected above the supports, where the negative moment reinforcement is composed by ordinary reinforcement bars only, i.e., without continuity prestress. This solution has been adopted in the French and Spanish HSLs for the construction of bridge decks with span lengths up to 38 m, because its construction is simpler, faster and cheaper then the construction of bridge decks with continuity prestress.
This technique has been most criticized because of the absence of prestress in the negative moment region. The adoption of this solution for railway bridges (where high variable loads occur, and both the rail safety and passenger comfort are highly dependent on structural stiffness) raises more doubts for some authors (it is said that important crack opening and loss of structural stiffness may occur due to the absence of prestress in the negative moment region). In spite of that, there is no sound evidence that this solution leads to a bad structural performance. The insufficiency of rules in design codes, regarding the specificity of precast structures, has contributed for maintaining doubts about the performance of this solution. Considering the relevance of continuous precast bridge decks for the construction of medium span bridges, it is urgent to clarify the doubts concerning their performance.
This project was also motivated by the need for improving the knowledge (particularly in Portugal) about the fatigue phenomenon in HSL continuous precast bridges. Indeed, for concrete structures, the fatigue phenomenon is only relevant if there is a high (cyclic) variable load / permanent load ra  |
Summary
This project focuses on continuous bridge decks constructed with precast beams (each beam having a length similar to the span length) transversally connected by a thin slab.
One particular type of continuous precast bridge decks, which has been criticized by the technical community, will be analyzed in this project: those constructed with precast girders monolithically connected above the supports, where the negative moment reinforcement is composed by ordinary reinforcement bars only, i.e., without continuity prestress. This solution has been adopted in the French and Spanish HSLs for the construction of bridge decks with span lengths up to 38 m, because its construction is simpler, faster and cheaper then the construction of bridge decks with continuity prestress.
This technique has been most criticized because of the absence of prestress in the negative moment region. The adoption of this solution for railway bridges (where high variable loads occur, and both the rail safety and passenger comfort are highly dependent on structural stiffness) raises more doubts for some authors (it is said that important crack opening and loss of structural stiffness may occur due to the absence of prestress in the negative moment region). In spite of that, there is no sound evidence that this solution leads to a bad structural performance. The insufficiency of rules in design codes, regarding the specificity of precast structures, has contributed for maintaining doubts about the performance of this solution. Considering the relevance of continuous precast bridge decks for the construction of medium span bridges, it is urgent to clarify the doubts concerning their performance.
This project was also motivated by the need for improving the knowledge (particularly in Portugal) about the fatigue phenomenon in HSL continuous precast bridges. Indeed, for concrete structures, the fatigue phenomenon is only relevant if there is a high (cyclic) variable load / permanent load ratio. Consequently, fatigue is not explicitly considered in the design of most concrete structures. For the afore mentioned reasons, there seams to be insufficient knowledge about the consequences of the fatigue phenomenon for HSL precast bridges.
Furthermore, currently existing design codes do not include practical procedures for fatigue verification in railway bridges designed for speeds exceeding 200 km/h, where the resonance phenomenon may be of relevance. So, in these cases, fatigue verification involves the computation of accumulated fatigue damage, from the dynamic stress history. Such computation has to be done for each point of the structure and for each expected traffic scenario, which may lead to very time consuming calculations. Therefore, the aim of the project is also to clarify the performance of these structures in what concerns to the fatigue phenomenon. In order to achieve that, several case studies will be analyzed in detail. It is intended to identify the critical points, where fatigue verification has to be done for each studied structure. Furthermore, it is intended to propose a practical procedure for structural engineers to conduct fatigue verification in this kind of structures. |