Partially prestressed railway bridge

Building and maintenance of the railway track and its sub- and superstructures in the Netherlands are commissioned by ProRail. In addition to the Eurocode ProRail prescribes a wide range of regulations and guidelines related to railway track. Amongst these are a few specifically for the design of civil structures. With respect to prestressed concrete structures ProRail deviates from the Eurocode by demanding fully prestressed concrete. Partially prestressed concrete nowadays is often used in structures carrying road traffic (although not in prefab beams or girders) and is allowed according to the Eurocode. Main reason for fully prestressed concrete for railway bridges would be that partially prestressed structures are more prone to effects of fatigue loadings. However, in prestressed concrete not only prestressing tendons but also reinforcement is applied. Longitudinal reinforcement is present, but is of no use in service phase when a concrete section is almost totally in compression. There may be some economical benefit to allow a lower amount of prestressing and to make use of the capacity of the present reinforcement. In this thesis an existing prestressed railway bridge is used as a case study to determine the minimum prestressing degree leaving all other parameters like concrete dimensions, material properties and applied reinforcement undisturbed. ProRails rule is therefore deliberately ignored. The existing structure is designed according to former Dutch national regulations. A comparative design is performed to determine main differences in design checks between those regulations and the current Eurocode. It turns out that according to the current Eurocode and regulations of ProRail the existing structure is not fully prestressed. It was according to the former Dutch national standards. This means that structures designed according to the Eurocode need to be bigger, heavier, stronger than they would be according to the former Dutch national standards. On one hand the loads on a bridge are higher and on the other hand the regulations of ProRail seem to have become stricter. Since the dimensions of this analysed structure approaches its limits, measures are not easily taken to meet the requirement of fully prestressing. By analysing this structure as a partially prestressed bridge, it turns out that circa 15% of the prestressing could have been saved. The present reinforcement is just about enough to meet the requirement of the limited crack width. It approaches the limits with respect to fatigue, but that is not leading in this case. It is just a slight reduction and the economical benefits regarding a trough bridge will not be that large. But it offers an opportunity to review ProRails rule for fully prestressed concrete. In that case future structures do not need to become bigger than they would be according to the former Dutch national rules. Furthermore existing structures could meet the current requirements according to the Eurocode as well. This may be beneficial in cases where current structures near their service lifetime and need to be reassessed.

Subject

railway bridge
partially prestressing
trough bridge
reassessment

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Student theses

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master thesis

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EJongstra_Masterthesis_20 ... cl_app.pdf

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