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Shear resistance of prestressed girders with stirrups in regions without flexural cracks

Abstract:

In the Netherlands, existing bridges are being assessed to investigate whether they are capable to resist the current traffic loads, which have increased since the bridges were designed. A substantial part of these bridges consists of prestressed concrete girders with a thin web and a low amount of shear reinforcement. Preliminary assessments show that it is demanding to demonstrate sufficient resistance for these bridges when the current guidelines (NEN 2005, Rijkswaterstaat 2013) are used. Especially the regions around the end supports, in which high shear forces are present, are frequently found to be critical. On the other hand, the flanges in these regions remain free of flexural cracks due to the presence of prestress and it is questionable whether the current guidelines are suitable to determine the shear resistance for these conditions. The variable angle truss model, as used in the current Eurocode (NEN 2005), was found to significantly underestimate the shear resistance of prestressed girders in regions without flexural cracks, especially for low shear reinforcement ratios. As a solution, a new model is proposed, referred to as Model B1, that is based on the Modified Compression Field Theory (Model B1). The application of Model B1 results in a much more accurate determination of the actual shear resistance. This is because the model considers the low longitudinal strain that is present in the region without flexural cracks. Moreover, the model is derived for all shear reinforcement ratios applied in practice. By using the proposed model, more substantiated judgements are possible about the structural safety of existing bridges that consist of prestressed girders with a thin web. In this way, the proposed model can help bridge owners in their decisions regarding maintaining, strengthening or renewal of their prestressed bridges.