Discrepancies in Shear Strength of Prestressed Beams with Different Specifications

Principal Investigator:

Cathy French, I.T. Distinguished Professor, Civil, Environmental and Geo-Engineering

Co-Investigator

  • Carol Shield, Professor, Civil, Environmental and Geo-Engineering

Project Summary:

Rating bridges using Virtis software and Load Factor Rating (LFR) often indicate problems with the shear capacity of prestressed concrete girders; the capacities predicted by these rating methods often indicate capacities much lower than design level capacities, even though the girders exhibit no sign of shear distress. As a consequence, the rating methods have made it difficult to discern the cases for which shear capacity may be a real concern. In order to identify the reasons for the discrepancies and inconsistency in rating results relative to observed performance of the prestressed bridge girders, researchers conducted an analytical research program. The purpose of this investigation was to resolve the discrepancies among the results obtained, to provide recommendations regarding when shear rating should be an issue of concern, and to suggest potential modifications to be used in the process of shear rating using the Virtis software. The report provides a brief description of the models that provided the basis for the AASHTO shear design provisions and descriptions of the provisions through the 2002 AASHTO Standard specifications. This is followed by a description of the Virtis-BRASS rating tool, which was verified with example bridges provided by Mn/DOT. To investigate prestressed bridge girders within the inventory that might be most at risk for being undercapacity for shear, 54 girders were selected from the inventory for further evaluation. Some of the 54 girders were found to have larger stirrup spacings than required at the time of design. These girders were subsequently rated and evaluated per the 2002 AASHTO Standard Specifications to determine the adequacy of the designs based on the LFR inventory and operating rating methods. Potential sources for increased shear capacity were also identified and reviewed.

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