Minnesota Department of Transportation Research Services Section
Although Mn/DOT inspection reports indicate that prestressed concrete bridge girders in service do not show signs
of shear distress, girders rated with the Virtis-BRASS rating tool and Load Factor Rating (LFR) have indicated that
a number of the girders have capacities lower than design level capacities. One of the reasons for the discrepancy
was suspected to be conservatism of the rating methods (i.e., LFR). Other suspected reasons included potential
flaws in the rating tools used by Mn/DOT (i.e., Virtis-BRASS software) including neglecting possible additional
shear capacity parameters (e.g., end blocks). 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, an analytical
research program was conducted. The report provides a brief description of the models that provide 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 identified and reviewed.
Minnesota Department of Transportation
Research Services Section
Dereli, Ozer; Shield, Carol; French, Catherine.
Discrepancies in Shear Strength of Prestressed Beams with Different Specifications.
Minnesota Department of Transportation Research Services Section.
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