Investigation of Shear Distribution Factors in Prestressed Concrete Girder Bridges

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:

As shear requirements for prestressed concrete girders have changed, some structures designed using older specifications do not rate well with current methods. However, signs of shear distress have not been observed in these bridge girders and they are often deemed to be in good condition. The primary objective of this research program was to investigate the accuracy of existing shear distribution factors, which are used to estimate bridge system live load effects on individual girders, and which provide recommendations on shear distribution to be used in Minnesota with four components: a full-scale laboratory bridge subjected to elastic and inelastic behavior, field testing of bridges, a numerical parametric study, and integration of results to develop a screening tool to determine which structures benefitted from refined analysis. Laboratory bridge inelastic testing indicated shear force redistribution after cracking and before ultimate failure. Use of elastic distribution factors is conservative for shear distribution at ultimate capacity. Elastic laboratory testing was used to validate the finite element modeling technique and study the behavior of a barrier and end diaphragm, which affected shear distribution; ignoring their effects was conservative. Parametric study results indicated that a ratio of longitudinal stiffness to transverse stiffness could be used as a screening tool. If the stiffness ratio was less than 1.5, shear demand from a simple, conservative grillage analysis may be more accurate than shear demand from American Association of State Highway and Transportation Officials (AASHTO) distribution factor methods. Grillage analysis shear demand results due to permit trucks may also be more accurate, regardless of the screening tool ratio.

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