Sharpe, Graeme Peter (2007-08). Reflective cracking of shear keys in multi-beam bridges. Master's Thesis. Thesis uri icon

abstract

  • Multi-beam bridges made from precast concrete box girders are one of the most common bridge types used in the United States. One problem that affects these bridges is the development of longitudinal or reflective cracks on the road surface because of failure of the shear keys. Some states have attempted to correct this problem by redesigning the shear key or adding post-tensioning, but the problem persists in many new bridges. The purpose of this study is to investigate why these shear key failures are occurring. This project studies two types of box girder designs, the common Precast/Prestressed Concrete Institute (PCI) box girder bridges and the Texas Department of Tranportation (TxDOT) box girder bridge. In the past, reflective cracking has occurred in bridges of both types. The analysis procedure involves finite element analyses of bridge models with realistic support and loading conditions, and comparing the PCI and TxDOT bridges. The results indicate that both PCI and TxDOT box girder have sufficient strength to resist cracking from vehicular loads, but uneven temperature changes and shrinkage strains cause high tensile stresses in the shear key regions and lead to reflective cracking. The analyses showed the highest stresses were often times near the supports, rather than at midspan. Past studies have proposed using larger composite deck slabs, transverse posttensioning, or full-depth shear keys to prevent shear key failure. Composite slabs were the most effective way to reduce high stresses in shear keys, and were effective for all loading cases considered. Post-tensioning and full-depth keys also showed a reduction in shear key stresses, but were less effective.
  • Multi-beam bridges made from precast concrete box girders are one of the most
    common bridge types used in the United States. One problem that affects these bridges
    is the development of longitudinal or reflective cracks on the road surface because of
    failure of the shear keys. Some states have attempted to correct this problem by
    redesigning the shear key or adding post-tensioning, but the problem persists in many
    new bridges.
    The purpose of this study is to investigate why these shear key failures are
    occurring. This project studies two types of box girder designs, the common
    Precast/Prestressed Concrete Institute (PCI) box girder bridges and the Texas
    Department of Tranportation (TxDOT) box girder bridge. In the past, reflective
    cracking has occurred in bridges of both types.
    The analysis procedure involves finite element analyses of bridge models with
    realistic support and loading conditions, and comparing the PCI and TxDOT bridges.
    The results indicate that both PCI and TxDOT box girder have sufficient strength to
    resist cracking from vehicular loads, but uneven temperature changes and shrinkage strains cause high tensile stresses in the shear key regions and lead to reflective cracking.
    The analyses showed the highest stresses were often times near the supports, rather than
    at midspan.
    Past studies have proposed using larger composite deck slabs, transverse posttensioning,
    or full-depth shear keys to prevent shear key failure. Composite slabs were
    the most effective way to reduce high stresses in shear keys, and were effective for all
    loading cases considered. Post-tensioning and full-depth keys also showed a reduction
    in shear key stresses, but were less effective.

ETD Chair

publication date

  • August 2007