Shear Key Performance in Multibeam Box Girder Bridges

This report describes a series of field tests of multibeam prestressed box girder bridges. The objective of the test was to investigate the in-situ performance of the grouted shear keys, located at the longitudinal joints between adjacent girders. The longevity of these joints can be a problem with this type of bridge; failure of the joint will typically not only compromise the load-sharing mechanism between adjacent girders, but also lead to the failure of the deck waterproofing system, with attendant corrosion problems. The tests consisted of monitoring relative displacements occurring across the intergirder joints, as well as bending strains in the girders themselves during passes of a preweighted tandem-axle dump truck, with axle loads typically in the range of 84.7 kN (19 kips). Relative displacements were measured with an especially designed transducer, having a resolution on the order of .00254 mm (0.1 mil), which was bonded to the underside of the bridge across the intergirder joints. Bending strains in the girders were measured with conventional foil strain gauges, having a gauge length of 38 mm (1.5 in.). All bridges tested exhibited relative displacements across at least some of the joints, which indicated a fractured shear key.