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Design Details - Tie Plate Connections
August 23, 1978
To alert personnel of Federal Highway Administration and State highway agencies of a detail area to be avoided in new designs for structural steel bridges, and to point out the need for examination of this detail in bridge inspections.
Some bridges in Pennsylvania have experienced fatigue cracking of the top tie plate of floor beam brackets which are cantilevered from the main longitudinal girders. Research by Lehigh University has defined and evaluated the nature of this distress under Fritz Engineering Laboratory Report 386.4 and a digest of the results is contained in the Transportation Research Board Record 607, page 56.
- The tie plate for the floor beam bracket is normally bolted or riveted to both the floor beam bracket and the interior floor beam to transmit the moment capacity of the bracket back to the floor beam. In the instances of distress, this tie plate has also been bolted or riveted to the top flange of the longitudinal girder.
- Due to the cumulative effect of the compression stress in the top flange of the longitudinal girder, a shortening of this flange occurs. This shortening is not shared by the exterior stringers which rest on the top flange of the floor beam bracket and, as a result, a longitudinal displacement occurs in the end of the floor beam bracket relative to the part of the tie plate which is riveted or bolted to the longitudinal girder.
- This relative displacement sets up inplane bending stresses of a repetitive nature under live load which are sufficient to fatigue the tie plate to the point of fracture.
- This problem is not in evidence in situations where the stringers frame into the floor beam bracket and the flange of the bracket is in contact with the bottom of the concrete slab.
- Designs on future projects which utilize this type of detail should avoid bolting the tie plate to the longitudinal girder.
- Existing structures which have this detail should be carefully examined for evidence of cracking and the need for repairs. It is recommended that the bolts or rivets which connect the tie plate to the longitudinal girder be removed, and that the holes be protected against corrosion through use of smaller bolts and suitable washers or plates.
W. J. Wilkes, Director
Office of Engineering