|TO:||K. N. Kirker/R. T. Shaefer/W. A. Whitney/Y. A. Mhatre|
|DATE:||March 20, 1995|
|SUBJECT:||Prestressed Girder End Detailing at Intermediate Piers|
During the construction of the prestressed concrete girder bridge SR 503 - 144th Street to Battleground (Contract 4483), serious cracking was discovered in the bottom flanges of the girder ends where they were temporarily supported at intermediate piers. The piers of this particular structure have lowered cross beams which are skewed 30 degrees. The girder ends terminate perpendicular to the longitudinal axes of the girders. The bottom flange cracks occurred after setting the girders on temporary oak blocks, and prior to placement of diaphragm or slab concrete.
The skew of the crossbeam and diaphragm with respect to the ends of the girders in conjunction with a limited cross beam width required that the oak blocks be placed under the extreme end of the girders. The observed cracks are attributed to high corbel shear action and insufficient development of the prestressing strands in the extreme end region of the girder bottom flange for shear friction transfer of the girder dead load reaction.
To prevent similar occurrences in the future, designers are advised to maintain and detail a minimum of 6 inches between the end of the girder bottom flange and the edge of the oak block. In some instances, this may require a slightly wider cross beam. In addition, the ends of the girders shall be skewed parallel to the centerline of the pier if the skew is more than 5 degrees. The temporary oak block supports can then be placed parallel to and a minimum of 6 inches from the girder end. In addition, the edge of the cross beam should be checked to be sure that it can handle the temporary load safely. An edge distance of at least 3 inches is recommended.
This policy shall be adopted on all current and future bridge design projects. Bridges presently under contract should be investigated also. This policy will be incorporated into our Bridge Design Manual as soon as practical.
cc (w/Attach): J. Fahoum, Bridge and Structures - MS 47340
J. Dornsife, Bridge and Structures - MS 47340
F. Higgins, Bridge and Structures - MS 47340
J. A. Weigel, Construction - MS 47354