Memorandum

 Bridge bearing replacement operations generally require lifting of superstructure elements using hydraulic jacks.  The designer is responsible for calculating anticipated lifting loads and stipulating these loads on the contract drawings.

Past experience has shown that actual lifting loads nearly always exceed calculated lifting loads.  This phenomenon may be caused by many factors including friction in the hydraulic jack system and underestimation of dead loads.  Past office policy has required that all hydraulic jacks be sized for 150% of the calculated lifting load stipulated on the drawings.  On two recent contracts, the actual lifting loads were as much as 160% of the calculated lifting loads.  On future jobs, contract documents shall require that all hydraulic jacks be sized for 175% of the calculated lifting load.  In all cases, the designer shall verify from manufacturers' literature that appropriate hydraulic jacks are available to operate within space limitations imposed by the design situation.   

If you have any questions regarding this issue, please contact Ralph Dornsife at

705-7199.

JK:dsb

KNK/RJD

cc:     J. A. Weigel, Bridge and Structures -  47340

         F. Higgins, Bridge and Structures - 47340