TO: All Design Section Staff FROM: J. Kapur DATE: February 26, 2002 SUBJECT: Hydraulic Jack Design Requirements
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. On two recent contracts, the actual lifting loads were as much as 160% of the calculated lifting loads. Past office policy has been somewhat ambiguous in what safety factor should be applied to jack capacities. Some designers were requiring that all hydraulic jacks be sized for 150% of the calculated lifting load stipulated on the drawings while other designers were requiring 200%. On future jobs, unless the Expansion Joints and Bearings Specialist approves a variance, contract documents shall require that all hydraulic jacks be sized for 200% 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.
This memorandum supersedes one issued October 23, 2001 stipulating a 175% factor. If you have any questions regarding this issue, please contact Ralph Dornsife at 705-7199.
cc: J. Weigel, Bridge and Structures - 47340
F. Higgins, Bridge and Structures - 47340