March 18, 2004
TO: All Design Section Staff - 47340
FROM: J. Kapur - 47340
SUBJECT: Design Policy Memo
Release of Temporary Strands
In Prestressed Girders
The top temporary strands are used to ensure stability of prestressed girders during shipping and handling. The timing and releasing sequence of temporary strands are important in both design and construction of prestressed girders. Due to a recent inquiry from the WSDOT Bridge Construction Office to eliminate some inconveniences experienced in bridge deck construction, the following sequence shall be considered in the design, detailing, and preparation of contract plans for prestressed girder bridges:
1. Top temporary strands shall be released after erection and bracing of prestressed girders, before casting
2. The time lapse between the release of top temporary strands and deck slab casting shall not exceed 60 days.
Contractor shall submit supporting calculations in case slab casting is delayed beyond 60 days.
3. The “A” dimension calculations should be changed accordingly to reflect the sequence of load application
and time intervals.
4. The revised default time intervals for creep calculations in the PGSuper program, reflecting these modifications,
- Release of top temporary strands: 90 days
- Casting Diaphragms: 90 days
- Casting deck slab: 120 days
- Weight of diaphragms: 30 days (90 to 120 days)
The Bridge Design Manual (BDM) and the WSDOT Standard Specifications will be modified accordingly.
The current practice of releasing top temporary prestressing strands right before casting deck slab, although theoretically correct, has resulted in some bridge deck construction concerns. Releasing of top temporary strands results in additional camber in prestressed girders affecting finished profile grade. Unbalanced release of top temporary strands may also result in cracking of intermediate concrete diaphragms. To eliminate these construction concerns, the top temporary strands shall be released immediately after placing, and bracing of prestressed girders on the piers.
If you have any questions regarding these issues, please contact Richard Brice at 705-7174
or Bijan Khaleghi at 705-7181.
cc: J. A. Weigel, Bridge and Structures - 47340
F. Higgins, Bridge and Structures – 47340