Design Memorandum



TO:                  All Design Section Staff

FROM:            Bijan Khaleghi
DATE:             October 3, 2007
SUBJECT:       New limits for Long Span Prestressed Girders


The intent of this design memorandum is to increase the shipping weight and prestressing limit allowed in design to reflect current industry capabilities. This design memorandum supersedes the design memorandum issued on December 4, 2001.

Long span prestressed concrete girder bridges may bear increased costs due to difficulties encountered during the fabrication, shipping and erection of such long one-piece girders. Providing an alternate spliced-girder design to long span one-piece pretensioned girders may eliminate the excessive cost through competitive bidding. The following procedure for alternative design of prestressed concrete girders in the Plans shall be followed:

  • All prestressed concrete girders with shipping weight less than 190 kips shall be pre-tensioned only (no alternative design).

  • All prestressed concrete girders with shipping weight between 190 and 240 kips shall include both pretensioned and post-tensioned spliced prestressed concrete girder alternatives as part of the PS&E, with post-tensioning to be applied before the casting of deck slab. Post-tensioning may be applied after the casting of deck slab at the option of the Contractor with approval of the designer of record.

  • All prestressed concrete girders with shipping weight exceeding 240 kips shall be spliced prestressed concrete girders, with post-tensioning applied after the casting of the girder closures and deck slab.

  • For pretensioned concrete girders, the total number of permanent prestressing strands (straight and harped) shall be limited to 100-0.6 diameter strands.

When a spliced prestressed concrete girder alternative is presented in the Plans, the substructure shall be designed and detailed for the maximum force effect case only (no alternative design for substructure).

Shipping long girder segments may also be constrained by highway route or local features in some cases. It is therefore prudent to verify delivery for unusually long girders.


Background:

Prestressed concrete producers and trucking companies have recently increased their capability to new limits of 100-0.6 diameter strands and 240 kips shipping weight. This memorandum brings our designs up to the industry standard. The 190 kips shipping weight limit corresponds to 175 foot WF83G prestressed concrete girders that have been comfortably shipped in the last few years. These new limits can be used without consulting local producers. Designs that exceed these limits may be possible, but only after consulting with local producers and trucking companies.

Designers should continue to check that girder lengths and heights (including reinforcement projecting from the top flange) do not preclude the girders from being delivered to the jobsite.

Temporary strands for shipping and handling of long span prestressed girders may be pretensioned or post-tensioned at the option of the producer.

Providing a spliced prestressed concrete girder alternative allows for fabricating, shipping and erecting the girders in smaller segments, making the construction more feasible for remote locations.


If you have any questions regarding these issues, please contact Bijan Khaleghi at 705-7181.

cc:   Mohammad Sheikhizadeh, Bridge Construction - 47354

       F. Posner, Bridge and Structures 47340