PG Splice - PTZ During Storage

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PG Splice - PTZ During Storage

I am modeling a spliced WF girder bridge using PG Splice 4.1.3. It seems as though the program is defining the PTZs as though it were in the final condition (continuous) for the storage stage. For example, during storage the program is saying that there are PTZs at the top of the girder while in storage (thus it is limiting the tensile stress during storage at the top of the girder to 0.200 ksi). This does not seem accurate. There are no top temporary strands, so no compression is being introduced into the tops of the girders during storage...only bottom straight strands. As such, the top ends of the girders during storage are not precompressed tensile zones and should not be limited to 0.20 ksi. Can you confirm this is how PG Splice is defining PTZs (i.e. it is not defining PTZs within each stage, but defines PTZs only from the final stage)? Thanks!

Rick Brice
PG Splice - PTZ During Storage

AASHTO LRFD 5.2 defines the precompressed tensile zone as "Any region of a prestressed component in which prestressing causes compressive stresses and service load effects cause tensile stresses." PGSplice identifies all locations where prestressing (pretensioning and post-tensioning) causes compression and where Service load effects cause tension. Locations where both effects are present are considered to be in the precompressed tensile zone. Service loads occurs when the bridge is in service (eg, open to traffic). This is distinctly different then using service load combinations to compute stresses at intermediate construction stages. The definition of precompressed tensile zone does not have a time component so all prestressing is considered, which is consistent with Service load tensile stresses. There is only one precompressed tensile zone arrangement by the LRFD definition. As of LRFD 8th Edition, PGSplice is correctly following the specifications.

Having said that, we have recognized that the notion of precompressed tensile zone is something that was developed for conventional precast pretensioned bridges and cast-in-place post-tensioned bridges. It does not scale well to spliced girder bridges. As you have found, some of the tension limits don't make sense. In some cases, tension occurs and the tension limit, as specified by the LRFD, is not applicable. To this end, WSDOT has proposed changes to the AASHTO T-10 committee to change the tensile limits in and that removes the phrase "precompressed tensile zone" so that tension is evaluated wherever it occurs. This change is currently being deliberated in the T-10 committee.

For additional information, see "Proposed Simplification to the AASHTO LRFD Bridge Design Specification Tensile Stress Requirements for Precast-Prestressed Girders", Brice, R., Pickings, R., Proceedings of the PCI Convention and National Bridge Conference, 2014

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