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Prestressed ConcreteOnline self-study

Prestressed Girder Fundamentals: Losses and Service Stresses

1 PDH · about 60 minutes · Course no. BR-P01 · Instructor: Michael Craig, PE

About the instructor

Michael Craig, PE is Senior Vice President and Southeast Director at Strinteg Corporation, leading bridge inspection and structural engineering across eight states and Puerto Rico. In 27+ years he has inspected over 4,000 structures and completed 3,000+ load ratings and 160+ load tests. He has managed repair and preservation design for complex bridges, including cable-stayed bridges. He holds PE licenses in twelve states, DC and Puerto Rico, SPRAT Level 1, and FAA Part 107 credentials.

Precast, pretensioned concrete girders carry a large share of the short- and medium-span bridges in the United States. Designing, checking, rating or inspecting them takes three skills: understanding how the precompression gets into the concrete, estimating how much of it is lost over time, and checking concrete stresses at transfer and in service against the code limits. This course covers each in turn. It explains pretensioning and post-tensioning, defines the components of prestress loss, and works through the elastic shortening and approximate time-dependent loss equations of the AASHTO LRFD Bridge Design Specifications. It then reviews the stress limits for strand and concrete and explains why the Service III load combination, rather than Service I, governs tension in the bottom of a girder. The main sources are two public worked examples. One is the FHWA Comprehensive Design Example for Prestressed Concrete Girder Superstructure (FHWA-NHI-04-043), which uses an earlier LRFD edition. The other is Illinois DOT Bridge Design Guide 3.5, LRFD PPC Deck Beam Design (May 2019), which applies the 2017 eighth edition. Measured-versus-predicted loss data come from a 2021 PCI Journal study of AASHTO Type II–VI girders. AASHTO LRFD and the PCI Bridge Design Manual are not freely available. Equations and limits are therefore quoted as these public sources reproduce them, with their article numbers, and any example that is not taken from those sources is labeled illustrative.

⬇ Download study guide (PDF)

Learning objectives

  • Explain pretensioning and post-tensioning
  • Identify prestress loss components
  • Check service stress limits
  • Recognize Service III tension

Outline

  1. How prestressing works
  2. Strand stress limits
  3. Components of prestress loss
  4. Elastic shortening
  5. Time-dependent losses: the approximate method
  6. Concrete stress limits: at transfer
  7. Concrete stress limits: in service and the role of Service III
  8. Time-dependent losses: refined methods
  9. Predicted versus measured losses
  10. Worked example: losses and stresses for a simple-span girder (illustrative)
  11. Reading prestressed girders in the field and in ratings
  12. Common mistakes and quick checks
  13. Transfer length, development length and the effective prestress
  14. Putting it together: the design and check sequence
  15. Glossary of terms used in this course
  16. Open the course (21 screens, 2 knowledge checks)
  17. Final quiz — 10 questions, 70% to pass

References

  1. AASHTO, LRFD Bridge Design Specifications, 9th Edition, 2020
  2. PCI, Bridge Design Manual, 3rd Edition (with revisions)
  3. FHWA, Comprehensive Design Example for Prestressed Concrete (PSC) Girder Superstructure Bridge with Commentary, US Customary Units, FHWA-NHI-04-043 (2003), fhwa.dot.gov/bridge/lrfd/fhwanhi04043.pdf
  4. Illinois DOT, Bridge Manual Design Guide 3.5 — LRFD PPC Deck Beam Design (May 2019, updated for the 2017 AASHTO LRFD 8th Edition)
  5. Dang, C. N., et al., 'Investigation of measured prestress losses compared with design prestress losses in AASHTO Types II, III, IV, and VI girders,' PCI Journal 66(3), May–June 2021