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CulvertsOnline self-study

Culvert Load Rating and Live Load Through Fill

1 PDH · about 60 minutes · Course no. BR-CU03 · 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.

Culverts are buried structures. The soil around them is part of the load path: it carries the earth load, spreads truck wheel loads before they reach the top slab, and pushes on the walls. That is why rating a culvert is different from rating a girder bridge. This course covers soil-structure interaction, the AASHTO LRFD live-load distribution through fill developed in NCHRP Project 15-29 (NCHRP Report 647), the dynamic load allowance for buried structures, live load surcharge, analysis models for reinforced concrete box culverts from simple 2D frames to 3D soil-structure models, and what field load testing has shown. It draws on NCHRP Report 647 Appendix C (proposed specification revisions, April 2009) and the Louisiana DOTD RC Box Culverts Testing and Rating Statewide Final Report (SDR Engineering Consultants, July 2020), which summarizes AASHTO MBE Article 6A.5.12, AASHTO LRFD provisions and NCHRP 15-54 findings. All worked numbers are labeled illustrative.

⬇ Download study guide (PDF)

Learning objectives

  • Explain soil-structure interaction
  • Distribute live load through fill
  • Rate concrete box culverts
  • Know when load testing helps

Outline

  1. Why culverts are rated differently
  2. Earth loads: EV, EH and ES
  3. Live load through fill: the basic rule
  4. The rectangular area equations
  5. Illustrative live load pressure calculation
  6. Dynamic load allowance and live load surcharge
  7. Analysis models for box culverts
  8. The rating factor applied to culverts
  9. What field load tests have shown
  10. Pipes and flexible culverts
  11. Inspection data that ratings need
  12. Where the NCHRP 15-29 provisions came from
  13. Illustrative shallow-fill strip calculation
  14. Illustrative earth load magnitudes
  15. Multiple cells, tandems and lane load
  16. Common rating pitfalls
  17. Putting it together: a rating workflow
  18. Glossary of culvert rating terms
  19. Open the course (24 screens, 2 knowledge checks)
  20. Final quiz — 10 questions, 70% to pass

References

  1. AASHTO, The Manual for Bridge Evaluation, 3rd Edition, 2018 (with interim revisions)
  2. AASHTO, LRFD Bridge Design Specifications, 9th Edition, 2020
  3. NCHRP Report 647, Recommended Design Specifications for Live Load Distribution to Buried Structures, 2010
  4. NCHRP Project 15-29, Appendix C, Proposed Revisions to AASHTO Specifications, CNA Consulting Engineers and Simpson Gumpertz & Heger, April 2009 (online appendix to NCHRP Report 647).
  5. SDR Engineering Consultants, RC Box Culverts Testing and Rating Statewide, Final Report, State Project No. H.009859.5, Louisiana DOTD, July 15, 2020.
  6. AASHTOWare, Bridge Design and Rating (BrDR) 7.6 New Features and Enhancements (culvert highlights citing NCHRP Project 15-54).