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Steel I-Girder Flexure Fundamentals (LRFD Section 6)

1 PDH · about 60 minutes · Course no. BR-ST01 · 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.

Steel I-girders are the most common steel bridge superstructure in the United States, and their flexural design is governed by Article 6.10 of the AASHTO LRFD Bridge Design Specifications and its Appendices A6 and D6. This course walks through the flexural framework: cross-section proportion limits, composite and noncomposite behavior, the compact and noncompact classifications, positive-flexure strength and the ductility check, negative-flexure flange local and lateral-torsional buckling, the web bend-buckling and load-shedding factor, the hybrid factor, constructibility checks during deck placement, and the service and fatigue limits on flange stress. Article and equation numbers follow the 9th Edition (2020); confirm the edition and interims your owner has adopted. Equations were checked against the Caltrans Bridge Design Practice Chapter 6.1 (Steel Design Theory), the Illinois DOT design guide for LRFD composite steel beams, and the FHWA Steel Bridge Design Handbook. The worked numbers in the example sections are illustrative and were computed for this course; they are not from a real bridge.

⬇ Download study guide (PDF)

Learning objectives

  • Classify sections: compact and noncompact
  • Explain composite and noncomposite behavior
  • Check constructibility
  • Relate lateral-torsional buckling to bracing

Outline

  1. How Article 6.10 is organized
  2. Proportion limits (6.10.2)
  3. Composite and noncomposite behavior
  4. Compact, noncompact and slender: what the words mean
  5. Classifying a composite section in positive flexure (6.10.6.2.2)
  6. Compact positive-flexure resistance (6.10.7.1)
  7. The ductility check (6.10.7.3)
  8. Discretely braced compression flanges (6.10.8.1 and 6.10.8.2)
  9. The moment gradient modifier C<sub>b</sub>
  10. Relating LTB to bracing layout
  11. Web bend-buckling (6.10.1.9)
  12. The web load-shedding factor R<sub>b</sub> (6.10.1.10.2)
  13. Hybrid sections and the hybrid factor R<sub>h</sub> (6.10.1.10.1)
  14. Constructibility (6.10.3)
  15. Service limit state: permanent deformation (6.10.4)
  16. Fatigue and fracture (6.10.5)
  17. A note on C<sub>b</sub> from Caltrans practice
  18. Illustrative example, part 1: proportions and the plastic moment
  19. Illustrative example, part 2: the bare steel girder during deck placement
  20. A flexural design workflow
  21. Common mistakes
  22. Summary and self-check
  23. Where Appendix A6 fits
  24. Open the course (29 screens, 2 knowledge checks)
  25. Final quiz — 12 questions, 70% to pass

References

  1. AASHTO, LRFD Bridge Design Specifications, 9th Edition, 2020
  2. FHWA, Steel Bridge Design Handbook, FHWA-HIF-16-002 (multi-volume), 2015
  3. Caltrans, Bridge Design Practice, Chapter 6.1 Steel Design Theory, October 2022 (dot.ca.gov)
  4. Illinois DOT, Bridge Manual Design Guide 3.3.4, LRFD Composite Steel Beam Design (idot.illinois.gov)
  5. FHWA, Steel Bridge Design Handbook, FHWA-HIF-16-002, Vol. 12: Design for Constructibility and Vol. 11: Design Example (2015)