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LRFD DesignOnline self-study

LRFD Limit States and Load Combinations

1 PDH · about 60 minutes · Course no. BR-D01 · 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.

Every LRFD bridge design check starts with the same inequality: the factored resistance must be at least the factored load effect. This course explains the reliability philosophy behind the AASHTO LRFD Bridge Design Specifications, the four groups of limit states (strength, service, extreme event and fatigue and fracture), the load factors in Table 3.4.1-1 and the permanent load factors in Table 3.4.1-2, the load modifiers for ductility, redundancy and operational importance, and a practical way to pick the governing combination for a member. Table values were checked against the current form of the tables reproduced in AASHTO's SHRP2 R19B implementation white paper, which shows the wind and fatigue revisions carried into the 8th (2017) and 9th (2020) editions. Example numbers are illustrative teaching values unless a source is named. Always check the edition and interim revisions your owner has adopted.

⬇ Download study guide (PDF)

Learning objectives

  • Explain the LRFD reliability philosophy
  • Identify strength, service, extreme event and fatigue limit states
  • Apply load factors and load modifiers
  • Select governing combinations

Outline

  1. The LRFD idea: reliability, not a single safety factor
  2. Table 3.4.1-1: the load combinations
  3. Table 3.4.1-2: permanent load factors
  4. The combinations one by one
  5. Load modifiers: ductility, redundancy and importance
  6. A workflow for finding the governing combination
  7. Using maximum and minimum factors correctly
  8. Illustrative example: an exterior bearing uplift check
  9. Illustrative example: comparing combinations for a pier cap
  10. Common mistakes and a review checklist
  11. Why the factors are what they are
  12. Which combinations usually matter, member by member
  13. Load abbreviations used in Article 3.3.2
  14. Open the course (19 screens, 2 knowledge checks)
  15. Final quiz — 10 questions, 70% to pass

References

  1. AASHTO, LRFD Bridge Design Specifications, 9th Edition, 2020
  2. AASHTO, Status of Implementation of SHRP2 Specifications to the AASHTO LRFD: Recommendations from Service Limit State Design for Bridges (SHRP2 R19B white paper)
  3. Nowak, A. S., Calibration of LRFD Bridge Design Code, NCHRP Report 368, Transportation Research Board, 1999
  4. Minnesota Department of Transportation, LRFD Bridge Design Manual, Section 3, Loads and Load Factors, January 2019
  5. Grubb, M., Updates to the AASHTO LRFD BDS Section 6, 8th Ed. (2017) and 9th Ed. (2020), presentation hosted by South Dakota DOT, 2019