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LecturesModule 10

Plate Girders

Built-up I-sections, tension-field action, stiffener design.

AISC Reference Box
  • AISC 360-22 — Specification chapter governing this topic
  • AISC Manual 16th Ed. — Design tables and worked examples

Lecture Notes

This module introduces plate girders. Lecture content here covers the governing physics, LRFD philosophy, and how the relevant AISC 360-22 chapter organizes the limit states.

Instructors can replace this text in Admin Mode. Each section is structured around: (1) behavior, (2) failure modes, (3) AISC limit-state equations, (4) design workflow, (5) detailing requirements.

A short comparison to ASD is included only where the resistance factor / safety factor relationship clarifies the LRFD design check.

Welded plate girder (stiffeners shown)
Welded plate girder with intermediate transverse stiffeners and tension-field action (AISC Ch. F + App. 6).

Formula Sheet

NameEquationAISC Ref
Design strengthφ Rn ≥ RuAISC 360-22 B3.1

Worked Example

Plate Girders

Given
Replace with project-specific given data (loads, geometry, material).
Load combination
Controlling LRFD load combination from ASCE 7.
Required strength
Compute required strength Ru from the controlling combination.
Limit states
  • Limit state 1
  • Limit state 2
AISC reference
AISC 360-22 — applicable chapter
Solution steps
  1. 1. Required strength
    Compute Ru.
  2. 2. Trial section
    Pick a trial from AISC shape tables Instructor should verify with official AISC Manual.
  3. 3. Check each limit state
    Apply φ Rn ≥ Ru for every governing limit state.
  4. 4. Iterate
    Resize until the most economical section satisfies all checks.
Final design decision
Select the lightest section that satisfies all LRFD limit states.
Common mistakes in this example
  • Skipping a limit state
  • Using the wrong φ factor
  • Forgetting serviceability checks

FE-Style Worked Examples (6)

Each example mirrors the NCEES FE Civil Reference Handbook style: brief givens, a labeled figure, AISC section reference, step-by-step numeric solution, and a single boxed answer.

Given
Span 80 ft, gravity wu=4 k/ft, no rolled W deep enough/economical.
AISC Reference
AISC Ch. F + App. F13
Step-by-step solution
  1. Decision
    Built-up I with h/tw > 260 → plate girder territory.
Answer Use welded plate girder, design per §F13.
Welded plate girder (stiffeners shown)

Practice Problems

  1. [E] Distinguish a plate girder from a rolled W-shape in one sentence.
  2. [E] State the h/tw threshold above which a section is a plate girder (~970/√Fy).
  3. [E] Name two functions of intermediate transverse stiffeners.
  4. [E] Define tension-field action.
  5. [E] State the AISC chapter for plate girders (Ch. F + Appendix 6).
  6. [M] Design flanges of a plate girder: M = 4500 k-ft, h = 60 in., Fy = 50 ksi.
  7. [M] Web slenderness check for 60 x 5/16 in. web, A36. Classify per §F13.
  8. [M] Intermediate stiffener spacing for Vu = 350 k on 60 x 3/8 web, A36.
  9. [M] Compute Vn with TFA for the previous problem; compare without TFA.
  10. [M] Bearing-stiffener pair at a 200 k support reaction; size 1/2 in. plates each side.
  11. [H] Design complete simply-supported plate girder, L = 60 ft, Mu = 5200 k-ft, Vu = 380 k.
  12. [H] Hybrid girder: A572 Gr 50 flanges, A36 web. Compute Rh per §F13.3.
  13. [H] Combined moment + shear: Mu/φMn + 0.625·Vu/φVn ≤ 1.375 (§G2.2).
  14. [H] Design fillet weld size between flange and web for shear flow VQ/I at supports.
  15. [H] Fatigue check on a flange splice, Δσ = 12 ksi, 2x10⁶ cycles per AISC Appendix 3.
Structured Clues
  • Web is slender when h/tw > 5.7√(E/Fy).
  • Intermediate stiffeners are required when shear demand exceeds φvCv1·Vy.
  • TFA (tension-field action) allowed only when transverse stiffeners exist.
Code References
  • AISC 360-22 §F4, F5, G2.2, Appendix 6

Quiz

1. Which AISC 360-22 chapter primarily governs plate girders?
2. In LRFD, the basic design inequality is:

Common Student Mistakes

  • Mixing ASD and LRFD load combinations in the same problem.
  • Using nominal strength Rn instead of design strength φRn.
  • Forgetting to check every limit state listed in the AISC chapter.

"Professor Explains" Script

Today we're talking about plate girders. Think of this topic as one step in the LRFD workflow: identify the demand, identify the limit states from the relevant AISC chapter, then check that φ·Rn is at least equal to Ru. We'll walk through the failure modes, the equations, and a worked example. Pay close attention to where the resistance factor changes — that's where students lose points on exams.