Definition of a deep plate girder (by flange-angle proportion) A plate girder is termed a “deep plate girder” when its overall depth is at least n times the depth of the vertical leg of the flange angles. The value of n is:

Introduction / Context:
“Deep” plate girders behave differently from shallow girders in terms of shear distribution and web slenderness. Traditional rules use proportions relative to flange-angle legs to categorize a girder as deep for detailing and stiffening guidance.

Given Data / Assumptions:

• Depth measured overall between flanges.
• Vertical leg of flange angles is the reference dimension.
• Looking for a conventional threshold ratio n.

Concept / Approach:
Classic steelwork texts often use a multiple of the flange-angle leg to define “deep.” A commonly cited threshold is around eight times the vertical leg dimension, indicating heightened attention to web stiffening and bearing details.

Step-by-Step Solution:
Adopt standard proportion → deep if depth ≥ 8 × (vertical flange-leg).Select n = 8.

Verification / Alternative check:
This proportion aligns with traditional detailing recommendations for stiffener spacing and bearing checks on deep webs.

Why Other Options Are Wrong:

• 2, 4, 6: too small to reflect “deep” behavior.
• 10: more restrictive than typical guidance and not the common threshold.

Common Pitfalls:
Equating “deep” only with span-to-depth ratio; this definition specifically uses flange-angle leg depth as the reference.

Final Answer:
8