Plate girders – unstiffened web criterion: A web plate is classified as “unstiffened” when the ratio of its clear depth to thickness is less than which value?

Introduction / Context:
Plate girder webs may be provided with transverse (and sometimes longitudinal) stiffeners to prevent shear buckling. When the web is sufficiently stocky, it can be treated as “unstiffened,” simplifying detailing and fabrication.

Given Data / Assumptions:

• Web plate in a plate girder.
• Clear depth d (between flanges) and thickness t are known.
• Buckling under shear governs the need for stiffeners.

Concept / Approach:

A practical limit compares the slenderness of the web panel via the ratio d/t. If d/t is small enough (stocky web), shear buckling is unlikely within service ranges, and transverse stiffeners can be omitted except at supports and concentrated loads. A commonly adopted threshold is d/t < 85 for mild steel webs, beyond which web stiffeners are recommended to prevent buckling.

Step-by-Step Solution:

Verification / Alternative check:

Traditional handbooks list web stiffening limits near 85 for mild steel and adjust with yield strength or serviceability constraints in modern codes.

Why Other Options Are Wrong:

• 35, 50, 60: too low; would over-conservatively require stiffeners in many practical webs.
• 90: slightly above common threshold; may risk shear buckling without stiffeners.

Common Pitfalls:

• Using overall depth instead of clear depth between flanges.
• Ignoring concentrated load panels which may still need stiffeners.

Final Answer:

85.