In a plate girder, why are stiffeners provided on or along the web plate?

Introduction / Context:
Plate girders achieve economy by using thin webs with flanges carrying most bending. Thin webs are susceptible to shear buckling and local crushing near reactions; stiffeners mitigate these issues.

Given Data / Assumptions:

• Thin web plate under shear/bearing demands.
• Transverse (vertical) and bearing stiffeners may be used.
• Longitudinal stiffeners may also be used in deep webs.

Concept / Approach:
Web panels buckle in shear if h/t is large. Transverse stiffeners subdivide the web into smaller panels to raise critical shear buckling stress. Bearing (end) stiffeners distribute concentrated reactions into the web and flanges, preventing local crushing.

Step-by-Step Solution:

Verification / Alternative check:
Check web slenderness and panel proportions; confirm shear buckling capacity with and without stiffeners per design provisions.

Why Other Options Are Wrong:
Flange compressive stress is governed by flange area, not web stiffeners; average shear stress is not reduced (capacity is increased); bearing-only statement is incomplete.

Common Pitfalls:
Omitting bearing stiffeners at heavy reactions; misplacing stiffeners relative to loads; ignoring weld details causing web distortion.

Final Answer:
To avoid buckling (bulking) of the web plate and provide bearing where needed