In a plate girder, why are stiffeners provided on or along the web plate?
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ATo reduce compressive stress in the flanges
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BTo reduce the average shear stress in the web
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CTo sustain local bearing stress only, not buckling
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DTo avoid buckling (bulking) of the web plate and provide bearing where needed
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ETo increase the steel grade without changing thickness
Answer
Correct Answer: To avoid buckling (bulking) of the web plate and provide bearing where needed
Explanation
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:
Identify web panel demand (shear and bearing).Provide vertical stiffeners at code-limited spacings to prevent shear buckling.Provide bearing stiffeners under concentrated loads and at supports.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