A contractor is framing a floor system using I-joists. The building inspector notes that web stiffeners are required at bearing points. According to CBC, when are web stiffeners mandatory for I-joists?
Correct Answer
D) When bearing stress exceeds the web capacity
CBC requires web stiffeners for I-joists when the bearing stress from concentrated loads or reactions exceeds the web's capacity to resist crushing or crippling, as determined by engineering analysis.
Why This Is the Correct Answer
I-joist webs are thin OSB or plywood panels optimized for bending resistance, not concentrated compressive loads. When the bearing stress from a concentrated load or reaction force exceeds the web's capacity to resist web crushing or web crippling at the bearing point, web stiffeners (vertical blocking between the flanges) are required. This is determined by engineering analysis per the manufacturer's specifications and CBC structural requirements — it is a performance-based requirement, not a prescriptive trigger based solely on length or load magnitude.
Why the Other Options Are Wrong
Option A: Only when bearing length exceeds 3.5 inches
Bearing length of 3.5 inches is a common prescriptive minimum bearing length for I-joists, not a trigger for web stiffeners. Longer bearing lengths actually reduce bearing stress; requiring stiffeners only when bearing length exceeds 3.5 inches contradicts engineering logic.
Option B: At all bearing points regardless of load
Requiring web stiffeners at all bearing points regardless of load would be unnecessarily conservative and is not the code requirement. Web stiffeners add cost and labor; they are only required when the engineering analysis shows the web cannot handle the applied stress.
Option C: Only when point loads exceed 1000 lbs
A 1,000 lb point load threshold is not a CBC-specified trigger for web stiffeners. I-joist manufacturers provide load tables for web stiffener requirements based on joist depth, span, and reaction force — no single universal load threshold applies across all products.
Memory Technique
I-joist webs are thin — they can crush under point loads. Stiffeners = vertical reinforcement at bearing. They are required when stress EXCEEDS capacity — always an engineering call. Remember: 'Web Stiffeners When Web Stress is Too HIGH' — performance-based, not prescriptive.
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