What is the minimum bearing length required for a 2x10 floor joist bearing on a wood beam according to CBC?
Correct Answer
D) 1.5 inches
CBC requires minimum 1.5 inches bearing length for dimensional lumber joists bearing on wood supports to ensure adequate load transfer.
Why This Is the Correct Answer
Option D (1.5 inches) is correct. The California Building Code (CBC), consistent with IBC and AWC NDS provisions, requires a minimum bearing length of 1.5 inches for dimensional lumber floor joists (like a 2x10) when bearing on wood supports. This minimum ensures adequate load transfer through bearing without crushing the wood fibers at the contact area. 1.5 inches corresponds to the actual thickness of a 2x dimensional member laid flat.
Why the Other Options Are Wrong
Option A: 2.5 inches
Option A (2.5 inches) is incorrect. This value is not the standard minimum for wood-on-wood bearing under CBC. It may be confused with masonry bearing requirements or misremembered from other code provisions.
Option B: 3.5 inches
Option B (3.5 inches) is incorrect. 3.5 inches is the actual width of a 2×4 stud or the net dimension of a 4× member, not a bearing length requirement. This figure likely appears as a distractor drawing on common lumber dimensions.
Option C: 4.5 inches
Option C (4.5 inches) is incorrect. This value exceeds any standard minimum bearing requirement for wood floor joists on wood beams and does not correspond to a CBC provision. It may be confused with masonry or concrete bearing requirements for heavier members.
Memory Technique
Wood-on-wood = 1.5 inches = the thickness of the joist itself (a 2x member's actual thickness). Think: 'the joist needs to sit on something at least as wide as it is thick.' For masonry support, double it to 3 inches.
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