According to Michigan residential construction standards, what is the maximum allowable deflection for a floor joist under live load?
Correct Answer
B) L/360
Michigan standards require that floor joists not deflect more than L/360 under live load to prevent excessive bounce and cracking.
Why This Is the Correct Answer
L/360 is the correct answer. This is the standard deflection limit for floor joists under live load as specified in the Michigan Residential Code and IRC. 'L' is the joist span in inches, and dividing by 360 limits deflection to roughly 1/3 of an inch per 10 feet of span. This limit prevents noticeable bounciness and protects finishes like tile from cracking.
Why the Other Options Are Wrong
Option A: L/240
L/240 is the allowable deflection limit for floor joists under total load (live + dead), not live load alone. Using L/240 for live load would allow too much movement and fail code requirements.
Option C: L/300
L/300 is not a standard floor joist live load deflection limit in Michigan or the IRC. It appears between L/240 and L/360 and may confuse candidates, but it is not the code-specified value for this application.
Option D: L/480
L/480 is a more restrictive limit sometimes applied to roof members with plaster or stiff finishes, not a standard floor joist live load criterion. Using this value would be overly conservative and is not required by code for typical floor joists.
Memory Technique
Think '360 for the floor you walk on' — 360 degrees in a circle, and you walk in circles on the floor. L/360 = live load on floors. L/240 = total (dead + live) load on floors. The stricter value (360) applies when the load is just live (people and furniture moving around).
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