What is the maximum allowable deflection for a floor joist with a span of 14 feet under live load conditions according to Michigan Building Code?
Correct Answer
B) L/360 (0.47 inches)
Michigan Building Code limits floor joist deflection to L/360 under live loads. For 14 feet (168 inches): 168/360 = 0.47 inches maximum.
Why This Is the Correct Answer
Michigan Building Code (adopting IRC/IBC standards) limits floor joist deflection under live load to L/360, where L is the span in inches. For a 14-foot span: L = 14 × 12 = 168 inches. Maximum deflection = 168 ÷ 360 = 0.467 inches, rounded to 0.47 inches. The L/360 standard for floors under live load ensures the floor feels stiff and does not cause cracking of finish materials.
Why the Other Options Are Wrong
Option A: L/240 (0.7 inches)
L/240 (0.7 inches) is the deflection limit used for roof members and ceiling joists under live load — not floors. It is a less restrictive standard that allows more deflection. Using L/240 for a floor would permit excessive bounciness and potential finish material damage.
Option C: L/480 (0.35 inches)
L/480 (0.35 inches) is a more restrictive standard sometimes used for floors with brittle finish materials (like tile or stone) or high-end applications. It is not the standard Michigan Building Code minimum for general floor live load deflection.
Option D: L/600 (0.28 inches)
L/600 (0.28 inches) is an extremely restrictive deflection limit not commonly required by code for residential floor joists. This level of stiffness exceeds normal code requirements and would only be specified for special conditions.
Memory Technique
Remember the deflection hierarchy: L/360 for FLOORS (live load) — the '3' in 360 looks like a floor plan with three rooms. L/240 for ROOFS (think '2' stories up = roof). Higher divisor = stiffer = stricter. L/360 > L/240 in strictness.
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