According to Alabama building codes, what is the maximum allowable deflection for a floor joist with a 16-foot span under live load?
Correct Answer
D) L/360 (0.53 inches)
Alabama building code limits floor joist deflection to L/360 under live load. For 16 feet: (16 × 12) ÷ 360 = 0.53 inches.
Why This Is the Correct Answer
The IRC and IBC both specify L/360 as the maximum allowable deflection for floor members under live load. For a 16-foot span: (16 ft × 12 in/ft) ÷ 360 = 192 ÷ 360 = 0.533 inches, which rounds to 0.53 inches. This strict limit ensures floors feel stiff and do not cause cracking in finishes below.
Why the Other Options Are Wrong
Option A: L/180 (1.07 inches)
L/180 (1.07 inches) is the deflection limit for roof rafters or non-plastered ceilings under live load — not floors. Using L/180 for floors would allow twice as much deflection as permitted, resulting in bouncy floors and potential damage to finishes.
Option B: L/300 (0.64 inches)
L/300 (0.64 inches) is sometimes used for roof members with attached plaster ceilings but is not the standard for floor joists under live load. It is more lenient than the required L/360.
Option C: L/240 (0.8 inches)
L/240 (0.8 inches) is the limit for floor members under total load (dead + live), not live load alone. Using this figure for a live-load-only question is incorrect; live load alone requires the stricter L/360.
Memory Technique
The deflection ladder (most to least restrictive): Floors live = L/360 → Floors total = L/240 → Roof with plaster = L/300 → Roof no plaster = L/180. For floors under LIVE load, always use the strictest: L/360.
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