According to the Florida Building Code, what is the maximum allowable deflection for a floor beam with a 20-foot span under live load?
Correct Answer
C) L/360 = 0.67 inch
The Florida Building Code typically limits floor beam deflection under live load to L/360, where L is the span length. For a 20-foot span: (20 × 12)/360 = 0.67 inches maximum deflection.
Why This Is the Correct Answer
The Florida Building Code establishes L/360 as the standard deflection limit for floor beams under live load to ensure structural integrity and occupant comfort. This ratio means the maximum allowable deflection equals the span length divided by 360. For a 20-foot span, this calculates to (20 × 12 inches)/360 = 240/360 = 0.67 inches. This limit prevents excessive floor movement that could cause cracking in finishes or create uncomfortable bouncing sensations.
Why the Other Options Are Wrong
Option A: L/240 = 1.0 inch
L/240 = 1.0 inch is too lenient and would allow excessive deflection that could cause structural damage, cracking in finishes, and uncomfortable floor movement that exceeds code requirements.
Option B: L/300 = 0.8 inch
L/300 = 0.8 inch is more restrictive than required but not the standard specified in the Florida Building Code, which specifically mandates L/360 for floor beam live load deflection.
Option D: L/480 = 0.5 inch
L/480 = 0.5 inch is overly restrictive and typically applies to roof members or special conditions, not standard floor beams, making it unnecessarily conservative and potentially uneconomical.
Memory Technique
Remember 'Floor = 360' and use the phrase 'Floor beams bend 360 ways but stay within limits' to recall that floor deflection uses L/360 ratio
Reference Hint
Florida Building Code, Chapter 16 - Structural Design, Table 1604.3 for deflection limits, or IBC Section 1604.3 which the FBC references
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