For a residential floor system with 16" joist spacing, what is the maximum allowable span for 2x10 Southern Pine joists (Grade #2) supporting a live load of 40 psf?
Correct Answer
B) 15'-2"
According to standard span tables, 2x10 Southern Pine #2 grade joists at 16" on center can span approximately 15'-2" for a 40 psf live load. Span tables account for deflection limits and bending strength requirements.
Why This Is the Correct Answer
Option B (15'-2") is correct because it represents the maximum allowable span for 2x10 Southern Pine #2 grade joists at 16" on center supporting a 40 psf live load according to standard residential floor joist span tables. This span limit is determined by both deflection criteria (typically L/360 for live loads) and bending strength requirements. The span tables are based on engineering calculations that ensure structural safety and serviceability under normal loading conditions.
Why the Other Options Are Wrong
Option A: 13'-6"
13'-6" is too conservative and represents a span that would be appropriate for either a higher live load requirement, lower grade lumber, or possibly 2x8 joists rather than 2x10s.
Option C: 16'-8"
16'-8" exceeds the safe span limit for these conditions and would likely result in excessive deflection or potential structural failure under the specified 40 psf live load.
Option D: 18'-0"
18'-0" is significantly over the allowable span and would definitely cause structural problems including excessive deflection, potential cracking of finishes, and possible structural failure.
Memory Technique
Remember '2-10-15': 2x10 joists span approximately 15 feet for typical residential floor loads (40 psf live load at 16" OC spacing)
Reference Hint
IRC Table R502.3.1(1) - Floor Joist Spans for Common Lumber Species, or Florida Building Code Residential Chapter 5 - Floors
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