According to span tables, what is the maximum allowable span for a 2x10 Douglas Fir floor joist spaced 16 inches on center with a live load of 40 psf?
Correct Answer
A) 16 feet 5 inches
Span tables for 2x10 Douglas Fir joists at 16 inches on center with 40 psf live load typically allow spans up to approximately 16 feet 5 inches, depending on grade and deflection criteria.
Why This Is the Correct Answer
According to standard span tables for Douglas Fir lumber, a 2x10 joist spaced 16 inches on center with a 40 psf live load can span approximately 16 feet 5 inches. This assumes standard grade lumber (typically #2 grade) and meets deflection criteria of L/360 for live loads. The span is determined by both bending strength and deflection limits, with deflection often being the controlling factor for floor joists.
Why the Other Options Are Wrong
Option B: 18 feet 8 inches
20 feet 1 inch is significantly over the safe span limit and would require either a larger joist size (2x12) or engineered lumber products to safely carry the specified loads.
Option C: 20 feet 1 inch
14 feet 2 inches is too conservative and represents a span that would be appropriate for either a smaller joist size (like 2x8) or higher loading conditions.
Memory Technique
Use '2-10-16-16' - 2x10 joist, 16 inches OC, approximately 16 feet maximum span for typical residential loading
Reference Hint
Florida Building Code Chapter 5 - Wood Frame Construction, or IRC Table R502.3.1(1) for floor joist spans
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