For a residential floor system with 16-inch on-center 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 feet 2 inches
According to standard span tables, 2x10 Southern Pine #2 joists at 16" o.c. with 40 psf live load can span approximately 15 feet 2 inches. Span tables account for deflection limits and bending strength requirements.
Why This Is the Correct Answer
Option B (15 feet 2 inches) is correct because it aligns with the standard span tables for Southern Pine #2 grade lumber. These tables are based on engineering calculations that consider both the allowable bending stress and deflection limits (typically L/360 for live loads). The span tables account for the specific properties of Southern Pine #2, including its fiber stress in bending and modulus of elasticity, when subjected to a 40 psf live load at 16-inch on-center spacing.
Why the Other Options Are Wrong
Option A: 13 feet 6 inches
13 feet 6 inches is too conservative and represents an unnecessarily short span that doesn't utilize the full structural capacity of 2x10 Southern Pine #2 joists under the given loading conditions.
Option C: 16 feet 8 inches
16 feet 8 inches exceeds the allowable span and would likely result in excessive deflection or stress that violates building code requirements for 2x10 Southern Pine #2 at this loading.
Option D: 18 feet 4 inches
18 feet 4 inches is significantly over the safe span limit and would create dangerous deflection and potential structural failure under the specified 40 psf live load.
Memory Technique
Remember '2x10 at 16 o.c. = about 15 feet' - the span in feet is roughly 1.5 times the joist depth in inches for typical residential loading
Reference Hint
Look up span tables in the Florida Building Code Chapter 5 (Wood Frame Construction) or IRC Table R502.3.1(1) for floor joist spans
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