Under Alabama building codes, what is the maximum allowable span for 2x8 floor joists spaced 16 inches on center using Southern Pine lumber?
Correct Answer
C) 13 feet 2 inches
Alabama building codes allow 2x8 Southern Pine floor joists at 16" o.c. to span a maximum of 13 feet 2 inches.
Why This Is the Correct Answer
The IRC/Alabama residential building code span tables for Southern Pine (No. 2 grade) 2x8 joists at 16-inch on-center spacing and standard 40 psf live / 10 psf dead load yield a maximum span of 13 feet 2 inches. This is derived directly from the code's lumber span tables.
Why the Other Options Are Wrong
Option A: 12 feet 8 inches
12 feet 8 inches is shorter than the allowable span and would result in an unnecessarily conservative (over-built) design. It does not correspond to any standard Southern Pine span table entry for this configuration.
Option B: 14 feet 6 inches
14 feet 6 inches exceeds the code-permitted span for this lumber size and spacing. Using this span would create a floor system that deflects excessively or is structurally unsafe under design loads.
Option D: 11 feet 10 inches
11 feet 10 inches is the approximate span for a smaller member or wider spacing (e.g., 2x8 at 24 inches o.c.), not 16 inches o.c. Mixing up joist spacing is a common source of error on span table questions.
Memory Technique
2x8 at 16" o.c. → '13-2': think of a baker's dozen (13) plus 2 extra inches as a bonus. '13 feet 2 inches' — the joist goes a '13-to-2' distance.
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