When framing a wood structure in California, what is the maximum span allowed for a 2x10 Douglas Fir floor joist spaced 16 inches on center with a live load of 40 psf?
Correct Answer
D) 16 feet 5 inches
Per CBC span tables, a 2x10 Douglas Fir joist at 16 inches on center can span 16 feet 5 inches with 40 psf live load.
Why This Is the Correct Answer
Per CBC span tables for 2x10 Douglas Fir-Larch No. 2 joists at 16-inch spacing with a 40 psf live load and 10 psf dead load, the allowable span is 16 feet 5 inches. This is a direct table look-up value and is the correct answer (option D).
Why the Other Options Are Wrong
Option A: 20 feet 1 inch
20 feet 1 inch exceeds the CBC-tabulated maximum for this size, spacing, and load combination. This span might apply to a larger joist size (e.g., 2x12) or a lower live load condition.
Option B: 14 feet 2 inches
14 feet 2 inches is too conservative and corresponds to a higher live load scenario or a lower-grade lumber designation than Douglas Fir-Larch No. 2.
Option C: 18 feet 8 inches
18 feet 8 inches is closer but still exceeds the allowed span for a 2x10 at 16 inches on center under 40 psf. This figure might be confused with a 12-inch spacing value.
Memory Technique
For 2x10 DF at 16 OC with 40 psf LL: '16-5' — the joist depth (10) reminds you the span is in the 16-foot range, and the '5 inches' is the distinguishing detail. Pair the number 16 (spacing) with 16 feet to anchor the approximate span.
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