A 2x10 Douglas Fir-Larch joist with a span of 14 feet is supporting a live load of 40 psf and dead load of 15 psf. According to span tables, what is the maximum allowable spacing?
Correct Answer
D) 16 inches on center
With a total load of 55 psf (40 LL + 15 DL) and a 14-foot span, a 2x10 Douglas Fir-Larch joist can safely span at 16 inches on center according to standard span tables. Closer spacing would be acceptable but not required.
Why This Is the Correct Answer
For a 2x10 Douglas Fir-Larch joist spanning 14 feet with a total load of 55 psf (40 psf live load + 15 psf dead load), standard span tables specify 16 inches on center as the maximum allowable spacing. This spacing provides adequate structural support while optimizing material usage. The combination of lumber grade, span distance, and load requirements determines this specific spacing requirement according to building codes.
Why the Other Options Are Wrong
Option A: 19.2 inches on center
19.2 inches on center exceeds the maximum allowable spacing for a 2x10 Douglas Fir-Larch joist under these load conditions. This wider spacing would not provide adequate structural support and could lead to excessive deflection or failure under the specified loads.
Option B: 24 inches on center
12 inches on center is overly conservative for this application. While this spacing would certainly be safe, it's not the maximum allowable spacing and would result in unnecessary material costs and labor. The span tables allow for wider spacing given the load conditions.
Option C: 12 inches on center
24 inches on center is far too wide for this joist size and load combination. This spacing would result in structural inadequacy, excessive deflection, and potential failure. Standard span tables do not permit such wide spacing for these conditions.
Memory Technique
Remember '16 for standard residential' - 16 inches on center is the most common joist spacing for typical residential floor loads with 2x10 lumber at moderate spans.
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