A floor system uses 2×12 joists spanning 16 feet with a live load of 125 psf. The joists are spaced 12 inches on center. What is the total load per linear foot on each joist?
Correct Answer
D) 165 plf
Each joist supports a 1-foot wide strip of floor. Total load = (Live Load + Dead Load estimate) × tributary width = (125 psf + 40 psf dead load) × 1 ft = 165 plf. The 40 psf dead load is typical for wood frame floor systems.
Why This Is the Correct Answer
With joists spaced 12 inches (1 foot) on center, each joist's tributary width is 1 foot. Total load per square foot = live load (125 psf) + assumed dead load (40 psf for wood frame floors) = 165 psf. Load per linear foot = 165 psf × 1 ft tributary width = 165 plf. The 40 psf dead load is standard practice for wood frame floor assemblies including sheathing, finishes, and framing self-weight.
Why the Other Options Are Wrong
Option A: 140 plf
140 plf would result from using only a 15 psf dead load estimate (125 + 15 = 140), which is too low for a typical wood frame floor system that includes sheathing, finish flooring, and self-weight of framing.
Option B: 190 plf
190 plf would imply a dead load of 65 psf (125 + 65 = 190), which is excessive for a standard wood frame floor and more typical of a concrete slab or heavy finish system.
Option C: 125 plf
125 plf ignores the dead load entirely and uses only the live load. Dead load must always be added to live load to determine total design load — using live load alone is a code violation and fundamentally incorrect structural practice.
Memory Technique
For wood frame floors, remember '40 for dead' — a 40 psf dead load is the standard assumption. Then: Total load = (LL + DL) × tributary width. At 12-inch spacing, tributary width = 1 ft, so the math is simple multiplication. '125 live + 40 dead = 165 total.'
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