A contractor is installing 2x10 floor joists spanning 14 feet in a residential project. What is the maximum allowable deflection under live load per CBC requirements?
Correct Answer
D) L/240 = 0.7 inches
CBC requires floor joists to have a maximum live load deflection of L/360, but total load deflection of L/240. For residential floors, L/240 = 14 ft × 12 in/ft ÷ 240 = 0.7 inches is the governing limit for total deflection.
Why This Is the Correct Answer
L/240 = 0.7 inches is the correct answer because CBC governs total load deflection for floor joists at L/240. For a 14-foot span: 14 ft × 12 in/ft = 168 inches ÷ 240 = 0.70 inches. While L/360 applies to live load only, the question asks for the maximum allowable deflection under the governing CBC limit, which is L/240 for total load on floor members.
Why the Other Options Are Wrong
Option A: L/360 = 0.47 inches
L/360 = 0.47 inches applies specifically to live load deflection limits for floors, not the governing total deflection limit. The calculation 168 ÷ 360 = 0.47 inches is a valid intermediate value, but L/240 is the controlling criterion for total load deflection per CBC.
Option B: L/300 = 0.56 inches
L/300 = 0.56 inches is not a standard CBC deflection limit for floor joists. This ratio is used for some roof applications, not residential floor systems. It falls between the live-load limit (L/360) and the total-load limit (L/240), making it incorrect for either use.
Option C: L/180 = 0.93 inches
L/180 = 0.93 inches is the deflection limit used for roof rafters with no finish ceiling attached (non-structural roofing members), not for floor joists. Using this more lenient limit for floors would allow excessive deflection that creates noticeable bounce and finishes damage.
Memory Technique
Memorize deflection limits with a simple story: '360 degrees in a circle = live loads go full circle (most restrictive for floors); 240 = total load for floors; 180 = roofs without ceilings (most lenient).' Floor joists must meet BOTH limits — L/360 for live, L/240 for total — the larger deflection number (L/240) governs as the ceiling.
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