A contractor is building a single-family residence with 2x10 floor joists spaced 16 inches on center spanning 14 feet. The building is located in Seismic Design Category D. What is the maximum allowable deflection under live load according to CBC?
Correct Answer
B) L/240 = 0.70 inches
For floor joists in residential construction, CBC allows deflection of L/240 under live load. With a 14-foot span (168 inches), the maximum deflection is 168/240 = 0.70 inches.
Why This Is the Correct Answer
CBC Table 1604.3 specifies deflection limits for floor members under live load as L/240, where L is the span length. With a 14-foot span (168 inches), the maximum allowable live-load deflection is 168 ÷ 240 = 0.70 inches. The Seismic Design Category D designation is contextual background and does not change the deflection limit for floor joists under live load.
Why the Other Options Are Wrong
Option A: L/360 = 0.47 inches
L/360 (0.47 inches) is the deflection limit for floor members supporting plaster or brittle finishes, or for roof members under live load. It is more restrictive than required for a standard wood floor joist under live load. Applying L/360 to this scenario is overly conservative and reflects confusion between floor and brittle-finish criteria.
Option C: L/300 = 0.56 inches
L/300 (0.56 inches) does not correspond to a standard CBC deflection limit category. It falls between L/240 and L/360 but is not a prescribed value in CBC Table 1604.3 for floor joists. Selecting it reflects guessing between two memorized values.
Option D: L/180 = 0.93 inches
L/180 (0.93 inches) is the deflection limit for roof members not supporting a plaster ceiling, under live load or wind load. It is less restrictive than the floor joist standard and would allow excessive bounce and serviceability issues in a residential floor.
Memory Technique
Remember the deflection hierarchy: L/360 for glass/plaster (fragile), L/240 for floors (firm footing), L/180 for roofs (can flex more). Think '360 degrees = full circle of protection for fragile things; 240 = solid floor standard; 180 = roof can breathe.'
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