Commercial Building Inspector B2Roof Ceiling ConstructionHard
A structural engineer is reviewing the design of a steel-framed roof for a large warehouse. The roof design includes a series of wide-flange beams spanning 60 feet. Due to the long span, the engineer calculates a required minimum moment of inertia (I) to control deflection. For a simply supported beam, what is the maximum deflection allowed by IBC for roof members supporting plaster or similar brittle finishes, when subjected to live load only?
- AL/240
- BL/480
- CL/360
- DL/180
Show answer & explanationAnswer & explanation
Correct answer: C. L/360
IBC Table 1604.3 permits maximum deflections for structural members. For roof members supporting plaster or other brittle finishes, the live-load deflection limit is L/360. This is to prevent cracking of these sensitive finishes. For total load, it would be L/240.
Why the other options are wrong
- A. L/240 is the total load deflection limit for roof members supporting plaster or brittle finishes, or live load only for floors.
- B. L/480 is a more stringent limit, often for floor members supporting plaster or brittle finishes under live load, or for very sensitive equipment.
- D. L/180 is typically allowed for roof members not supporting plaster or brittle finishes, under live load only.
Roof Beam Deflection Limit (Brittle Finishes)
The maximum permissible deflection for roof members supporting brittle finishes (like plaster) under live load, specified as L/360 by the IBC to prevent cracking.
- L = span length.
- Prevents finish damage.
- Different limits for live vs. total load.
Memory trick: Plaster on the roof, L/360 is the proof, for live load, no droof.