The maximum deflection limit for roof members supporting a plaster ceiling due to seismic or wind loads is what ratio?

The maximum deflection limit for roof members supporting a plaster ceiling is established to ensure that the ceiling remains intact and functional, particularly under various load conditions such as seismic or wind forces. In this context, a deflection limit expressed as a ratio of the span (L) is used to quantify permissible deflection.

The ratio of L/360 indicates that the maximum allowable deflection should not exceed 1/360th of the span length. This standard is widely adopted because it effectively balances comfort and structural integrity. For ceilings, particularly plaster types, excessive deflection could lead to cracking or a compromised appearance, as well as potential structural issues over time.

In contrast, ratios like L/180 or L/240 might allow for greater deflection, which is typically more acceptable for floors or other structural elements where cracking is less critical. The L/400 limit would be more stringent than necessary for plaster ceilings, which do not require such low deflection norms. Therefore, the choice of L/360 for ceiling structures provides a reasonable balance, ensuring the ceiling's performance and longevity while maintaining aesthetic quality.