Engineering - For beginners, Part 3

[RAB]

Key Elements for Deflection Ratios

Common minimum ratios for deflection in residential construction.
 
These are the minimum deflection values from the most widely used residential codes; most designer/engineer/builders have used these for base values with out much change for a number of years.
 
Rafters having no finished ceiling      1/180
Interior walls with non-brittle finish  1/180
Interior walls with brittle finish         1/240
Exterior walls with non-brittle finish  1/360
Floors and finished ceilings                1/360
Exterior walls with brittle finish          1/361 (not a typo)

1/360 = 1" of deflection in 360" or ½" of deflection in 180"

  **General Rule about Deflection Ratio**
As the finish becomes more brittle and the application is more noticeable a higher deflection ratio is needed.

Example based on finish: Sheet rock compound and plaster should have a support structure higher than 1/239.
Example based on application (structural movement): Wall assemblies around doors and windows should not deflect more than 1/360 or 3/16" in 76".
Example based on application (perceived vibration): Floor joists should not deflect more than ½" per 180" (1/360) of span

  **Ryan's Rule of Floor Deflection**
1/360 minimum ratio should be used in closets and spare bedrooms. Deflection ratios around 1/480 have best cost –to- deflection perception value.

  **Ryan's Rule of Door and Window Header Deflection**
1/8" maximum total deflection regardless of size.

Interested readers – this is a multipart series
Engineers please excuse the simplification that this forum dictates.

Ryan B