I'm not familiar with the BOCA, but in the '97 UBC the limits are 0.025 or
0.02 (depending of period) times the story height (1630.10.2). Thereby, if
your story height is in inches, your limit is in inches, if you use a story
height in feet, you get feet as your limit:
0.02(74') = 1.48'
0.02(888") = 17.76"
And of course, 17.76" = 1.48'. This is just a limit of 2%, units don't
matter as long as you stay consistent. That is why there is no metric
specific reference for that equation, as would be required if the 2% was a
feet to inches specific equation. 1.48" would be awfully small for a 74'
tall building. The 2% value refers to a drift level arrived at via research
as a reasonable level for a building to sustain. Buildings that reach 3% or
4% tend to do poorly, but of course, these levels are vast generalizations.
"Ritter, Mike" wrote:
> Please help me with a quick question. I am looking at some calcs
> prepared by another engineer, who calculated the allowable story drift
> for a three story building from BOCA and ran the number based on story
> height in inches. In other words, the building height is 74', but he
> used 888 inches. This gives the allowable story drift to be
> I've always used the story height in feet, which would give an allowable
> drift of about 1.48". I was going to show my colleage the code section
> which says story height is in feet, but could not find it in BOCA, UBC,
> SBC, Farzad Naiem, or any other text books. Does anybody have a
> reference section showing that the story height, by definition, should
> be in feet? I know it must be there, but I'm in a rush and can't find
> Thanks again for the help!
> Michael Ritter, PE