It's amazing how such a simple issue, which was clarified earlier, has
turned into a huge discussion item. The drift is always a percentage of the
story height, hence unitless. There are no changes from 94 to 97 UBC in the
definition of building drift. The only change is the allowable limit, which
by first inspection seems excessive. But that is only because the calculated
deflection (at strength level) gets multiplied by 0.7R.
I sure hope that I am not missing something here, otherwise we have been
checking drift calculations incorrectly for almost 9 months now!
Ben Yousefi, SE
San Jose, CA
From: SEConsultant [SMTP:seconsultant(--nospam--at)earthlink.net]
Sent: Thursday, March 23, 2000 1:12 PM
Subject: RE: Quick/Easy Question for Seismic List
It's a shame that the evolution of the code equations have not been
documented so that these issues are simply not questionable. I
think this is a failing of the code writers (again).
There is a general opinion that empirical formulas often use
constants and that the final units are assumed. For example, the
of the diaphragm deflection formula (for blocked diaphragms)
chord deflection. We had a number of debates that this was an
formula and that it was assumed in inches to match the other three
components of the formula. However, one or two engineers responded
list and recreated the derivations of the formula from the basic
formula to the one used in the code. The constants assumed to be
were, in fact, conversion factors.
I am not convinced that at first glance we should make assumptions
the final units should be. It is the responsibility of the code
insure that the formulas and glossary of terms is written so that
engineer understand what units of measurement is expected as input.
way, the engineer knows whether or not he or she needs to convert
This one short thread has had very strong opinions from each side.
thought that it did not matter if the we were to consider the height
or inches, Shafat's opinion is that the formula is unit dependent
therefore it matters.
I'm still confused and think the answer should come from the "horses
Dennis S. Wish, PE
From: Paul Crocker [mailto:PaulC(--nospam--at)ckcps.com]
Sent: Thursday, March 23, 2000 10:44 AM
Subject: Re: Quick/Easy Question for Seismic List
Why do you assume that if you put feet into the equation that it
other than feet as its result? It is also acceptable to use meters
height. Surely this could not be so if the 0.02 was calibrated to
inches conversion. If the EERI web site still has the field report
earthquake in Columbia last year (?) posted, it might be worthwhile
as it briefly discussed observed correlations between drift and
which largely agreed with the pre-existing body of research.
anticipated loads and building use, getting too close to the 1.48'
be appropriate, but I do not see it as a code violation. Also, if
close to the drift limit, drift will exceed the threshold beyond
analysis cannot be ignored, which may cause problems of its own.
though, if a maximum inelastic drift limit approach is taken, a
for a 74' building is not surprising. Surely moment frames would
possible if a 1.48" level of stiffness was code required. In fact,
longest shear walls could possibly comply with that. Try to
74' building under extreme loading and imagine if that would be