# RE: Quick/Easy Question for Seismic List

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Quick/Easy Question for Seismic List
• From: "Yousefi, Ben" <Ben.Yousefi(--nospam--at)ci.sj.ca.us>
• Date: Thu, 23 Mar 2000 13:42:20 -0800
```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

-----Original Message-----
Sent:	Thursday, March 23, 2000 1:12 PM
To:	seaint(--nospam--at)seaint.org
Subject:	RE: Quick/Easy Question for Seismic List

Paul,
It's a shame that the evolution of the code equations have not been
documented so that these issues are simply not questionable. I
personally
think this is a failing of the code writers (again).

There is a general opinion that empirical formulas often use
arbitrary
constants and that the final units are assumed. For example, the
first term
of the diaphragm deflection formula (for blocked diaphragms)
represents the
chord deflection. We had a number of debates that this was an
empirical
formula and that it was assumed in inches to match the other three
components of the formula. However, one or two engineers responded
to the
list and recreated the derivations of the formula from the basic
deflection
formula to the one used in the code. The constants assumed to be
unitless
were, in fact, conversion factors.

I am not convinced that at first glance we should make assumptions
as what
the final units should be. It is the responsibility of the code
writers to
insure that the formulas and glossary of terms is written so that
the
engineer understand what units of measurement is expected as input.
This
way, the engineer knows whether or not he or she needs to convert
the
formula.

This one short thread has had very strong opinions from each side.
When we
thought that it did not matter if the we were to consider the height
in feet
or inches, Shafat's opinion is that the formula is unit dependent
and
therefore it matters.

I'm still confused and think the answer should come from the "horses
mouth".

Regards,
Dennis S. Wish, PE

-----Original Message-----
From: Paul Crocker [mailto:PaulC(--nospam--at)ckcps.com]
Sent: Thursday, March 23, 2000 10:44 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Quick/Easy Question for Seismic List

Why do you assume that if you put feet into the equation that it
yields
something
other than feet as its result?  It is also acceptable to use meters
for
story
height.  Surely this could not be so if the 0.02 was calibrated to
make a
feet to
inches conversion.  If the EERI web site still has the field report
from the
earthquake in Columbia last year (?) posted, it might be worthwhile
this,
as it briefly discussed observed correlations between drift and
damage/failure,
which largely agreed with the pre-existing body of research.
Depending on
your
anticipated loads and building use, getting too close to the 1.48'
drift may
not
be appropriate, but I do not see it as a code violation.  Also, if
you get
too
close to the drift limit, drift will exceed the threshold beyond
which
P-Delta
analysis cannot be ignored, which may cause problems of its own.
Overall,
though, if a maximum inelastic drift limit approach is taken, a
limit of
1.48'
for a 74' building is not surprising.  Surely moment frames would
never be
possible if a 1.48" level of stiffness was code required.  In fact,
only the
longest shear walls could possibly comply with that.  Try to
visualize 1.48"
in a