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
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".
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 yields
other than feet as its result? It is also acceptable to use meters for
height. Surely this could not be so if the 0.02 was calibrated to make a
inches conversion. If the EERI web site still has the field report from the
earthquake in Columbia last year (?) posted, it might be worthwhile to read
as it briefly discussed observed correlations between drift and
which largely agreed with the pre-existing body of research. Depending on
anticipated loads and building use, getting too close to the 1.48' drift may
be appropriate, but I do not see it as a code violation. Also, if you get
close to the drift limit, drift will exceed the threshold beyond which
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
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"
74' building under extreme loading and imagine if that would be possible.