In my opinion, this issue reflects a disturbing trend in modern
code-writing. Items are changed without being carefully considered
and coordinated; by the time some errors are noticed, other people
(not the original proponent) have "rationalized", reinterpreted, and
accepted the error. The 10/Lw factor (in the calculation of the rho
factor) is a good example of this flawed process.
In the present case, the error is more easily and obviously proven.
The code drafts that were voted on and approved included both
equations. During subsequent editting, typing, and equation
renumbering ICBO inadvertently dropped the second equation (now 30-7).
Therefore, the building code (as legally binding, but not as
published) has always excluded Equation 30-7 from drift calculations;
ICBO is simply correcting the printed copy. If there is now a concern
that drift calculations in near-source areas should be modified to
account for a "fling" effect, such a significant technical change must
be proposed, considered, and either accepted or rejected.
Equation 30-6 (and NOT Equation 30-7) "was originally derived to match
the old 3 percent minimum lateral force requirement of California's
Riley Act"; it's easy to confirm the math. When the implied ASD basis
of the old Riley act is taken into consideration, Equation 30-7
requires between 2.7% and 21% base shear; this is quite a departure
from a straight 3%. I agree with Martin that "California's Riley Act
... certainly has no meaning outside California." I would go so far
as to say that the Riley Act, which was a political reaction to the
1933 Long Beach earthquake, probably doesn't have a place in modern
earthquake engineering. Drift limits may be a good idea, but more and
more complicated formulations which are calibrated to a political
decision made 68 years ago seem foolish to me. Surely we've learned
something in the intervening years. Let's decide what we've learned;
carefully consider, craft, and coordinate modern provisions; and thus
improve the quality, coherence, and useability of our codes.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
From: Gerard Madden [mailto:GMadden(--nospam--at)mplusl.com]
Sent: Monday, February 05, 2001 10:44 AM
Subject: RE: Drift per 1997 UBC
Fair enough Ben, thanks for clearing up Mr. Hoopers role in this
I guess the question is :
Did the limitation on Eq. 30-7 intentionally get removed because of
the zone 4 pulse wave considerations Martin & Mark mentioned, or was
it an oversight? Also, can we expect the IBC to follow suit?
I couldn't find anything on this topic in the 1999 Bluebook.