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RE: Drift per 1997 UBC

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This is getting really interesting. We recently had a project in San Jose
that the designer provided us a letter from ICBO, dated January 18, 2001 ,
stating that eqn. 30-7 should indeed be disregarded! ( the design firm,
incidentally, is where John Hooper works) I think ICBO should try to get
their house in order before any more conflicting opinions are issued in the

	-----Original Message-----
	From:	Allen Adams [SMTP:aadams(--nospam--at)]
	Sent:	Monday, February 05, 2001 8:38 AM
	To:	seaint(--nospam--at)
	Subject:	Drift per 1997 UBC

	Regarding ignoring the lower bound limit of Formula (30-7) when
calculating Drift in Zone 4, at one time ICBO took the position that Formula
(30-7), like Formula (30-6), could be ignored. At the time they said an
"Editorial Correction" would be published. However, ICBO has since changed
their position, before the correction was published. Based on that, the
original wording of UBC '97 is correct, and Formula (30-7) is not to be
ignored when calculating the forces to be used for drift. Here is the email
addressing the issue, received from ICBO on December 18, 2000:
		   ----- Original Message -----
	   From: <spandle(--nospam--at)>
	   Sent: Monday, December 18, 2000 3:02 PM
	   Subject: Story Drift Limitation from Gerald B. Neville

	    Dear Mr Hicks:
	    This is further "clarification" on the 1997 Uniform Building
Code     limitations on the calculation of story drift. . .Section 1630.10.
	       In a letter dated May 20, 1998, I indicated that both Formula
(30-6)     and (30-7) may be disregarded in the calculation of story drift.
Subsequent to that "editorial Correction", we had the Seismology
Committee of the Structural Engineers Association of California     review
the issue.  SEAOC expressed the opinion that only Formula (30-6)     may be
disregarded. . .as currently indicated in code Section 1630.10.3.      They
indicated that Formula (30-7) is a realistic lower limit that only
applies to tall structures in Seismic Zone 4. Considering the drift
sensitivity of the taller structures located in regions of very high     EQ
ground motion effects, drift calculations based a minimum design     lateral
force is appropriate.
	       It is noteworthy that both lower bound design lateral force
equations     apply for the drift calculations in the 2000 International
Building     Code...neither may be disregarded!
	       Gerald B. Neville
	    Senior Staff Engineer
	    Education Services, ICBO

	Since IBC 2000 also says that it is not to be ignored, I would be
surprised if ICBO changed their position again and said that for the UBC it
can be ignored. (Note that except for taller buildings it is not likely to
control anyway.)
	Allen Adams, S.E.
	RAM International