From: Allen Adams <aadams(--nospam--at)ramint.com>
Date: Mon, 05 Feb 2001 08:37:31 -0800
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 -----
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
Allen Adams, S.E.
>From: "Mike Valley" <mtv(--nospam--at)skilling.com>
>Subject: RE: Drift per 1997 UBC
>ICBO issued an erratum on January 18, 2001 (which has not yet been
>published) which changes this section to read as follows:
>"1630.10.3 Limitations. The design lateral forces used to determine
>the calculated drift may disregard the limitations of Formula (30-6)
>and (30-7) and may be based on the period determined from Formula
>(30-10) neglecting the 30 or 40 percent limitations of Section
>1630.2.2, Item 2."
>- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>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: Friday, February 02, 2001 12:50 PM
>Subject: Drift per 1997 UBC
>Does anyone know why the drift check requires you to use the minimum
>base shear for buildings in Zone 4? What I'm talking about is UBC
>1630.10.3 Limitations where it states that the limitations of equation
>(30-6) may be disregarded when checking drift. No mention is made of
>equation (30-7) which is the zone 4 minumum. There was no distinction
>in the 1994UBC based on zone.
>This can greatly impact design for buildings with periods greater than
>3 seconds in zone 4.
>Any comments are appreciated.
>Gerard Madden, P.E.
>Middlebrook + Louie, Structural Engineers
>71 Stevenson Street, Suite 2100
>San Francisco, CA 94105