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Drift per 1997 UBC

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     From:     Gerard Madden [SMTP:GMadden(--nospam--at)]

     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

     This can greatly impact design for buildings with periods greater
than 3 seconds in zone 4.


From: "Mike Valley" <mtv(--nospam--at)>

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."

------------------------ My 2 cents worth;
The errata has issued by ICBO , apparently after ICBO reviewed their notes of
some agreements that were made during the code hearings.  Since I wasn't there I
cannot comment.

The 30-7 equation was originally derrived to match the old 3 percent minimum
lateral force requirement of California's Riley Act, which certainly has no
meaning outside California.

However, the current wording of the UBC does have logical reason for existence,
as follows:

First, comparing the 94 vs 97 UBC, in the 94 UBC the lateral force used when
checking for drift was divided by the period to the 2/3 power, whereas the 97
UBC simply divides by period.  Take a moment and make a simple calculation of
what this means to a building with a 3 second or longer period, and you will
realize that the 97 UBC has already given these buildings a big break in the
lateral force used to check drift requirements.  Despite the complaints of some
engineers that the code requirement was unfair, they were already getting a

Second, there has been some discussion, notably by Prof.s Heaton and Hall of Cal
Tech that ground motion in zone 4 areas may include lurching effects in the form
of a velocity pulse.  This would not be good news for long-period structures
like high-rises.  Including the 30-7 limitation provides some slight reserve for
these structures.

Third, the UBC equations used to check for drift are based on an estimation of
the response of the structures first mode period only.  High-rise structures in
the 3 second range are certainly strongly affected by higher modes.

 Real earthquakes have an upper limit on the peak ground displacement, which
causes the spectrum to drop off in the long period range, rather than leveling
out as suggested by Eqn 30-7.  However, the location where this drop off occurs
is depended on event magnitude, fault distance and ground motion characteristics
(such as lurching).

Structures that are affected by the long-period design requirements of the UBC
include base isolated structures and some types of flexible non-building
structures (such as tanks with sloshing modes).  I have more sympathy for
engineers designing these structures who complain that the UBC is perhaps too
conservative for their conditions.