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# RE: 1997 UBC Section 1630.10.3 Limitations on Story Drift

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: 1997 UBC Section 1630.10.3 Limitations on Story Drift
• From: "Briceno, Cairo" <Cairo.Briceno(--nospam--at)hdrinc.com>
• Date: Wed, 23 Feb 2005 13:54:39 -0600

```I give me my two cents on this.

When the UBC 1997 code was transitioning to be in compliance with the
NEHRP provisions a lot of engineers believe that the NHERP minimum base
shear(0.1Sd1*I*W=0.11Ca*I*W) was too low for Zone 4 projects. They
introduced a specific value for the minimum base shear formula 30-7. In
their opinion that should be absolute minimum for zone 4 in all
calculations.

Projects outside zone 4 shall have the minumum base on 30-6.

Zone 4 Drift calculation: In the spirit of the code they allow to
calculate the base shear base on true period of the structure but it has
to be larger than the minimum establish by 30-7.

Zone 3 drift calculation: The code writers eleminate the basic shear
minimum and allows to use of the base shear calculation base on the true
period of the structure. This was view as a break for zone 3 projects
due to near fault effects

The espirit of of the language of the 1994 UBC was : 1) Use the true
period of the structure. 2) eliminate the code base shear minimum and
allow the base shear calculated base on true period.

The two code does not have the same objectives. Plan checkers will stick
to the interpretation of the code. UBC 97 is more conservative than
UBC94 for zone 4 projects. For zone 3 projects you can raised the bar as
EOR and required a minimum base shear (formula 30-7)

Back to real life software for Zone 4 projects must considered formula
30-7 as base shear minimum in compliance with the code.

-----Original Message-----
From: Henry Gallart [mailto:hgallart(--nospam--at)sideplate.com]
Sent: Wednesday, February 23, 2005 9:23 AM
To: seaint(--nospam--at)seaint.org
Subject: 1997 UBC Section 1630.10.3 Limitations on Story Drift

It has come to my attention on a current project that we are looking at
for a client that the building's base shear could be affected by as much
as 26% depending on the correct interpretation of the code.  Since the
1997 commentary of the blue book did not specifically address this, I
thought I would ask my fellow structural engineering community.

Section 1630.10.3 for checking building story drifts specifically
excludes the minimum base shear of Formula (30-6), but does NOT
specifically exclude Formula (30-7) used for Seismic Zone 4.  Does
anybody know the background to this particular section to know if the
intent was to exclude or not exclude Formula (30-7)? A major lateral
designing computer software program has interpreted that Formula (30-7)
is also excluded when checking drift base shear. I want to be sure they
are right.  If not, then get it fixed.

Thanks again,

Henry A. Gallart, S.E.
Senior Vice President
SidePlate Systems, Inc.
23422 Mill Creek Drive, Ste 215
Laguna Hills, CA 92653

tel: 800-475-2077 (USA)

tel: 949-305-7889 (Outside USA)
fax: 949-305-6395
www.sideplate.com

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