# Question on "Seismic Zone Factor" (Z) in UBC

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: Question on "Seismic Zone Factor" (Z) in UBC
• From: Ed Marshall <ed.marshall(--nospam--at)agra.com>
• Date: Thu, 25 May 2000 09:54:38 -0700
Title: Question on "Seismic Zone Factor" (Z) in UBC

By comparing 1997 UBC Figure 16-2 to Map 9-2 of ASCE 7-95 (or 1994 NEHRP maps):  Generally Zone 0 is Av < 0.05, The zone 0 to 1 border follows the Av = 0.10 contour, zone 2A corresponds to Av > 0.1 but less than 0.20,  zone 3 is GENERALLY 0.20 to 0.040, zone 4 is GENERALLY greater than 0.40.  The match up is pretty good in the Eastern US while there is substantial differences in the western US.  Of course Av is NOT ground acceleration but the effective peak velocity-related acceleration coefficient.  See the commentary to the 1994 NEHRP Recommended Provisions (FEMA 223A).

Ed Marshall, PE

-----Original Message-----
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Sent:   Thursday, May 25, 2000 11:30 AM
To:     seaint(--nospam--at)seaint.org
Subject:        RE: Question on "Seismic Zone Factor" (Z) in UBC

Bill, Todd, and Jake:

Thanks for responding to my cry for help.  I too think that Z (seismic zone
factor) itself may not exactly be the ground acceleration divided by g
(9.81m/s^2).  Although the code seems to read this way, it does not
explicitly say this.

Regardless of the true definition or meaning of Z, my real problem is how to
properly do a seismic design when all the information I have about the site
is a design ground acceleration.  The UBC uses "zones" which must have
definable ground acceration limits.  What I really want to find out is what
these accel limits are for the various zones (1, 2A, 2B, 3, and 4).  Once I
can place the design ground accel into a zone then the rest falls into
place.  It's this first step that I have no knowledge of.