From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Thu, 25 May 2000 11:05:31 -0400
Everything about seismic zoning is fictitious!
At first, when there was no Seismic Zone 4, and SEAOC proposals were only for
California, there was no Z factor. Because the UBC covered other states, it
included a Z factor with Seismic Zone 3 (the highest) being 1, compatible
with SEAOC's California recommendations.
When a Seismic Zone 4 was included, Z for that zone was 1; for Zone 3, Z =
3/4; for Zone 2, Z = 3/8; for Zone 1, Z = 3/16, and other factors adjusted
accordingly so that the answers came out the same, but the arithmetic became
Then, someone suggested that Z reflect the seismic zone so that it would be
easier to remember, so for Zone 4, Z = .4; Zone 3, Z = .3; Zone 2B, Z = .2;
Zone 2A, Z = .15; Zone 1, Z = .1, and all the other factors adjusted so that
the answers came out the same as previously.
Then, it was determined that Seismic Zones should begin where the USGS maps
showed those accelerations comparable to Z to begin, forgetting that the
USGS maps reflected accelerations of only 25 percent of what their research
showed would occur. In response to criticisms from SEAOC, USGS brought out
maps that showed the actual accelerations that would occur in California Zone
4 country were actually 3.9g, not .4g. The published reason that USGS gave
for dividing the actual accelerations by 4 was that California Engineers were
experienced in designing for earthquakes and used ductility to accommodate
the difference. (Politics also has an effect on the USGS maps, believe it or
Hope this helps.
A. Roger Turk, P.E.(Structural)
Jake Watson wrote:
>> What version of the UBC are you using? The "Z" is antiquated and
merely in the new codes for reference. Correct me if I am wrong (I am
way out on a limb in my knowledge) but in the 94 UBC things changed and
"Z" was no longer used in the base shear calc. The current ('97) base
shear calc is V=(Cv*I)/(R*T). If you divide V by "g" then you will have
the design acceleration.
Hope this helps
Jake Watson, E.I.T.
Salt Lake City, UT
> Wayne Stewart wrote:
> This is a questions regarding seismic design under the Uniform
> Building Code.
> It is my understanding that the magnitude of the seismic zone factor
> (Z) in the UBC is equal to a fraction of the acceleration due to
> gravity (9.81m/s^2). As a practicing Canadian engineer in Edmonton,
> Alberta, I am not really familiar with UBC and would appreciate
> feedback from someone who is familiar with this code.
> Please confirm that the relationship between design ground
> acceleration (in m/s^2) and "Z" is simply the design ground
> acceleration divided by 9.81m/s^2.
> Wayne Stewart, E.I.T.
> Walters Chambers and Associates Ltd.
> Structural Engineering Consultants
> Suite 1330, 10130 - 103 Street
> Edmonton, AB T5J 3N9<<