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Re: Question on "Seismic Zone Factor" (Z

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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 
more complicated.

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)
Tucson, Arizona

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.
> Thank-you.
> Wayne Stewart, E.I.T.
> Walters Chambers and Associates Ltd.
> Structural Engineering Consultants
> Suite 1330, 10130 - 103 Street
> Edmonton, AB  T5J 3N9<<