I am puzzled by the last paragraph of your email. I am not aware of the
maps or references you are referring to when you talk about the USGS map
values being divided by 4 or of the criticisms from SEAOC about map values
being too low. To my knowledge, no such references exist. Would you
please provide the references you are describing for the benefit of me and
other users of this site?
As far as the discussion of zone factors goes I refer readers to the USGS
web site at http://geohazards.cr.usgs.gov/eq Some of the points that have
been discussed are addressed, but perhaps not fully answered, in the FAQs
on the site. One useful report on the subject, OFR 95-596, "USGS Spectral
Response Maps and their Relationship with Seismic Design Forces in Building
Codes", may be downloaded from the site. This report traces maps and code
changes starting with the first USGS national probabilistic map in 1976,
with values that most certainly were not divided by 4, through 1994. More
recent maps and their relationship with IBC 2000 are discussed in the
latest edition of Spectra.
Our site also includes the probabilistic maps and the MCE maps (the latter
maps should be up by the weekend) plus a lot of other information. Check
E. V. Leyendecker
At 11:05 AM 5/25/00 -0400, you wrote:
>Everything about seismic zoning is fictitious!
>At first, when there was no Seismic Zone 4, and SEAOC proposals were only
>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
>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
>experienced in designing for earthquakes and used ductility to accommodate
>the difference. (Politics also has an effect on the USGS maps, believe it
>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<<
Edgar V. Leyendecker Telephone: 303-273-8565
U.S. Geological Survey Fax: 303-273-8600
Address for mail: Physical address:
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Denver, CO 80225-0046