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RE: IBC 2000 Seismic maps

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That is a good point.  And you're right, the word that I should have used is
"precise", not "accurate".  It is much more precise to look up based on
Latitude/Longitude, but zip codes are much easier to get.

When you enter a zip-code, the program knows the lat/long of the geographic
center of that zip code.  It then looks up the "S" values for that lat/long.
This is why both the web site and the CD-ROM that comes with the 2000 IBC
state that if your structure is in an area where the seismic hazards vary
greatly you need to use the lat/long lookup, not the zip.

If your structure is on the border between two adjacent zip's, one option is
to look up both zip's and average the two.

Most of my work is in Kansas City where the variation is minimum.  All of
the zips downtown return a S1 value of 6.0.  The Ss values vary between 12.1
and 12.2 (<1% deviation).  Therefore, I could easily just use standard
values of 6.0 and 12.2, and never look up KC again.  However, I like to have
a printout with the project name, location, S values, soil factors, and
design spectra for each project to put in with my calc's.  So I spend the 10
minutes it takes to look up the zip codes, enter the information, and print
the sheet.

On a side note, the 12.2 value is actually higher than what most people
would get from interpolating on the large maps.  Downtown KC is located
between the 12.1% cross (Pleasant Hill, MO - location of a US weather radar
site) and the 10% contour line.  Who would think that the value in between
12.1 and 10.0 is 12.2?  And yes I realize that it's only a variation of
0.001% g in an area where the seismic loads are comparable to wind.

Jason W. Kilgore, P.E.
Leigh & O'Kane, L.L.C.
(816) 444-3144

-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)]
Sent: Saturday, March 16, 2002 4:17 PM
To: seaint(--nospam--at)
Subject: RE: IBC 2000 Seismic maps

Jason Kilgore wrote:

. > For more accurate Ss and S1 values, click on "Hazard by Zip Code" or
. > "Hazard by Lat/Lon". Then enter the USPS zip code or Latitude/Longitude
. > and the page will return values that are more accurate than reading of a
. > large-scale map.

And how accurate is the value you get when one building has one value, and
another building on an adjacent lot has another value because they are in
different ZIP code areas?

The answers that you get with such precision are wholly inaccurate.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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