[CEUS-earthquake-hazards] Revised seismic design criteria forNew Madrid

[Jay Crandell]

Nicolas,
 
Thanks for sharing what's going on with the USGS maps.  I'm a little
confused.  Sounds like the "real risk" is lower than previously expected so
some other changes are being made to prop the map's design ground motions
back up to a level that "feels good".  For example, what is the justfication
for changing the hazard basis of the map from 2% in 50-years to 1% in 50
years.  Why not 5 to 10% in 50-years instead?  We have a good calibration in
California for what has worked in the past with reasonable success.  Why are
we not using this as a hard calibration point for risk and then sticking
with a risk consistent application of that calibration across the country?
It still seems like the map is wondering around in the abyss of relativism
and politics and someone needs to calibrate it to an "absolute" condition
that has worked by experience.  Is my description fair?
 
I also have a question regarding directionality of ground motion.  If the
worst case horizontal ground motion will be used for the design map, then
how will the probability of that ground motion being aligned with the
worst-cast orientation of any given building be handled?  It would seem that
if the maps change in this regard, then a probability adjustment factor of
less than one is needed to avoid a systematic conservative bias in the
design provisions.  This same concern is addressed by a wind load
directionality factor of 0.85.  But, maybe the bias is intended for seismic?
 
Thanks,
 
Jay
 
ARES Consulting
5095 Sudley Rd
West River, MD 20778
410-867-9617
410-867-9618 fax
 

  _____  

From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf Of
Nicolas Luco
Sent: Thursday, January 22, 2009 6:59 PM
To: Central and Eastern U.S. Earthquake Hazards Listserve
Subject: Re: [CEUS-earthquake-hazards] Revised seismic design criteria
forNew Madrid



Another clarification:  In addition to the risk coefficients mentioned
below, there are "maximum direction" amplification factors of 1.1 and 1.3
for, respectively, 0.2- and 1.0-second spectral acceleration that are
applied to the 2008 USGS hazard maps in deriving updated design maps for
building codes.  Furthermore, the 2008 hazard maps themselves are different
than the 2002 (and 1996) hazard maps, as are the corresponding deterministic
"caps" that are applied to the respective hazard maps.  Thus, the updated
design maps based on the 2008 hazard maps are not simply lower than previous
design maps by a factor of 0.7 near New Madrid and Charleston.  In fact, in
some cases the new design map values may be about the same as the previous
ones. 

Again, please let me know if you have any questions/comments 

----------------------------------------------------------------------------
Nicolas Luco
Research Structural Engineer
U.S. Geological Survey

Mailing Address:
PO Box 25046, MS 966
Denver, CO 80225

Physical/Overnight Address:
1711 Illinois Street, Room 426
Golden, CO 80401

Phone: (303)273-8683
Fax: (303)273-8600
----------------------------------------------------------------------------





Seth Stein <seth at earth.northwestern.edu> 
Sent by: ceus-earthquake-hazards-bounces at geohazards.usgs.gov 


01/14/2009 12:13 PM 


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Subject
[CEUS-earthquake-hazards] Revised seismic design criteria for New
Madrid

	




Readers may be interested in the latest proposal for seismic building
design criteria. In it "the 2008 USGS hazard maps should not be
substituted for the model building code design maps nor should they be
used with ASCE/SEI 41 or 31 for seismic rehabilitation or evaluation."

Specifically, there are a set of scale (risk) factors to apply to the
2008 USGS hazard maps to produce design maps: "Resulting risk
coefficients are generally 0.85-1.15, but as low as 0.7 near New Madrid
and Charleston".

The USGS disclaimer

http://earthquake.usgs.gov/research/hazmaps/products_data/2008/disclaimer.ph
p 


gives  information about how the hazard maps are modified to
produce design maps. For additional information, see

http://bssconline.org/2008AnnualMeeting/080910--Luco1@BSSC_Annual_Mtg(Posted
).pdf
http://bssconline.org/2008AnnualMeeting/08MeetingRecap.html
-- 
Seth Stein
William Deering Professor
Department of Earth and Planetary Sciences
1850 Campus Drive
Northwestern University, Evanston, IL 60208
(847) 491-5265 FAX: (847) 491-8060 E-MAIL: seth at earth.northwestern.edu
http://www.earth.northwestern.edu/people/seth

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