[CEUS-earthquake-hazards] reply to Joe Tomasello; buildings codes and earthquake hazard
Goldfarb, Lawrence
GoldfarbLP at cdm.com
Fri Feb 15 10:54:11 MST 2008
I agree with Allen Jones. I recall the BSSC workshops that I attended
in the early 90's where we discussed similar topics that have a huge
impact on the codes and subsequently the costs associated with designing
and retrofitting structures. Further, I have had the experience of being
in several earthquakes on the west coast. There is no correct answer
here, but the information being presented (especially open discussions
on input used to develop the maps) provides pertinent information for
engineering practitioners such as myself to be able to weigh these
important points. Nobody is being judged here. I would like to see the
forum remain open so that all ideas and viewpoints can be heard.
Larry Goldfarb
-----Original Message-----
From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
[mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf
Of Allen Jones
Sent: Friday, February 15, 2008 12:14 PM
To: ceus-earthquake-hazards at geohazards.usgs.gov
Subject: Re: [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings
codes and earthquake hazard
Speaking for myself, I find these discussions very interesting and
informative. I find think this sort of intellectual debate healthy and
provides a perspective missing from publications. I encourage you to
keep the discussion public and appreciate everyone's efforts in taking
the time to respond.
Allen Jones
________________________
Allen Jones, PE, PhD
South Dakota State University
Department of Civil Engineering
CEH 148, Box 2219
Brookings, SD 57006
Direct: 605-688-6467
Fax: 605-688-6476
On Fri, 15 Feb 2008, Arthur D Frankel wrote:
> Zhenming,
>
> I am sure many people on the bulletin board are getting tired of
> these exchanges.
>
> I presented similar comparisons at the Applied Technology Council
> workshop in Memphis in March 2005. As I recall, you and Joe Tomasello
> were in attendance. So I think you have seen these comparisons.
>
> Of course, I don't see any contradiction in what I've said. I think
it
> is reasonable to compare code values with the expected ground motions
> from the next 1811-12 type earthquake and with ground motions
> estimated for the
> 1811-12 earthquakes using intensity observations.
>
> I think further discussion between us on these issues should be made
> off of the bulletin board.
>
> -Art
>
>
> Art Frankel
> U.S. Geological Survey
> MS 966, Box 25046
> DFC
> Denver, CO 80225
> phone: 303-273-8556
> fax: 303-273-8600
> email: afrankel at usgs.gov
>
>
>
> "Wang, Zhenming" <zmwang at email.uky.edu>
> 02/15/2008 08:35 AM
>
> To
> Arthur D Frankel <afrankel at usgs.gov>
> cc
> James Cobb <cobb at uky.edu>, "Keifer, John D" <kiefer at email.uky.edu>,
> "ceus-earthquake-hazards at geohazards.usgs.gov"
> <ceus-earthquake-hazards at geohazards.usgs.gov>
> Subject
> RE: [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings
codes
> and earthquake hazard
>
>
>
>
>
>
> Art,
>
> This is the first time we see these comparisons:
>
> ?When I talk to code committees and other groups, I compare the
> relative level of protection that designing to different probability
> levels of ground shaking will provide to buildings. This can be
> assessed by comparing the ground-motion values for the probability
> levels in the building codes to the median ground motions expected
> when the next 1811-12 type New Madrid earthquake occurs and by
> comparing code values to intensities observed in the Memphis area from
the 1811-12 earthquakes.?
>
> The selected design ground motion should be consistent with the
> scientific facts. However, these comparisons seem to be contradictory
> to your early
> statements:
> 1. ?In fact, we release seismic hazard curves (a range of ground
> motion, from 0.0 to 10g or larger) for a grid of sites across the
> nation, so that users can calculate the ground motions at any
> probability level they choose.? ?It should also be reiterated that the
> national seismic hazard maps are based on the average hazard curves
> from a variety of input models and attenuation relations; they are not
worst-case maps.?
> 2. ?It is not correct to compare the intensity observations from
> 1811-1812 with the probabilistic hazard maps that also include the
> hazard from earthquakes closer to St. Louis.?
>
> Thanks.
>
> Zhenming
>
> From: ceus-earthquake-hazards-bounces at geohazards.usgs.gov
> [mailto:ceus-earthquake-hazards-bounces at geohazards.usgs.gov] On Behalf
> Of Arthur D Frankel
> Sent: Thursday, February 14, 2008 3:35 PM
> To: ceus-earthquake-hazards at geohazards.usgs.gov
> Subject: [CEUS-earthquake-hazards] reply to Joe Tomasello; buildings
> codes and earthquake hazard
>
>
> Joe,
>
> The USGS policy is to support the process of the Building Seismic
> Safety Council (BSSC) establishing probability levels and design
> procedures for the national model building codes, such as the
> International Building Code. The BSSC membership consists of a large
> group of engineers and stakeholders. The BSSC is a council of the
> National Institute of Building Sciences. The code development process
> of the BSSC is funded by FEMA. The design procedures are published in
> the NEHRP Recommended Provisions for the Development of Seismic
> Regulations for New Buildings, which is written by the BSSC/NIBS and
published by FEMA.
>
> I think a key responsibility of the USGS is to provide the best
> scientific information to decision makers. Part of this scientific
> information is assessment of the ground motions from the 1811-12
> earthquakes and estimation of the ground motions for the next 1811-12
type earthquake.
>
> When I talk to code committees and other groups, I compare the
> relative level of protection that designing to different probability
> levels of ground shaking will provide to buildings. This can be
> assessed by comparing the ground-motion values for the probability
> levels in the building codes to the median ground motions expected
> when the next 1811-12 type New Madrid earthquake occurs and by
> comparing code values to intensities observed in the Memphis area from
the 1811-12 earthquakes.
>
> For example, the value of ground motions around 1 Hz with a 10%
> probability of exceedance in 50 years (10%/50) is substantially lower
> than the median 1 Hz ground motion expected for the next 1811-12 type
> earthquake. The new Memphis code adopted in 2006 uses the 10%/50
year
> ground motions from the 1996 vintage of the national maps (the 2002
> maps are higher). Here I am considering 1 Hz spectral accelerations
> (S.A.), which are used for the design of buildings with about 10
> stories. For a site in Memphis (35.15 N; 90.05 W), the 10%/50 value
> of 1 Hz spectral acceleration is 0.16g (from the 1996 maps and using
> an amplification factor of 2.4 for class D stiff-soil site relative to
> firm-rock site from the NEHRP amplification factors). This is much
> lower than the median 1 Hz S.A. of 0.36g expected in Memphis from a
> scenario earthquake with moment magnitude 7.7 located on the portion
> of the current New Madrid seismicity trend northwest of Memphis (using
> the stiff-soil amplification factor from the NEHRP factors). This
> calculation of the expected spectral acceleration is based on the
> average of the five attenuation relations used in the 2002 national
> maps. If the next large New Madrid earthquake was a moment magnitude
> 7.4, the calculated median 1 Hz S.A. at Memphis would be 0.29g for a
> stiff-soil site, still much higher than the 10%/50 value (0.16g) from
the 1996 maps.
>
> The 10%/50 values for 1 Hz S.A. from the 2002 hazard maps would still
> be significantly lower than the scenario ground motions. For 5 Hz
> S.A., the expected values of the median ground motions for a M7.7
> earthquake are more sensitive to assumptions on the nonlinearity and
> attenuation of sediments in the Mississippi Embayment.
>
> The International Building Code (IBC) uses spectral accelerations that
> are
> 2/3 times the values with a 2% probability of exceedance in 50 years
> for most of the nation (there are some areas where IBC uses the median
> deterministic ground motions, depending on the level of the
> probabilistic motions). For 1 Hz S.A., the 2006 IBC specifies a value
> of 0.42g for Memphis (stiff-soil site). This is similar to the median
> value of 0.36g expected for a M7.7 earthquake (see above).
>
> In my presentations, I also compare the code values to the ground
> motions estimated from intensity reports in the Memphis area from past
> earthquakes. Here I use peak ground accelerations (PGA) rather than
> spectral accelerations, because intensities are generally correlated
> in the literature with PGA?s or peak ground velocities.
>
> Intensities in the Memphis area during the 1811-12 earthquakes have
> been assigned as intensity VIII by Hough et al. (2000) and as
> intensity X by Johnston (1996), depending on their interpretation of
earthquake effects.
> Intensity VIII corresponds to a peak ground acceleration between about
> 0.34 and 0.65g, based on the work David Wald did for Shakemap. This
> range is consistent with the calculated median PGA at Memphis of 0.39g
> for a
> M7.7 earthquake determined from the average of 5 attenuation relations
> used in the 2002 hazard maps (using the NEHRP amplification factors).
> The calculated PGA for a M7.4 earthquake is 0.32g, close to the range
> of the PGA?s estimated for intensity VIII.
>
> The new Memphis code procedure of using the 10%/50 values from the
> 1996 maps results in a PGA of 0.23g (for a stiff soil site), which is
> substantially lower than the range of ground motions estimated from
> the intensities reported in Memphis during the 1811-12 earthquake
> sequence (0.34-0.65g for intensity VIII). A similar value of PGA
> (0.22g) is found by taking the 5 Hz S.A. with 10%/50 and dividing by
> 2.0, which is the factor relating PGA to 5 Hz S.A. derived for
> M7.4-7.7 earthquakes from the average of the five attenuation
relations.
>
> Using a PGA that is 2/3 times the PGA with 2% probability of
> exceedance in 50 years (2%/50), which corresponds to the procedure
> used for spectral accelerations in the 2006 International Building
> Code for the Memphis area, gives a PGA value of 0.50g for Memphis
> (stiff soil site), which is in the range of the values estimated from
> the 1811-12 intensities. This is similar to the PGA value of 0.47g
> derived from the 5 Hz S.A.in the IBC divided by a factor of 2.0 to
> convert to PGA. So, there is evidence from intensity data that the
> ground motions specified in the IBC have been experienced in Memphis
> during the 1811-12 earthquakes
>
> In summary, the 1 Hz spectral accelerations with a 10% probability of
> exceedance in 50 years, as used in the current Memphis code, are
> substantially lower than the median 1 Hz spectral accelerations
> expected for the next 1811-12 type earthquake. The 10%/50 value of PGA
> is probably lower than the ground shaking experienced in Memphis
> during the 1811-12 earthquakes, based on intensity data.
>
> The 1 Hz spectral accelerations specified in the International
> Building Code (2/3 times the motions with a 2% probability of
> exceedance in 50
> years) are similar to the median 1 Hz spectral accelerations
> expected for the next 1811-12 type earthquake. The PGA with 2/3 times
> the value with 2%/50 is probably comparable to the PGA experienced in
> Memphis from the 1811-12 earthquakes, based on intensity data.
>
> -Art
>
>
>
> Art Frankel
> U.S. Geological Survey
> MS 966, Box 25046
> DFC
> Denver, CO 80225
> phone: 303-273-8556
> fax: 303-273-8600
> email: afrankel at usgs.gov
>
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