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Re: re NEHRP Provisions - 1994, 1997

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Ron,

Thanks much!  I knew that I could probably find out about it in the
commentary but took the lazy approach by asking someone to summarize for
me!  Thanks for letting me be lazy!  ;-)

I did know about the 475 year return period (10% probability of exceedance
in 50 years) in the 91/94 as opposed to the approximately 2500 year return
period (2% probability of exceedance in 50 years).  What I was sure was if
the new method was somehow "calibrated" to produce the same "design" forces
or if the new provisions were actaully more severe.

Your response helped greatly.  Thanks again.

Scott

At 06:32 AM 01/13/2000 -0800, you wrote:
>
>
>
>ON 1/12/00 - Scott Maxwell wrote:
>
>
>*****************
>
>Can someone out there give a description of the changes in philosophy
>between the earthquake provisions in NEHRP 91/94 as opposed to NEHRP 97?
>
>*****************
>
>There is a very major change in philosophy of the older NEHRP Provisions
and the
>1997 edition.  This philosophy change is largely based on the adoption of new
>ground motion maps that were published by USGS following a multi-year
study of
>seismic hazards throughout the United States and US Territories.
>
>In early editions of the NEHRPs - the basic intent of design was to provide
>structures with a high degree of confidence that they would provide "life
>safety" for ground motions having a  mean return period of approximately 500
>years.   It was believed (based largely on California practice and
customs) that
>such structures would be able to survive any earthquake likely to effect them
>without collapse.  This is because in California, the maximum credible ground
>motions were believed to be only modestly larger than those with a 500
year mean
>return period.
>
>However, based on hazards studies conducted by USGS in the late 1980s and
early
>1990s, it became clear that in much of the US, particularly in the eastern
US,
>the levels of shaking that would occur with mean return periods of a few
>thousand years would be substantially more severe than those with a mean
return
>period of 500 years (2 to 3 times greater).  Given this, the BSSC committees
>realized that designing for "life safety" for 500 year motion would not
provide
>any significant protection against collapse for infrequent, but very large
>earthquake events, such as the 19811-1812 New Madrid earthquakes, in the
eastern
>US.
>
>Therefore, in the 1997 NEHRP Provisions the design philosophy was changed.
>Rather than designing for "life safety" for 500 year ground shaking, it was
>decided to design for Collapse Prevention (that is a state of non-collapse or
>near collapse) for the maximum level of ground shaking that could
reasonably be
>expected to occur at a site.  This level of ground shaking is termed the
MCE or
>(Maximum Considered Earthquake) shaking - not the maximum capable or
credible.
>This is because it is recognized that stronger earthquake shaking could
occur,
>it is just very unlikely that it would occur and that it does not make
sense to
>consider such stronger shaking in design.  In most of the US MCE ground
shaking
>is taken as that level of shaking with a mean expected return period of
>approximately 2,500 years.  In regions with major active faults, such as
the San
>Andreas in California, MCE shaking is taken as a conservative estimate
(150%) of
>the shaking that would be expected to occur at a site should the
characteristic
>earthquake the fault is capable of producing, occur.
>
>
>So - in the 1997 Provisions, ground motion maps provide MCE ground shaking
>levels.  This is a major change from past maps and will result in much more
>intense shaking levels for most sites.  The MCE shaking levels are than
reduced
>by a  factor of 1.5 (the margin presumed inherent in the R values) so that
the
>old values of R could be retained.
>
>Please note that a complete description of all of this is contained in teh
>commentary to the NEHRP Provisions, FEMA-304, which can be obtained either
from
>BSSC or FEMA.
>
>Ron Hamburger
>Chair,
>Building Seismic Safety Council
>Technical Subcommittee 2.
>
>
>
>