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re NEHRP Provisions - 1994, 1997

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ON 1/12/00 - Scott Maxwell wrote:


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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?

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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.