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Re: 10% - 50 year motion

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On 1/17, Larry Oeth wrote:

>I'm working on a subcommittee of the SEA Oregon reviewing some topics in =
>the draft FEMA 356 - Seismic Rehabilitation of Buildings.  The question =
>arises as to how the level of risk was deamed appropriate for design =

>We all know the ground motion parameters (10%/50yr  &etc.), but WHY were =
?they chosen?  Is there an actuarial basis?  Cost benefit?  10 old guys =
>around a table?

Any thoughts or ideas out there?

The 10% - 50 year definition of design ground motion can be traced back to the
ATC3.06 report, published in the mid-1970s.  Prior to that time, the design
earthquake was taken as an event producing the most severe ground shaking "ever
anticipated" at a site, without specific definition as to how often this might
occur.  In the ATC3.06 project (forerunner to the current SEAOC Blue Book and
also the NEHRP Provisions) an attempt was made to determine the acceptable risk
of failure for new building design, by comparing risks associated with other
hazards, e.g. airplane crashes, fires, etc. Concurrently, the project decided
that on average, ground shaking with a 0.4g pga was representative of a
approximately a 500 year return period for seismically active regions of
California and that this appeared to be about the level of ground shaking that
then current codes were capable of providing protection for.  Also, it was
reasoned that the "average" building might have a life of 50 years, and a 10%
chance of experiencing a design event during the life of the building, might be
appropriate.  Thus, in the 1988 UBC and 1985 NEHRP Provisions, 10% - 50 year
ground shaking became annointed as the "design event."

At about the same time, work began on developing a series of seismic evaluation
tools - e.g. ATC-14, then ATC-22 and finally FEMA-178 (now FEMA 310).  These
evaluation procedures gravitated towards evaluating existing buildings for the
same hazard that new buildings are designed for (but accepting a somewhat higher
risk of failure - thus the 75% reduction in ground motion in ATC-14, ATC-22, etc
relative to then current codes).

FEMA -273 (the forerunner of FEMA 356) stayed with the same 500 year definition
because by the mid 1990s, when the ATC-33 project (development effort for
FEMA-273) was in progress, it was generally accepted that 500 years was the
magic number.  Interestingly, at about the same time this decision was made,
BSSC and USGS were concurrently developing the 1997 NEHRP Provisions (basis for
IBC, ASCE-7, NFPA-5000, etc) using new ground motion maps  In the 1997 NEHRP it
was decided that while 500 years might be an apporpriate return period for
design events in zones of high seismicity, it would not be adequate to capture a
repeat of major events (e.g. New Madrid, Charleston) that had occured in
historic times. Thus, it was decided to abandon 500 years, and instead, to base
design on a Maximum Considered Event (typically with a 2,500 year mean return
period), capped by a conservative estimate of the maximum level of shaking
likely to be produced by known active faults in the region.  ATC-33/FEAM-273
though aware of this, was quite advanced at the time, and stayed with 500 years
for Life Safety, while adopting the MCE for Collapse Prevention.

The commentaries to all of the above documents provide a good paper trail
indicating how all these decisions were made and rationalzied at their
respective times.