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Re: Need feedback from those using strict interpretation for Woodstructures

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Lynn Howard,

In the subject email message, "Need feedback from those using strict 
interpretations for Woodstructures, you stated: "We are supposed to be 
designing for the earthquake that has a 10% chance of reoccurring every 50 

According to the 1997 UBC, Division IV-Earthquake Design, the correct 
definition of the "Design Basis Ground Motion is that ground motion that has 
a 10% chance of being exceeded in 50 years as determined by a site-specific 
hazard analysis or may be determined from a hazard map."   This Design Basis 
Ground Motion has a MEAN RETURN PERIOD of 475 year with a mean annual 
probability of exceedance of .002 or .2%.  There is nothing in this 
definition that addresses the question of the recurrence time for the design 
basis ground motion.

Earthquakes are defined by their probability of being exceeded in a certain 
number  of years or their MEAN RETURN PERIOD, not their chance of recurrence 
in so many years or their recurrence period like the time for the return of 
Haley's comet.

Frank E. McClure      FEMCCLURE(--nospam--at)    July 31, 1999.