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Re: Northridge Earthquake a Major "Event"

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Lynn and Ben's arguments are valid for structures that remain essentially
elastic, rock about their base, or otherwise respond without significant
permanent degradation of strength and stiffness.

But most modern structures rely on ductility to some extent, so we can
expect permanent damage in design-level events. It takes time to degrade and
produce permanent damage/degradation in well-designed and built structures.
Longer duration earthquakes with accelerations comparable to Northridge can
generate higher damage rates than Northridge.

Lynn and Ben's comments reflect our profession's over-emphasis on elastic
design and forces in the design process. We are only implicitly addressing
other aspects of actual performance in most designs at this time.

I don't think Lynn and Ben's statements are entirely incorrect, but I
suggest you keep an open mind. Its our nature to judge acceptability based
principally on the most recent events and based on the measures we happen to
be most familiar with. New research and recent guidelines developments are
beginning to incorporate more comprehensive measures.

Fred Turner, Staff Structural Engineer, California Seismic Safety
Commission, 1755 Creekside Oaks Drive Suite 100, Sacramento, CA 95833
916-263-0582 Work Phone, 916-263-0594 Fax fturner(--nospam--at)

-----Original Message-----
From: Lynn <lhoward(--nospam--at)>
To: seaint(--nospam--at) <seaint(--nospam--at)>
Date: Monday, October 04, 1999 4:06 PM
Subject: Re: Northridge Earthquake a Major "Event"

>Thanks to Mr. Hamburger, Mr. Turner, Mr. Greenlaw and Mr. Valley for
>your responses to my posting.
>I basically agree with most of what was said by these people.  However,
>:)  my point I think is still valid.  And perhaps I made no attempt to
>make a point, other than a very broad one.
>Richter scale magnitude is a measure of the total energy released from
>an earthquake.  It has no direct correlation to ground motion.
>Obviously, a 9.0 earthquake will see more ground motion than a 1.0.
>However, the difference can not be as clear when discussing the
>difference in ground motion from a 6.7 to a 7.3 earthquake.
>The depth of the epicenter, the soils strata a building is sitting on,
>as well as MANY other factors are important in predicting the actual
>ground motion a building will see during a design level event.
>There were in fact buildings close to the epicenter that experienced
>STRONG ground motion that performed up to a level of what I would call
>life safety.  From my recent readings, it appears life safety has been
>defined differently by various groups.
>If I took that actual spectral curves from actual data during the
>Northridge earthquake and plugged them into the computer analysis
>instead of the UBC curves, guess what, virtually none of the buildings
>close to the epicenter that experienced strong ground motion would
>figure as being able to perform adequately during this event.  Some did
>indeed failed.  And there are very valuable lessons to be learned from
>that.  But the lessons that I think are being missed are the ones that
>tell us why some of these buildings that were subjected to very high
>ground accelerations were able to come through this event successfully
>when judged from a life safety perspective.
>What I was responding to was a comment that the Northridge Earthquake
>was not a big enough earthquake to really test these buildings.  My
>position is that there were many areas that did sustain very large
>ground accelerations, and if it wasn't a Code level earthquake, it was
>very close to one.  For areas that were subjected to these strong
>motions, you cannot dismiss the evidence that many buildings performed
>adequately due to the fact that the earthquake was only "moderate".
>Yes, we can all visualize and find earthquakes that have happened that
>are larger and produced higher ground motion with a longer duration.
>But do these very large events fall into the 10% probability of
>exceedence in 50 years.  For some areas they probably do, but you have
>to be very close to the San Andreas Fault.  I know that for our area
>(Santa Barbara) an 8.0 earthquake that last 2 minutes is considered to
>have a very low chance of occurrence.
>Again, thanks for all the great responses.