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RE: Overturning under seismic load

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Bottom line, we don't understand with any confidence what the "real" loads
are the foundation level.  Several codes have actually given reductions
beyond the "R" we now use.  I believe the older UBC's used a "j" factor to
reduce overturning loads.  Real world experience has proven than using the
"R" values at the foundation is conservative for design.  How many buildings
have you read about that fall over because of foundation stability?  The
only ones I know about have had liquefaction problems.  Get a copy of FEMA
310 and read the commentary on overturning stability.  It will both
enlighten and confuse you.

One comment in addition to Eli's,  engineers often forget that the
earthquake starts at the ground.  You can't get energy dissipation in the
frame above until the ground (and the foundations) move.  In my opinion, the
short duration of the load in a direction never lets the building move far
enough to "overturn" the building.  Remember they are impulse loads.  It may
start to lean one direction, then the ground moves the other way and brings
the building back.  So if you comply with "R" level loads and keep
deflections to a minimum, some movement will occur, but not enough for the
building to tip over.

Just a few rambling thoughts,

Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: Zhoub002 [mailto:zhoub002(--nospam--at)hawaii.rr.com]
Sent: Wednesday, August 25, 2004 2:38 PM
To: Sherman, William
Subject: Overturning under sesimic load



I don't understand why the building code like UBC 1997 allows reduced
seismic load (i.e. 1/Rw) to be used for evaluating overturning. "Rw" is
a coefficient associated with ductility, but I can't imagine a
"ductile" overturning.

Ben Z.




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