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FW: omega at foundations

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I would agree.
 
I don't quite understand the rationale behind the demarkation of "foundation system" and "superstructure" when it comes to seismic design, although I admit that I may be in the minority.  It seems that many engineers design the superstructure of a building based on a ductile mechanism, assuming that certain members are going to go inelastic and detailing them accordingly, then checking that the strength of the elastic members (or yield mechanisms) exceed that required to yield the ductile members.
 
BUT, when they get to the foundation that all goes out the window.  I believe it was quite common (based on older codes) for designers to use the reduced seismic loads for the foundation design straight out of the code, instead of 1) making the foundation capable of sustained yielding through the use of ductile detailing or 2) making sure that the foundation is capable of resisting the actions associated with the yield mechanism of  the superstructure above.
 
I've spoken with a couple of engineers whom I hold great respect for wrt seismic design, and the answer I have gotten is that the foundation has inherent overstrength due to the interaction with the supporting soil, and that it is just too complicated to properly account for this, and that the proof of this is that there are few examples of failed foundations (how one is to see such a failure is beyond my ken).  I still hold that a rational analysis, even based on fuzzy assumptions, is superior to just hoping that it will be OK
 
So, I would say that the ENTIRE foundation should be included in the seismic design program, by either ensuring ductile capacity or basing the design actions on the superstructure mechanism.
 
My $.02
 
T. Eric Gillham PE
-----Original Message-----
From: Mark Pemberton [mailto:Markp(--nospam--at)lbdg.com]
Sent: Wednesday, May 30, 2001 12:40 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: omega at foundations

But what about the shear and moment within the footing for
a continuous grade beam?
 
Mark Pemberton, P.E.
-----Original Message-----
From: HAWNENG(--nospam--at)aol.com [mailto:HAWNENG(--nospam--at)aol.com]
Sent: Friday, May 25, 2001 8:10 PM
To: Seaint(--nospam--at)seaint.org
Subject: Re: omega at foundations

Mark,

Our company does a lot of engineering for schools and hospitals.  We are on a
first name basis with most of the folks at DSA in Sacramento.  We factor the
loads by Omega when designing the anchorage from the column or brace to the
foundation, but not for the overall stability of the footing or to check the
soil pressure.  We only engineer the anchor bolts, base plate and welds for
the omega force level.  

If  the folks at DSA  in Sacramento are reading this, I hope I am not "giving
away" any secrets, as I have a back check appointment up there next week.

Greg Showerman, S.E.
Modesto, CA