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Re: Foundation design for seismic

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I submitted an abstract to SEAOC that addressed two improvements to the
foundation situation.  1.  Cut a JOINT between sidewalk and building (it
should be a law) in high seismic risk zones.  2.  I submitted a design for a
very large knee braced frame for the first interior bays below grade.  Tom
Harris did the evaluations of the abstract and I have never heard the
results.

Greg
-----Original Message-----
From: Wladimir bassett A. <wbassett(--nospam--at)tmx.com.ni>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Wednesday, May 19, 1999 3:34 AM
Subject: Re: Foundation design for seismic


>T. Eric Gillham PE wrote:
>>
>> Here is a question that has been on my mind for quite some time.  I have
>> asked other engineers knowledgeable in seismic design, and gotten
differing
>> opinions.
>>
>> Here it is:
>>
>> Does the UBC have in it language that either explicitly or implicitly
>> requires the foundation of a structure (say a mat foundation for example)
be
>> designed for the CAPACITY of the superstructure?
>>
>> Looking at UBC94, I have found some sections, notably 1921.2.1.4 and
>> 1921.2.2.3, that together seem to indicate this is the case.
>>
>> For the record, I feel strongly that the foundation SHOULD be designed to
>> either yield the superstructure, or as an less favorable option be
designed
>> as a ductile foundation.  This only makes sense to me, as we are assuming
>> (usually) that the superstructure will be yielding during a major EQ, so
the
>> use of reduced seismic forces (Rw=12 for a SMRF) will NOT give the
expected
>> demands on the foundation.
>>
>> For example, a SMRF supported by a mat foundation.  It seems logical that
>> the punching shear capacity of the mat should exceed the maximum expected
>> axial load in any of the columns, NOT just the code level axial loads
>> obtained from an elastic analysis.  Otherwise, the mat foundation is in
>> danger of failing in punching shear before the superstructure does all of
>> its work, and the design assumptions which were made would likely become
>> much less appropriate.
>>
>> In the end, I really don't mind if the UBC doesn't explicitly require
this,
>> as I do it anyway.  But, in some cases, where I am for example checking
>> another company's design, having code support becomes much more
important.
>>
>> Anyone have any thoughts on this one?
>>
>> T. Eric Gillham PE
>> GK2 Inc.
>> PO Box 3207  Agana, Guam  96932
>> Email - gk2(--nospam--at)kuentos.guam.net
>> Ph:  (671) 477-9224
>> Fax: (671) 477-3456
>>
>
>I think that this is the realm of seismic design. In fact, the
>foundations are the gateaways for the seismic forces to enter into or
>act upon the building, so the design (foundation), no more no less
>should be in accordance with the philosophy being used for the
>super-structure design.
>
>                         W. Bassett A.
>