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Alex:

My point was that code level forces, such as .75(1.4D+1.7L+1.7E) are NOT the
axial loads that will be expected during the actual design earthquake, if
Rw=12 for calculating Vb.  The expected axial loads on exterior columns (not
interior, mind you) will be a function of the strength-deformation
characteristics of the structure, more specifically the maximum net shear
that the beams which frame into the column line can input.  Thus, 1.4D+1.7L
will, in all likelihood, NOT control.

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
-----Original Message-----
From: Alex C. Nacionales <alexcnac(--nospam--at)easycom.net>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Friday, May 21, 1999 3:19 AM
Subject: Re: Foundation design for seismic


>All forces (using all loading combinations) resisted by the superstructure
>should be transmitted to the foundation no more no less. Axial loads due to
>1.4DL+1.7LL forces are bigger than  .75(1.7LL+1.4DL+1.7E) so the large
>moments caused by the later combination will cause failure before any
>punching shear failure.
>Past earthquakes also has taught lessons that led to code revisions which
>are mostly
>in the columns and beams, which are the most affected by previous
>earthquakes except for some buildings that collapsed due to poor soil or
>liquefaction of soil.
>
>My humble opinion.
>
>Alex C. Nacionales, C.E.
>Iloilo City, Philippines
>
>
>----- Original Message -----
>From: T. Eric Gillham PE <gk2(--nospam--at)kuentos.guam.net>
>To: seaoc list <seaint(--nospam--at)seaint.org>
>Sent: Wednesday, May 19, 1999 5:28 AM
>Subject: Foundation design for seismic
>
>
>> 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
>>
>>
>>
>
>
>