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Foundation design for seismic

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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