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Re: Foundation design for seismic

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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)
> 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.