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Re: Foundation design for seismic

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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
>
>
>