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RV: Foundation during Earthquake

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-----Mensaje original-----
De: raul labbe <rlabbe(--nospam--at)ctcinternet.cl>
Para: seain(--nospam--at)seaint.org <seain(--nospam--at)seaint.org>
Fecha: Lunes, 02 de Julio de 2001 04:47 p.m.
Asunto: Foundation during Earthquake

Fellow Engineers,
 
I require some comments from you on this issue :
Usually most of the seismic codes or guidelines, are
specifying a minimum compressed area under 
footings/foundations, when subjected to overturning
moments arising from the quake. This may be 75% to
80% of the foundation plan, pending on the Code.
( I don't remember exactly, I think AASHTO is asking for
80% for footings of bridge piers ).
Now, this requirement, is independent and additional to
the checking of safety against overturning, e.g. Mr/Mo > 1.5 or 2.0
where Mr = Resisting Moment, and Mo= Overturning M.
It happens, now and then in design, that this second requirement is
fully accomplished, but not the first one, e.g. having a soil compressed 
area less than 75%.
If this is the situation, what is the real effect upon the whole structure ?
Is this allowable ?
Thanks in advance.
 
Raúl 
S.S.Eng.