I don't think you can safely ignore seismic earth pressure for a retaining
wall, although I have met designers who don't think it's an issue. The
Mononobe-Okabe formula, which is the one in the AASHTO standard bridge spec,
can be used for retaining walls subject to active earth pressure which are
free to slide a little at the base, but I think you need to modify it a
little if you have a rigid wall, as is probably the case if your wall is
doubling as a building foundation wall. I don't think it is conservative for
at-rest loads. Bowles' comments in his Foundation Design textbook seem to
regard the entire subject of predicting seismic earth pressure as a bit like
voodoo, and not worth investing too much confidence in.
There is a pretty good discussion of all this in Steve Kramer's
"Geotechnical Earthquake Engineering", Prentice-Hall, 1996. His methodology
for seismic pressure on rigid walls is based on research by Wood in 1973,
and there doesn't seem to be much else out there you can hang your hat on
for easy to use, day-to-day design. For deep walls, the seismic soil
pressures predicted by these equations can get pretty hefty.
Some papers I have read about seismic loads on embedded structures suggest
that sometimes the soil damps the seismic forces, and other times it makes
things worse, depending on interaction effects with the structure. Unless
you want to get into elaborate finite element modeling, it's kind of a
crapshoot what to assume for design.
From: J. Karim Hosseinzadeh [mailto:jkh(--nospam--at)jamkar.com]
Sent: Monday, May 01, 2000 1:09 PM
Subject: Re: Retaining Wall Loading
I know this answer is late, but I now have a similar design situation and I
am pondering against my past experience, whether I should combine earth
pressure with seismic. UBC 97 Code sections 1612.2.2 & 1612.3.3 don't say
that H "should be" combined with seismic, what it says is, "where H is to be
considered" it should be added to the basic load combination. Now the
question is "where" do we consider H. I have always been taught to ignore
seismic forces acting on the portion of the building which lies underground
such as a basement. The reasoning is that the inertia forces of the
earthquake and the ground movement are in opposite direction. Therefore the
retaining wall either moves away from the retained earth or pushes against
it. At either case the load acting behind the wall becomes less severe than
the original earth pressure.
I would be interested to know if there are any hard rules about this
J. Karim Hosseinzadeh
JAMKAR ENGINEERING, INC. Tel. 949 250 0522
Newport Beach, California Fax 949 250 1527
----- Original Message -----
Sent: Wednesday, April 19, 2000 5:03 PM
Subject: Retaining Wall Loading
> Are seismic loads applied to a retaining wall in combination with the H
> UBC97 section 1612.2.2 & 1612.3.3 indicate that the H load should be
> with seismic. ASCE 7-95 does not give load combination that include E & H.
> Could anybody shed light on this? Thanks a lot!
> Rainier C. Catubig, S.E.
> San Diego, California