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RE: Retsining Wall Seismic

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The force doesn't sound out of line from what I've seen in references.
Formulas for the total force for unyielding walls in Kramer's "Geotechnical
Earthquake Engineering" are given as the unit weight of the soil times the
seismic factor times HxH times a coefficient which is upwards of 1.1. If
your seismic factor was .5g and you were using 120 pcf for the soil weight,
that would give you a total force of 66H^2 lb per foot of wall, or an
average incremental seismic wall pressure (psf) of 66H. The resultant is
usually more than halfway up the wall as well, upwards of 0.6H. These
numbers were based on experiments with confined soil in boxes on a shake
table performed back in the 1970's.

Up here in the (beautiful Pacific) Northwest, I have seen recommendations
from geotechnical engineers on the order of 25H for the wall pressure on
yielding walls. Whether you should use really use these numbers for design
(and how you use them) is another question. I think some structural
engineers regard this all as bogus, but I don't see how you can ignore it. 

I question whether these numbers should be used for stability calculations
on cantilever walls. The wall isn't going anywhere unless the soil fails,
and I think reaching that conclusion is more complex than meets the eye.
Seismic loads are dynamic, not static phenomenon, and what ends up happening
depends on duration, period, and a host of other issues. Soil is not exactly
the homogeneous, elastic material analysts love, and for short periods of
time, we have all seen vertical cuts in soil that ought to fail but manage
to stand for a while.

This is a subject some of the folks on this list seeking thesis topics might
want to explore. There's not a lot of guidance out there.

-----Original Message-----
From: lrhauer(--nospam--at)earthlink.net [mailto:lrhauer(--nospam--at)earthlink.net]
Sent: Wednesday, January 29, 2003 7:38 AM
To: seaint(--nospam--at)seaint.org
Subject: Retsining Wall Seismic


I assume all engineers in California are now designing to the 2001 CBC since
that code was adopted November 1st by most, if not all, jurisdictions. Has
anyone complied with sec. 1611A.6 and 1630A1.1, paragraph 5 re. earthquake
forces on retaining walls??

I am presently designing a building with 12.5 feet of retained earth on 3
sides. The soils engineer has given me seismic horizontal earth forces of
69(HxH) #/ft for restrained walls and 27(HxH) #/ft for cantilevered walls,
in addition to the EFP. These formulas produce forces on the walls, and
building, which are impractical, (impossible?), to design to. Any comments?

Larry Hauer, S.E.


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