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# RE: Seismic earth pressue force increment

• To: "'seaoc '" <seaoc(--nospam--at)seaoc.org>
• Subject: RE: Seismic earth pressue force increment
• From: "Sprague, Harold O." <SpragueHO(--nospam--at)bv.com>
• Date: Thu, 2 Oct 1997 13:34:00 -0500

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Lynn,

I know your problem.  There is no agreement among soils engineers on how
to calculate the seismic induced soil lateral load.  I tried to make it
happen in the last cycle of the BSSC, but was met with torches and
pitchforks carried by the soils engineering community.

There is the Mononobe-Okabe equation (used by AASHTO), but the accuracy
is a matter of contention, and they result in rather large loads.  Some
soils engineers claim that there is no seismic induced soil lateral

In your particular case, I do not understand how the weight of the
building enters into the equation.  I believe you need to use the 0.4
factor because the induced dynamic lateral force is from the surrounding
soil and is not a function of the structural response.  You really
should get back with the soils engineer.  Like I said earlier, there is
no agreement in the geotech community.  Every soils engineer draws their
own conclusions and recommendations.

Good luck,
Harold Sprague
Black & Veatch
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From: Lynn Howard
To: seaoc(--nospam--at)seaoc.org
Subject: Seismic earth pressue force increment
Date: Thursday, October 02, 1997 9:36AM

We are designing a partially buried concrete box !water reservoir), and

the Soils Engineer has supplied us with the following design criteria
for the seismic earth pressure forces.

The dynamic lateral force increase due to earthquake shaking can be
estimated for drained, level backfill conditions using the following
equation:
dynamic lateral force increment, pounds/ft = 1/2 * (soil unit weight) *
(wall height) * (site acceleration).

He continues by adding the following statement after the above equation:

For purposes of estimating the resultant lateral force increment, we
recommend assuming a ground acceleration of .4g and a soil weight of 125
pcf.  Assuming a 10 ft. high wall, the dynamic lateral force increment
is estimated to be 2500 pounds per foot of wall.  The resultant can be
assumed to act at a distance of .6 of the wall height up from the bottom
of the wall and the resulting dynamic lateral earth pressure
distribution can be assumed to be an inverted triangle ( 0 pressure at
bottom, maximum pressure at top).

My question is, this suggest that we take the weight of the building
times .183 (concrete shear wall structure), plus .4 times the weight of
the soil in the profile distribution he describes.  The site is in
seismic zone 4, so a site acceleration of .4 is assumed.  However, going
through the formulas to come up with our base design shear, we only
design for .183g's of the building weight.  Should the same concept
apply to the weight of the soil?  Do we really have to use a .4g factor
(the actual ground acceleration), or when we design our shear walls and
structure for shear and overturning forces, due we use a .183 factor.

Any input would be appreciated.

Lynn

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