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Re: Seismic earth pressue force increment

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: Seismic earth pressue force increment
• From: Joe McCormick <jmccormick(--nospam--at)proaxis.com>
• Date: Thu, 02 Oct 1997 13:49:37 -0700

```At 07:36 AM 10/2/97 -0700, Lynn wrote:
>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.

I would design the walls for the earth pressure plus the seismic load using
0.4g for out-of-plane forces.
In my opinion the reduction of acceleration used to compute building base
shear is not applicable in this
case since the building is not experiencing the free-field acceleration at
its base like a normal concrete
building, but is rather undergoing complex "soil-structure interaction", and
respsonse is not only due to the
structure alone, but also from the soil.  Who knows if the lateral seismic
earth pressure effectively drops off when the concrete tank goes beyond it's
elastic range?  Isn't that the assumption we make when we design buildings
for a lower static equivilent load:  building goes beyond elastic, energy
disipates, damping increases, period shifts, and self-weight acceleration
forces diminish....all pretty
good assumptions for a structure being shaken from a fixed base undergoing
response from its own accleration and velocity, but not necessarily true
when the building is buried in the soil and the seismic loading is coming
from a mass that behaves in a notoriously non-linear fashion, and is
semi-infinite in size.  Just my opinion though, and I know there are MANY
others on this subject!

Joe McCormick

```