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# Re: Seismic earth pressue force increment

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: Seismic earth pressue force increment
• From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
• Date: Thu, 2 Oct 1997 14:32:41 +0500

```>From Lynn Howard:

> 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
> = 1/2 * (soil unit weight) * (wall height) * (site acceleration).

Please note that the wall height should be squared in the above equation, i.e.
the seismic lateral soil force is calculated similar to the lateral force due
to the equivalent fluid pressure of soil; however, a lateral coefficient equal
to the ground acceleration is used in lieu of an active soil pressure
coefficient.

> My question is...Do we really have to use a .4g factor (the actual ground
> acceleration)...?

The formula described gives the estimated actual lateral soil force, similar
to ZICW (without the R-factor).  To account for ductility, I would recommend
applying an R-value of 4 to this force - based on UBC recommendations for
"distributed mass cantilever structures" (under Nonbuilding Structures) - as
applicable to a cantilever retaining wall.  (Note: ACI 350 is working on a
design code for liquid retaining structures and the draft document recommends
an R-value of 4.00 for buried fixed base concrete tanks.)

It should also be noted that a water-retaining structure will experience
dynamic fluid pressures as well - this is normally determined using "Housner's
method", involving "impulsive and convective" fluid pressures.

```