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Re: Seismic earth pressue force increment

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Responding to the below:  

What he suggests is not unreasonable, but the intent is more for the 
local out-of-plane design of the wall, which you are designing (based 
on the new 97 UBC strength provisions) for around 0.8 x Wt (or so) 
anyway.  For the overall base shear, the geotech probably 
conservatively says to use the same thing, but for your tank, this 
may become a very large part of the overall base shear.  As long as 
the entire force will be resisted by the structure, it seems like a 
reasonable approximation to use E = (V-struct + V-soil)/Rw, where 
V-struct is the so-called unreduced force = ZICW, for the overall 
structure analysis, while still designing the local wall using the 
geotech's recommendation.-----------MWJ

> From:          Lynn Howard <lhoward(--nospam--at)>
> Subject:       Seismic earth pressue force increment

> 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.
Martin Johnson
EQE International, Inc.