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Re: Seismic earth pressue force increment

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I would agree with Martin Johnson's approach for calculating the building
base shear,  except for the V-soil value.  I would not reduce the V-soil by
using the 0.4 seismic factor before reducing the base shear by Rw.  I would
instead include a 45 degree wedge of soil projected up from the base of the
wall as part of the mass of the building.  V= ZICW/Rw  except W= building
weight plus soil wedge weight, since you are looking for a soil/building
interaction.  In reality the angle of the soil wedge may be less, say between
30 - 45 degrees depending upon the soil, but it is probably best to be
conservative and use the 45 degree wedge. 

 I would think this approach is reasonable since the tank wall is designed to
transfers the shear from the acting soil pressure to the diaphragms at the
top and bottom of  the wall.   The diaphragm at the top of the wall then
transfers the shears to the cross walls.   I am assuming that any dynamic
soil pressure occuring below the 45 degree line measured upward from the base
of the wall does not load the wall or building.  If soil only occurs on three
sides of the buried tank, than I would also include the soil active pressure
in addition to the calculated seimic base shear V.

I would also use the geotects recommendations for the local design of the
basement wall in addition to any other acting loads (soil active pressure,
surcharge, etc.)

My two cents worth

Michael Cochran
Brian L. Cochran Associates
BCASE1356(--nospam--at)aol.com


<< Subj:	 Re: Seismic earth pressue force increment
 Date:	97-10-02 11:50:31 EDT
 From:	mwj(--nospam--at)EQE.COM (Martin Johnson)
 Reply-to:	seaoc(--nospam--at)seaoc.org
 To:	seaoc(--nospam--at)seaoc.org
 
 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)silcom.com>
 > 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.
 > 
  >>