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Re: Seismic earth pressue force increment
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- Subject: Re: Seismic earth pressue force increment
- From: "Martin Johnson" <mwj(--nospam--at)EQE.COM>
- Date: Thu, 2 Oct 1997 08:48:14 PST
- Priority: normal
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. > Martin Johnson EQE International, Inc. mwj(--nospam--at)eqe.com
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