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RE: Seismic earth pressue force increment
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaoc '" <seaoc(--nospam--at)seaoc.org>
- Subject: RE: Seismic earth pressue force increment
- From: "Sprague, Harold O." <SpragueHO(--nospam--at)bv.com>
- Date: Thu, 2 Oct 1997 13:34:00 -0500
Lynn, I know your problem. There is no agreement among soils engineers on how to calculate the seismic induced soil lateral load. I tried to make it happen in the last cycle of the BSSC, but was met with torches and pitchforks carried by the soils engineering community. There is the Mononobe-Okabe equation (used by AASHTO), but the accuracy is a matter of contention, and they result in rather large loads. Some soils engineers claim that there is no seismic induced soil lateral load. In your particular case, I do not understand how the weight of the building enters into the equation. I believe you need to use the 0.4 factor because the induced dynamic lateral force is from the surrounding soil and is not a function of the structural response. You really should get back with the soils engineer. Like I said earlier, there is no agreement in the geotech community. Every soils engineer draws their own conclusions and recommendations. Good luck, Harold Sprague Black & Veatch ---------- From: Lynn Howard To: seaoc(--nospam--at)seaoc.org Subject: Seismic earth pressue force increment Date: Thursday, October 02, 1997 9:36AM 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. Any input would be appreciated. Lynn
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