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 Subject : Ground Acceleration for Seismic Lateral Earth Pressures.. 02/26/2021 03:39:05 PM 
Kelly Cabal
Posts: 21
Posting this question on behalf of a CalGeo member:

What is the current standard of practice in determining the ground acceleration to be used for calculating the additional seismic lateral earth pressure for a retaining wall. Is it Sds/2.5 or PGAm or something else? The site is located near Los Banos, CA.

For the same site what should be used to determine the seismic load for slope stability analysis.
 Subject : Re:Ground Acceleration for Seismic Lateral Earth Pressures.. 02/27/2021 03:03:39 PM 
Doug Schwarm
Posts: 2
It's hard to go wrong if you design out of the AASHTO LRFD Bridge Design Specifications. Appendix A11 has a pretty concise summary, and it incorporates Anderson's new thinking about general limit equilibrium in NCHRP Report 611. The horizontal seismic component is either the site-adjusted PGA or half the site-adjusted PGA, depending on whether you can tolerate 1-2 inches of permanent deflection. AASHTO designs bridges for the 1,000-year event (7% chance of exceedance in 75 years). It may be that you're working on a building and your building official will require you to us the 2% in 50 yr (2,500-year) event. The "Sds/2.5" approach assigns building ductility reductions to structures that typically are not ductile like a building. I've seen it used in Puget Sound by engineers trying to report low PGA's and hoping that the building official doesn't call them on it. Sds/2.5 is explicitly incorrect, plus you can get a lower PGA by following Caltrans (who use the BDS) and claim the shorter exceedance interval.
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