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Re: Seismic mass

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This is a prime candidate for a two stage analysis.  The upper structure would be analyzed separately as sitting on a rigid base, then the base analyzed as a separate structure with an applied mass. 
I would not distribute any of the base weight into the flexible upper structure.
The base structure is interesting, in that it is essentially the ground (only six feet high and a solid block).  Seismic response would be attenuated by the soil fill, and the actual accelerations will be almost indistinguishable from the ground, like a bridge abutment.
Current theories of seismic soil contribution to retaining wall load are mostly predicated on cantilevered walls with a soil failure plane being able to develop.  Seismic soil loading on a restrained wall is not properly quantifiable.
I doubt a seismic design is even necessary for the base structure, and you could include soil or not, it will not make an appreciable difference.
I won't comment on the wall design.
----- Original Message -----
Sent: Tuesday, February 07, 2006 10:55 AM
Subject: Re: Seismic mass

Paul and others:
Another twist to that question..........I have been asked to design a retaining wall for a house that is in the FEMA flood plane of Riverside County.  The county has a spec sheet they give out.
6'-0" retained height,    8" cmu  #4's @ 16oc at center of wall,  footing 12" wide 36" deep
4" slab on grade at top of wall,  house sits on top of this.
EFP 30psf;  1500psf soil Bearing;  Passive 250pcf;  soil 115pcf;   solid grout no inspection.
County claims it is only good 10km from Type B fault)
I cannot get it to calc out as restrained let along cantilevered.
My question is doing a lateral on the project would you do it as two story and use only the cmu wall weight in the analysis or.........
Do the house calc separately and at as load at top of rigid floor slab.
Would you at some weight for the dirt in the analysis for the Retained structure ??
Joe Venuti
Johnson & Nielsen Associates
Palm Springs, CA