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 If you want the physics answer, the forces your structure sees are not due to displacement, but due to acceleration. Velocity imposes no force on an object, only changes in velocity.  By the physics of the problem, the mass of your walls will contribute to the forces they must resist. Is your question what is allowable by code? We all know the code does not necessarily mimic the physics, so the answer there may vary. I'm _not_ aware of an exception where the code allows you to ignore building mass below grade. Don't take my word for it, though; in my part of the country things generally blow over far more frequently that the earth shakes them down, so I have a far better knowledge of wind than seismic. `Jordan` Me wrote: I have a two story steel stud shearwall building over a single level, fully subterranean, mildly reinforced parking garage.  Building plan is basically 70 ft. x 200 ft.   Transverse concrete shearwalls have been added at the subterranean parking level.   What is the seismic mass contribution of the perimenter concrete basement walls to the design of the transverse basement walls? The mild reinforced parking deck will be considered in the mass calculations, but subterranean walls, in our opinion, do not contribute to the lateral mass, even of the transverse walls. The design lateral forces of the building result from seismic displacement, but since the displacement of the garage occurs with the seismic displacement, wouldnt the contribution of subterranean mass be negligible?   As an analogy.....If you were to do a COF calculation on slab seismic base shear resistance, it would be similar to multiplying the slab mass times the base shear coefficient prior to the coefficient of friction calculation.     This is not a seismic soil increment question, just a massing question.   Appreciate the response....     Ahhh...imagining that irresistible "new car" smell? Check out new cars at Yahoo! Autos. ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********