basements and seismic base shear

huangyhg

basements and seismic base shear
i am designing an apartment building with three levels of wood framing above 1 story of below grade parking. i expect to use 12" thick concrete basement walls on all four sides of the basement parking garage. the top of the 8" thick concrete podium slab supporting the three levels of wood framing is expected to be about 4" to 8" above grade. will i have to design the basement walls to act as concrete shear walls, or can i assume that all of the base shear will be transferred into the earth at the grade level? the plan dimensions of the building are about 75' x 300'. the building codes do not really address this issue, so your help is greatly appreciated. thanks, kevin.
asce 7-02, section 9.7.5 "foundation requirements for seismic design categories d, e, and f." discusses basement walls a bit. aci 318-02 chapter 21 (esp. 21.10) is a good place to check as well, and says this:
"base of structure 鈥?level at which earthquake motions are assumed to be imparted to a building. this level does not necessarily coincide with the ground level."