Foundation Seismic Dead Load

[dtvankirk]

I have a 4-story steel structure with a basement level that is 20-ft below finish grade. The basement walls are 15" thick poured concrete walls. When calculating the seismic dead load of the structure (for the purposes of calculating seismic design base shear) should you include the weight of the foundation walls in the dead load calcs? Does the fact that these walls are underground have any impact on the seismic design calculations?

 

[bjb]

I'm not in a high seismic area, so take my response with a big grain of salt. It's my understanding that for calculating the seismic base shear, you only need to consider the weight of the structure above the ground level. I think this is because it is the inertia of the above ground structure to the ground motion that results in the seismic shear. However, the basement walls would need to be designed for the inertia of the retained soil and their own weight. The foundation walls also have to transfer the accumulated seismic forces from the superstructure into the earth. I'm no seismic expert, but that's my understanding.

 

[dtvankirk]

Thanks. That's also how I understood it. Just had to make sure.