It's a displacement based design. You will need to find differential displacements between base slab and roof slab due to EQ motions. You will find all the principles described in this paper:
Thanks, it appears to concur with my line of thought on this subject. I have used compression only springs in the past. Finding non-linear spring data is probably a challenge. I have heard of FLAC and see another one SHAKE is there one you know of and could recommend with regards to ease of use and processing time for complex finite element (not frame)structures.
1. Following Wang's paper no soil lateral springs needed. Equivalent (pseudo) earth pressure will be applied to mimic displacements at the top of the box.
2. Geotechnical engineer may use SHAKE to compute free-field displacements in the soil.
3. FLAC is a great program for excavations and minig, but it requires a capable person who will be working with it full time, going to the training, conferences and talking to the support (which is very good). Learning curve is VERY steep. I would leave it to FLAC experienced Geotech to supply you with equivalent loads/displacements. Also FLAC is weak in modeling frame elements it's more catered for plates and bricks.
