Buried Parking Garage Seismic Design

[Jackson26]

I am in the process of designing a one-story parking garage that is completely below grade (with the exception of the access ramp).
There is 4'-6" of soil on top of the "roof" slab.
The garage is located in California with Seismic Design Category D.
The walls around the perimeter of the garage retain soil and also act as shear walls.
There will be a condition in the life of the garage where one side is completely excavated for the construction of a light rail (3 sides will retain soil)
My question is this:
When calculating the effective seismic weight for shear wall design, does the weight of the soil above the structure need to be included?
Note that the soil is not tied to the structure in any way like a planter would be. Just loose soil.
Thoughts?

 

[Greenalleycat]

I would not personally include the weight of the soil for lateral considerations

My assumption from what you've said is that the reinforcement in the wall will be governed by earth pressure + seismic earth pressure out-of-plane loads
Adding some additional roof weight isn't really good to change anything I think (even though technically there is some amount of lateral load from interface friction)
The roof itself will be designed for the soil weight etc - you should consider vertical EQ loads in combination with gravity for this element IMO

Side question, you say that after one side is opened up for light rail there will be -3- sides left retaining soil, implying all 4 sides retaining soil prior to that....how is that a garage? How do you get in to this thing!

 

[KootK]

I would be inclined to include the weight of the overburden as seismic weight and combine that effect with the seismic lateral earth pressures that would apply, similar to retaining wall design where the walls are not able to rotate significantly.

More liberal approaches would include:

1) Assume that the overburden just pushes into the adjacent soil, assuming such soil exists (this only applies in one direction for your situation) or;

2) Take only a portion of the overburden as seismic weight like we do with storage sometimes.

In my heart of hearts, I could totally see:

a) The overburden not moving independently from the diaphragm as we assume with storage and;

b) The transition between overburden and adjacent soil developing gaps at those locations for various reasons.

 

[Greenalleycat]

Oh wait, is that 4 foot and 6 inches, or a range varying between 4" and 6"?
I read it as 4" to 6" and thought it wasn't very much so would ignore it...4' 6" is a different beast

I would include some nominal contribution - in practice this is a complex problem of underlying soil period -> building period -> soil above roof period -> interface friction transferring loads
I mainly see this affecting your base shear transfer, I don't think it will affect your wall reinforcement or overall stability

 

[structSU10]

In snow country the code makes you count 20% of the snow weight in seismic mass. I would think snow is much less cohesive than soil tends to be. I would include it.

 

[HTURKAK]

My points,

- Clause 12.7.2 Effective Seismic Weight. ...
...
5. Weight of landscaping and other materials at roof gardens
and similar areas.
In your case, IMO, 80 % of soil would be included to effective seismic wt.

- However, the horizontal soil earth pressure + seismic earth pressure will govern for the seismic design of the perimeter basement walls . The contribution of soil wt . will not affect the shear wall in plane reinf.
- The roof slab shall be designed for gravity and vertical seismic effects .
- The design seismic force may be calculated using (ELF) procedure.

- I will suggest you to look for detailed info :
Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments ((NCHRP report )



He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48