Should Basement weight be considered for Seimic weight?

[brick2020]

I have a 4 basement with G+11 storey structure, in a moderate seismic zone.
I have considered the superstructure height from the ground level to determine my time period and considered only the weight of the Superstructure to calculate my base shear. The basement footprint is larger than the building tower footprint, and is surrounded by stiff retaining walls. The soil is fairly rigid, SBC of 450 kN/m^2 at 15m(bottom of raft)

My question is: Should I consider the weight of the basement slabs to be part of my seismic weight? Any references plz?

 

[CELinOttawa]

No; Seismic weight is the active mass above the ground floor.

With a significant podium, or certain soil conditions, you should be designing your foundation walls for seismic loading. Still does not make the foundation and portions of the structure which are underground part of the seismic weight.

Any design example for your loadings code should show this... Where are you, and what code are you using?

 

[KootK]

I used to agree with CEL and my engi-heart still wants to. However, the latest and greatest out there is pointing the other way. I've got a gaggle of papers on this issue that I'll share when I get back to the office. Presently, in your situation, I'm using the foundation level as the seismic base but not including the below grade structure as seismic weight. My local soils are not usually very stiff. And the inherent inconsistency of the method does make me want to barf a bit.

Thanks for starting this thread. I'll be monitoring it's progress with great interest.[pre][/pre]

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Archie264]

Presently, in your situation, I'm using the foundation level as the seismic base but not including the below grade structure as seismic weight.

Kootk, that statement confuses me a bit. Are you saying that you model it with one or more zero-weight stories?

 

[KootK]

Yup. The mass below grade gets zeroed out. JoshPlum has a special modelling term for this that I can't remember.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Archie264]

Ok, thanks for the clarification. I'll be watching this thread, it looks like it will be an interesting one.

 

[KootK]

I've also been making some liberal use of ASCE's two stage procedure. for buildings with stiff bases structures. Here's some discussion fodder.

Link
Link
Link
Link
Link


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Residual mass vectors! Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.