Seismic load on basement 1

[WWTEng]

Single story with a basement. 40'x120'. Masonry shear walls and concrete basement walls. IBC 09, SDC=B.

The question I have is about the basement level. Are there any special provisions to determine the seismic force on the basement level or is its similar to a level about grade? Do I need to consider some weight of the soil as seismic weight?

 

[CELinOttawa]

I do not know american codes, but the first storey load is the base shear of the structure. The load from all storeys above is assumed to be transfered into the ground at ground level.

The only time anyone checks parts of basements walls is if they have large or significant amounts of windows. The compressive strut action means that normal basements work by inspection.

 

[CELinOttawa]

Sorry, one thing: Depending on your site class and earthquake loads, you may have to deal with an out of plane load on the wall. I'll let an Engineer familiar with your IBC help you there as I couldn't if I wanted to...

If you are dealing with out of plane seismic loads it is likely you'll be permitted higher compressive strengths in your masonry owing to cyclical loading, but most engineers reduce/remove the added capacity due to the axial load on the wall. It is typic

 

[Qshake]

I agree with the out-of-plane load due to the soil and wall for the basement. I'm confused by the windows in the basement though. Perhaps this is different than what I'm thinking of...

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[johnbridge231]

out of planee bending may be critical.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[CELinOttawa]

Qshake: Architectural designs with 50% and 70% strip windows (by width) in the top 3' of the foundations in order to get natural light into the basement... I've not only seen them, I've seen them built. *sigh* Clearly the seismic load will be transferred through magic into the soil two feet below.

 

[WWTEng]

Thanks all for the input. But I am still unclear (I have never done seismic analysis for a building with a basement).

I found this article in StructureMag

http://www.structuremag.org/article.aspx?articleID=993

I think I am going to assume my base at grade level. I'll design the masnory shear walls for shear due to roof and perpendicular walls (half the height). But what about the floor which is precast floor planks. Wouldn't this floor+basement walls (perpendicular to direction) generate some seismic loads? Shouldn't I design the basement walls to resist these shears?

 

[CELinOttawa]

All loads, including anything above grade, go into your seismic loads.

Check your code to see if you need to do a seismic retaining wall design for our of plane loads.

 

[CELinOttawa]

So I admit to being more than a little curious as to whether or not you'll need to consider out of plane wall seismic forces on the foundation. It seems not, though I stand to be corrected. See below:

Seismic wall pressures (Sec. 1802.2.7)
– SDC ≥ D
– For design purposes treat these loads as “E” not “H"

From:

http://seabc.ca/documents/seminars/ASCE_SEI_7_05.pdf

- Bottom of page 6.