Seismic Force on retaining wall

[taoski]

I'm trying to determine what, if any lateral pressures due to earthquake forces I need to include on a building basement wall (out of plane) for a SCD-D building. ASCE7-05 11.8.3 requires the geotechnical investigation to report the above mentioned forces, which seems to imply that it has to be designed for.

But is this really the case? Do I need need to include seismic effects for a basement wall design, and what part of the code requires it? The closest I can find is in section 15.6.1, but that would only apply to free-standing site walls.

Thanks, Daniel.

 

[civilperson]

If the walls remain in effective contact with the soil, (retaining walls and backfilled basements), then Kp can be calculated from Monobe-Okabe equation. Both soil impacted walls and other structural walls must be designed for ASCE 7-05 12.11.

 

[taoski]

Thanks, but that's not exactly what I'm wanting to know.

Which code provision addresses soil effects on seismic loads for walls? 12.11 addresses out-of-plan forces without regard to soil.

 

[kslee1000]

In lieu of getting into complicate soil-structure dynamic interaction, we designed a basement by simply resolving the base shear into a couple against the walls. The pressure form for the couple was two opposite triangles (one been regular, the other was upside down). On top of the couple, Pp was added to the wall that was pushed into the soil, and Pa was added to the wall that was separating from the soil. Also a case was made to represent the soil-wall had separated completely, in such case, there was no pressure on the leaving wall.

Look out for ground water, it behaves differently from the soil.

Your geotech shall be able to provide better information if a more precise analysis is required/desired.

 

[theonlynamenottaken]

Good eye... there isn't really any statements requiring inclusion of seismic forces into soil retaining structures. A few states have recognized this in their amendments to IBC 2006. For instance, one of the amendments:

Basement, foundation and retaining walls in SDC D, E, and F:
Yielding walls (3/8)*kh*g*H^2
Non-Yielding kh*g*H^2
kh= Sds/2.5
Point of application = .6*H

I've seen alot of the proposed amendments to future IBC editions include provisions for seismic effects on soil retaining structures. I myself do include seismic effects on my soil retaining structures if they present any risk to public safety... e.g.

- Seismic effects for basement walls and retaining walls supporting foundation surcharges
- Ignore seismic effects on site retaining walls, etc