Seismic basement pressure

[penpal97]

I'm trying to design a fairly deep basement and I am not quite sure how to design for seismic soil pressure. People have told me Monoabe-Okabe does not apply and I should look at ASCE 4-98 Seismic Analysis of Safety-Related Nuclear Structures, where they use the Wood Elastic Method.

The Wood Elastic Method calculates the dynamic soil pressure exerted on the basement wall. Do I also need to account for static earth pressure (as opposed to active, or even passive earth pressure) in the load combination? And what about the otherside of the building, does that have no soil pressure whatsoever because the soil is "moving" away from the structure?

 

[civilperson]

Both sides of the basement are receiving the forces transmitted by seismic acceleration. M-O is accepted for retaining walls, for 16 foot basement walls and for above ground tanks backfilled with soil. What new rule negates Monoabe-Okabe from use? (I am sure there are new methods using elastic half space models of soil.)

 

[penpal97]

ASCE 4 Commentary - "The Monobe-Okabe method of determining dynamic soil pressure was developed for dry, cohensinless soils under the assumption that sufficient wall yielding occurs to produce minimum active pressure and that, at this point, a soil wedge approaching incipient failure, with maximum shear strength mobilized along the sliding surface, is formed."

The basement does not move, therefore does not allow the soil to develop the "wedge". I am guessing the Wood method assums the wall does not move and that the soil is basically slamming into the basement wall.

So do I need to include static earth pressure (either static Ko, Ka active, or Kp passive) as well as the dynamic pressure?

 

[kslee1000]

The following is my past practice to design basement wall subjects to seismic loading.

On one side of wall only:
- Assume the soil is in at-rest state at incipient of an earthquake event (due to instant change in directions, the passive failure mechanism is unlikely to happen).
- Add the dynamic earth pressure (should be an up-side-down triangle)to the at-rest pressure, and inertia force of the wall (note that all forces are in same direction, and effects of static & hydrodynamic pressures from ground water, if present, should also be considered).
- Analyze/design a strip of the wall that is supported by top & bottom slabs.

 

[penpal97]

thank you kslee1000. what about for overall overturning and base shear? Do you apply any static type loads on the other wall?

any one else have any thoughts?

 

[kslee1000]

Active pressure on the opposite wall should be justified as the wall is leaving the soil. Overturning and base shear (or sliding - usually is ignored) calculations should be same as for earth retaining structures.

All these assumptions are based on structure surrounded by thick layer of sound subgrade material. Otherwise, you shall consult with a geotechnical engineer to explore the potential failure mode before jumping into all of these.