Soil Seismic Load Questions

[DCBII]

Consider a multistory structure with a basement. Assume the geotechnical engineer has given you a soil seismic load against the basement wall. That soil seismic load is resisted largely by the ground floor diaphragm at the top of the basement wall, which carries it out to the basement shear walls. How does this seismic earth force affect the vertical distribution of seismic forces? I would assume it is usually ignored when calculating the vertical distribution, and then added on afterward to the ground floor diaphragm, but I wanted to get the community's input.

Also, does the R value of the basement walls affect the seismic earth force? For example, would this force be the same in an ordinary reinforced shear wall as in a special reinforced shear wall? I would guess that most engineers just take the force at face value and don't modify it based on R.

 

[Deker]

The vertical force distribution starts from the seismic base which may vary depending on your basement configuration. See this article for discussion on how to locate the seismic base: Link. Soil seismic load is additive to but does not affect the distribution of building lateral forces.

Soil seismic load is based on peak ground acceleration reduced by a few factors that depend on the nature of the loading rather than the ductility of the lateral system. As such, the load is independent of the lateral system used. See this paper for some background: Link.

 

[HTURKAK]

-Is this a multistory structure with a single basement ? If so, My suggestion will be, neglect the passive thrust of the surrounding soil and perform your analysis without considering the soil effect for the superstructure. The dynamic soil loading may be considered for the perimeter basement walls

- If the geotechnical engineer has given me a soil seismic load against the basement wall, i would ask to furnish the dynamic horizontal passive springs and vertical springs

- Suggest you to look CH .19 SOIL–STRUCTURE INTERACTION FOR SEISMIC DESIGN.

- and the following document at

https://www.nehrp.gov/pdf/nistgcr12-917-21.pdf

- If this is a real question , will you pls post more info ( basement and upper storey plans , section, seismicity etc..) to get more valuable responds ?