Seismic Loads on Site Walls

[jdgengineer]

I'm curious how others design site walls and retaining walls for seismic loading in seismic category D, E, F.

Chapter 18 of the IBC requires seismic soil pressures for retaining walls >6' tall. Chapter 15 of ASCE 7 requires seismic loading on cantilevered site walls due to it's self-weight (R=1.25) where the walls is 6' above grade.

My questions for you:

For a retaining wall that is 7' tall, do you include both the soil seismic and the cantilevered wall self weight seismic? If you don't, want if the wall was covered in a heavy veneer? If you don't still, want if the wall cantilevered above the retaining, would you apply it to just the portion above grade?

The conservative option is obviously to apply both, but I'm not sure that was the intent and wanted to see how others interpret it.

 

[jdgengineer]

An image for reference.

 

[HTURKAK]

...do you include both the soil seismic and the cantilevered wall self weight seismic?

This is true ..The seismic load on a cantilever retaining wall is calculated by adding the soil seismic load and the wall self-weight seismic load.
Your second approach is the reasonable approach.
The commentary section of ASCE 7 C11.8.3 Additional Geotechnical Investigation Report
Requirements for Seismic Design Categories D through F states ;
'....Resource Paper RP 12, “Evaluation of Geologic
Hazards and Determination of Seismic Lateral Earth Pressures,”
published in the 2009 NEHRP provisions (FEMA 2009).'
you can download the subject doc. free of charge . I have downloaded and attached.

I will suggest you to look ( NCHRP Report Seismic Analysis and Design of Retaining Walls, Buried Structures, Slopes, and Embankments-Transportation )

 

[Greenalleycat]

https://www.building.govt.nz/assets/Uploads/building-code-compliance/b-stability/b1-structure/geotechnical-guidelines/module-6-earthquake-resistant-retaining-wall-design-version-1.pdf

Go to page 43 of the PDF (page number in bottom right corner is 35, they don't align)

I agree with JDG's sketches
The seismic soil coefficient Kae needs to be calculated and will be higher than your standard Ka values - this is to be applied in addition to soil self weight

 

[jdgengineer]

Thank you for the responses. I'm surprised the building code doesn't define how to handle the wall inertia and the dynamic soil seismic pressure well.

A lot of literature seems to only account for the dynamic soil seismic pressures (neglecting wall inertia)

I see some reference to using kh = SDS/2.5 to determine the wall inertia forces.

Some references mention that wall inertia and dynamic soil are not in phase so it's potentially overly conservative to combine them at their full values.

ASCE 7, Chapter 15 isn't very clear. Section 15.6.8 refers to Section 15.6.1 for earth retaining structures which gives no mention to wall inertia forces which seems to imply to only design for dynamic soil seismic loads.