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# Seismic loads and retaining wall/building interaction

## Seismic loads and retaining wall/building interaction

(OP)
I have a 2 story U shaped building with a 14' retaining wall all along one side. The second floor rigid diaphragm will have to be supported on the top of this wall. I am in IBC seismic category "C".

I can either:
1)Design the wall as a cantilever, tie the building in at the top, and use the wall to support the seismic forces from the building (both levels)

2)Design the retaining wall seperate from the building (as a cantilever), use moment frames in the building to support its own lateral, but still tie the floor in for use as a shear wall in the opposite direction.

What do you guys think is the better way, considering  differences in the seismic actions of the different structures?

### RE: Seismic loads and retaining wall/building interaction

haynewp,
Hi.
I feel I will vote for your second approach of keeping both the structures independent and I would even think of not providing any shear wall or stiff connection, instead think of some sliding or flexible connection at II storey level.

The reason I am less comfortable with the first method is the earthquake forces on one structure affect the design of the another. While designing the retaining wall you need to consider the inertia forces of the building transferred to the wall. Similarly, while designing the building, you can not overlook the inertia of the wall and dynamic forces of backfill acting through the connection on the building. To make the matter complicated, the wall will be subjected to considerable moments as the dynamic pressures act one way and inertia forces exerted from the building act to the opposite direction.

I would prefer to keep it simpler and do them independently. As far as connection is concerned, it can be a simply supported beam type, which will not take any earthquake loads.
Regards,
flame

### RE: Seismic loads and retaining wall/building interaction

haynewp - is the retaining wall similar to a basement wall of the building?   From your description that's what I'm imagining.  If so, I would think that the basement wall would be built initially free of earth pressure...then the question is, do you backfill prior to installing the diaphragm?

If they need to backfill during construction - then it could be designed as a stand-alone retaining wall.. Once the diaphragm is in place, the behavior of the wall would switch from cantilever to simple span.  I would worry about the ability to get a true slip joint between the top of wall and floor that would work.  I would tie the two together and analyze as one system.  (your option 2)

Design the cantilever action for construction-type soil loading (perhaps a reduced equivalent fluid pressure than you would see long-term.  This would get you the vertical, earth-side rebar.  Then, with the floor tied in you can analyze with the long-term equivalent fluid pressure force from the retained earth and design for the vertical inside face rebar.

### RE: Seismic loads and retaining wall/building interaction

(OP)
Thanks for the posts, and yes, I am thinking about putting a note not to backfill until the building is completely finished and tied into the retaining wall, if this is possible for them.

That way I don't have to worry about the expense of a 14' cantilever wall, and I can design it as restrained. Also, I think I may be able to use the retaining wall as a shear wall in all directions. For earth pressure, seismic, and building seismic loads. The building is "U" shaped, like this:

http://www.angelfire.com/linux/haynewp/index.html

You can see the top of the 14' retaining(basement) wall in this second floor plan view (in red) all along the rear of the "U". I just hope the 3 1/4" concrete diaphragm is stiff enough to properly transfer all the forces to the walls to act as shear walls. Any other suggestions?

### RE: Seismic loads and retaining wall/building interaction

My \$.02 - I agree with JAE.

### RE: Seismic loads and retaining wall/building interaction

I forgot to mention my imagination of different foundations for each of the structure - wall & the building, in the post above. Cant help, I am in hills.

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