Resisting Lateral Forces at Top of Foundation Wall

[JNEnginr]

Here's a Question...

Say I have a basement, 12 ft tall from slab on grade basement slab to top of plywood sub-floor. Floor construction is 24" deep floor trusses.

One the other side of the basement wall is all fill, and instead of a wood floor system, i have slab on grade at almost the same elevation as my plywood sub-floor.

My original thought was to design as a cantilevered retaining wall with all surcharges as required.

Owner comes back and says no cantilevered retaining wall, they'll just temporarily shore the wall.

So then lateral forces at the top of the wall must get resisted in one of two ways: send the earth force at the top of the wall into the plywood diaphragm and block the hell out of it..

Or develop the top of wall forces into the high concrete slab on grade with dowels.

Any thoughts on the things i should check when choosing the latter?

Thanks,

 

[XR250]

I would try to use the slab somehow for the permanent load. 12 ft. of backfill is huge for a floor diaphragm. You also have to get the load out of the diaphragm once it is in there.

 

[KootK]

They key thing that you should consider is the degree of permanence of the exterior slab on grade that your building will be reliant on for stability. The most common solution is to rely on the adjacent wood floor diaphragm.

Other stuff to check:

1) resisting soil drag.
2) tension in SOG.
3) SOG to wall connection...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Would your slab be a tension member dragging load to the soil or a beam that spans to perpendicular walls somewhere?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I've never had an issue getting a wood floor to resist the backfill pressures. Even on tall walls, I've had them temporarily brace the wall during backfilling procedures because they usually want to backfill at least partially before the wood floor is in place.

 

[mike20793]

Agreed with jayrod. Simpson has some nice connectors for this application. Keep an eye on diaphragm shear, though.

 

[Teguci]

Don't develop the load into the slab on grade that is sitting on top of your retained soil - super bad idea.

 

[DETstru]

I would put the load into the wood diaphragm.

Depending on the extents of the SOG and what it's connected to (other than your retaining wall), it could cave in with the soil.

 

[JNEnginr]

Slab on grade extends about 30ft, then terminates atop the buildings exterior foundation wall. I would then probably dowel SOG into top of that wall. That wall is only 4ft deep, and I'd imagine would have some slidding resistance to help out.

KootK, I'd imagine the beam to the perpendicular wall condition.

Jay and Mike, I agree it can be done that way (into wood diaphragm) but just checking on the other option.

Teguci, care to elaborate?

 

[kgengr]

I would design this as fixed-base & pinned-top and take the load into the wood diaphragm, distribute to shear walls.

 

[HouseBoy]

How long is this wall?
What is span of the 24" floor trusses and are there perpendicular walls there that could buttress the subject wall?
I often try to get the wall to span horizontally when I am not happy with the capacity of the plywood floor assembly.

How far across the building/floor to the other side of the plywood floor?

I like kgengr's idea but for a fixed base, the footing needs to be able to prevent rotation so...I think there is some judgment there. Depending on the soils, that might be tough to get comfortable with (without getting close to the wide footing the owner is already saying "no" to).

 

[KootK]

KootK, I'd imagine the beam to the perpendicular wall condition.

So the beam is actually spanning along the wall to some kind of support rather than dragging the load into the SOG as tension? I've done something similar for sections of basement wall behind stairwells where there is no diaphragm to lean on. I've done it by casting a concrete girt a couple of feet below the exterior slab as a second pour. That way, it becomes an element that is quite obviously an important part of the building structure and not subject to later removal. It can be a building envelope problem as you can imagine.

I believe that some of the trepidation that you're hearing here regarding the SOG solution comes from concern for mobilizing soil friction which would just end up getting transferred back to the wall again as additional soil pressure. You ultimately end up needing to reach out quite a ways beyond the wall for reliable tension resistane. I tried to convey the notion in the sketch below. Let me know if it's inadequate. I'm finding this a difficult thing to describe for some reason. I guess I'm saying something along the lines of "circular load path".

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JNEnginr]

Wall is about 90 ft long, no walls to act as buttresses.

So is there a concrete (ha) reason for not using the slab to resist the forces?

 

[Teguci]

1. You will need to ignore any resistance of the first 18 ft or so of the slab since it is being resisted by the retaining wall you are trying to support - pulling yourself up on a falling rope
2. Slab on grade will need to be designed to ACI 318 standards - it is now a structural slab and not just a slab on grade
3. Consider the possibility that a sewer line or something will need to be trenched and the slab will be cut.
4. You will only get a small amount of shear friction from the weight of the slab on the soil.
5. It is fairly standard to put this lateral load into the plywood flooring system.
6. You will need to develop the reinforcement on both sides of any control/construction joints - don't use dowels to allow volume change.

Not saying I wouldn't do it (I have), but you should have some very good reasons to support this uncommon solution.

 

[KootK]

7. Strain compatibility. You might need 1/2" or more of axial displacement in the SOG to resist the lateral load imposed on it. How much load will be absorbed into your diaphragm once that movement occurs?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JNEnginr]

Koot,

What you're saying definitely makes sense, i get the circular pattern that develops.

But what if instead of an actual "beam", i just say the slab is a series of smaller beams, whose depth is the slab depth, and whose width is the bar spacing in a sense.

If i did that, I should continue this tension rebar all the way to the other side of the slab and into that foundation wall, correct? The tension force needs to be carried the whole way across the slab via rebar, right?.

 

[KootK]

It might be time for another sketch to illustrate what you're suggesting. Based on what you've told us above, my understanding is that the SOG is acting as a tension tie and not as any kind of beam/girt (be it a big one or a bunch of little ones. If that's true, then you just need to go far enough past the retained soil with the designated tension rebar to feel comfortable that you've developed your soil friction beyond the vertical line in my sketch above. I've never seen it before but I suppose you could even introduce a shear key into your SOG kinda like we use sometimes on retaining walls. Like I said at the top though, I'm not a big fan of the system in general.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[HouseBoy]

He's saying the slab is a tension member that is ANCHORED AT THE FAR SIDE into the foundation wall over there. No soil friction would be involved.

 

[KootK]

So lateral resistance will be provided by SOG attachment to an entirely separate building? I'm liking this less and less all the time.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

The full at rest earth pressure from the 4 ft wall will only be about 1/3 of the tension reaction required for the 12ft wall. Even if you put your 4 ft wall into full passive pressure, you are going to have a tough time resolving the loads (you may want to provide grade beams to put some moment into your 4 ft tall shear key).