Basement Wall Pressures 5

[hemiv]

Hi all, I'm working on a little project to stabilize and push back my bowing basement wall. Going to go with an I-beam at 4-5' centers, with an angle foot plate anchored to the basement slab and an angle top piece connected to my joists. I'll have a bolt going through the top angle to apply pressure to the beam during dry seasons and hopefully slowly bring the wall back somewhat plumb. (Like this

https://www.midamericabasementsyste...tion-repair-products/wall-repair-system.html)

What kind of loads should this system be designed for?

I need to bring the wall in almost an inch, so for a 7' basement wall that's about 1.1%, so I took a look at at-rest pressures.

Say, Ko=0.8
unit weight=125pcf
c=800psf (not sure about this one)

Then, the horizontal earth pressures I need to overcome are: 0.8*(7'*125pcf)/2 + 2*800psf*sqrt(0.8) = 1781psf at the bottom of the wall. Top of the wall would be 1431psf.

Say 5' tributary width on a simple span, and I end up with a required moment of 44.5 k-ft, requiring a Zx=10.7in3 or 17.8in3 with the ASD factor. End reactions are 25.5k. The modulus is not as surprising to me as the end reactions. Most systems I've seen are not sizing their connections anywhere near enough for that kind of load. Seems to be mostly done with (2) 1/2" concrete screws, and done in a 4-6" slab. So I figure I must be off somewhere.

 

[hemiv]

MTNClimber -

I'm not looking for anyone to tell me exactly why this is happening, just curious to what degree that can actually be done. Can any geotech pinpoint an exact cause? Seems to me like it would always be a combination of all factors, so structural decisions should be made accordingly. Thanks for your input regarding soil shrinking, too.

PEinc -

Thanks, noted. I think I have enough joist span to make it work. Bowed walls are on the south side. And no, I don't have any drainage issues around there.

Edit: By joist span, I mean I have enough to widely distribute the loads so that the pressure at the back will not be that high.


oldestguy -

Of course, this being an internet forum, the nastiness was bound to come out at some point. I'm actually learning a lot here; I am a structural engineer with ears that work quite well.

 

[PEinc]

You said, "By joist span, I mean I have enough to widely distribute the loads so that the pressure at the back will not be that high."

You won't know if you "have enough" of everything required until you determine the required jacking force for the wall while considering passive resistance, active pressure, at-rest pressure, expansive soil pressure, hydrostatic pressure, and eccentric joist loading. Walls do not push back easily. A big bulldozer might not be able to push back that much earth. How do you think a block wall will stand up to the total jacking force? The force could be significant when applied to a wood-framed house with probably unreinforced, block foundation walls. The load will be per linear foot of wall. For example, just passive resistance could be about 12 KLF (Pp = 0.5 x 0.120 x 4.0 x 7' x 7'). At a jack spacing of maybe 5' the load could be 60 kips or 30 tons. Now multiply that by the number of jacks. The distributed load to the structure will be the same but will be concentrated at a few individual, nailed, floor joists with some type of nailed, plywood flooring providing the distribution. I can just picture the probable floor cracking, doors racking, windows cracking, and drywall or plaster cracking. I doubt you will get much engineering assistance from the salesman who will sell you that vertical, jacking post system.

http://www.PeirceEngineering.com

 

[hemiv]

PEinc -

Well that's the point of my entire thread - trying to understand the loads so I can properly analyze the rest of the house. Saying "I think I have enough" doesn't mean I'm just crossing my fingers - I can do the proper analysis once I know the loads.

The thing that alarmed me to begin with is that I did in fact calc high loads, similar to your 60k. But, again, if you look around this method isn't that uncommon and I'd think the marketplace would have run them out if folks were destroying their homes using them.

In any case, I really do want to understand the soil behaviors at play. What kind of consideration can be given to the incremental approach? In other words, are loads required to adjust the wall at once higher than over a 2-5 year period.

 

[phamENG]

I'd think the marketplace would have run them out if folks were destroying their homes using them.

Don't be so sure. There's a foundation repair company in my area that should have been run off years ago, but instead they're the biggest boys in town. I've lost count of the number of times I've been called out behind them for a second opinion only to find that they only need to add a floor joist under a wall parallel to the joists that the framer missed 70 years ago - no need for the $60,000 underpinning. Or that the cracking in their brick is rust jacking due to corroded brick lintels, not foundation settlement.

Some of these companies do good work - the one I mentioned has done some really good work and really helped some people - but they're selling a product and they're good at it. Most of the their customers don't know anything about construction or engineering, and can be easily fooled or scared into buying something. I'm not saying that this group is doing it with this system, but I think we've all seen contractors who won't scruple over robbing somebody blind - so long as it was done legally and under contract. Sometimes, the invisible hands move really slowly.

 

[PEinc]

"...if you look around this method isn't that uncommon and I'd think the marketplace would have run them out if folks were destroying their homes using them."

Let the buyer beware! The world is full with thousands of "tools." It is up to you as a designer or contractor to pick the right tool for the job at hand. The product that you linked to is just a tool. I don't think it is the right tool for this job.

Edit: Consider the magnitude of the loads that need to be jacked. Then, look at the small, closely spaced bolts that attach the vertical post to the unreinforced concrete slab and the wood joists. I haven't run any numbers but it seems to me that the limiting factors to this tool's capacity are the connections of the steel angles to the joists and the connection of the vertical posts to the thin, concrete, basement slab.

http://www.PeirceEngineering.com

 

[MTNClimber]

hemiv - I think there are a bunch of us that are skeptical that this system can push your wall back, and with good reason: soil is not easy to push around. MidAmerica even states "Straightening potential depends on several factors, including soil conditions outside the foundation." We don't really know what the soil conditions are in your case but have made educated guesses. If you get a 5-6' shrinkage crack in the soil directly against the foundation maybe it will work. I personally haven't seen that deep of a shrinkage crack at a residence.

I personally think this method could stop further bowing but I'm not sold on being able to straighten the wall with the soil still present. PEinc brought up a lot of items that should be taken in consideration. If you acknowledge these risks and still want to proceed, then go for it. But if you want to be more cautious, maybe take some of the advice provided.

 

[MotorCity]

Since this is an unreinforced block wall, I would see any attempt to push the wall back going horribly wrong due to:

1. "Overpushing" the wall back. You walk a real fine line between pushing the wall back to vertical and bowing the wall outward.
If this happens, it would be sudden and catastrophic.

2. What are you going to jack against without risking damage to some other part of the house?
I would avoid using wood floor joists as any part of jack supports.

Speaking of the joists, that's another force you will have to overcome to jack the wall back in place. You would be trying to move a wall with lateral and vertical load on it.

I would simply try to shore the wall from getting any worse.

 

[PEinc]

The joist are also attached to the same wall that you are trying to push back. You would have to disconnect the joists from the top of the bowed wall or just "let'r rip!" MTnClimber, MototCity, and I are on the same page here.

http://www.PeirceEngineering.com

 

[hemiv]

Alright y'all, thanks for the advice. I will likely proceed with installing beams primarily for preventing further movement. Digging out would be tricky due to utilies very close to the house, but I will likely do some of that as well.

If I install the bolt jack mechanism, I'll tighten it with a torque wrench and limit it to an allowable determined by analysis. If I dont't get much movement over time, oh well - I'm at least preventing further movement. Per MotorCity, I won't get greedy and go for full plumb in order to prevent a total reversal.

I am still interested in hearing from someone whether or not, or to what degree, these loads are time dependent. i.e, Seems to me it would take much more force to move them in, say, one day than in, say, 2-5 years. Just like it takes more force to mechanically compact soils than it does to let them settle on their own. I realize I may be wrong about them, but I would be great if someone could clarify that.

 

[SlideRuleEra]

I am still interested in hearing from someone whether or not, or to what degree, these loads are time dependent. i.e, Seems to me it would take much more force to move them in, say, one day than in, say, 2-5 years.

The force needed to restore the wall to near vertical is not time dependent. This is because the problem is more than soil mechanics, see my sketch below.

1) As the wall bows inward, soil outside the wall both settles (under gravity) and self-compacts.

2) For the existing condition, soil level outside the wall is generally "lower" than it was when the wall was constructed.

3) To "restore" the wall to near vertical, the applied horizontal pressure has to both overcome soil resistance and "do work" (in a mechanical engineering sense) to raise the overall soil level outside the wall.
Note: Yes, the soil may slowly compact somewhat from the applied horizontal force but basically it's volume remains unchanged and the only way for it to move is "UP".

Until the magnitude of the applied force is adequate to both overcome soil resistance and "do work" (raise the outside soil level), not much (other than maybe some soil compaction) is going to happen... no matter how much time passes. Gravity never takes a day off.

http://www.SlideRuleEra.net

 

[MTNClimber]

I'm guessing your site is in a semi-arid or arid location and therefore unsaturated soil mechanics are at work. In unsaturated soil mechanics, you usually don't have to worry too much about the clay being drained vs. undrained since the pore water pressure is almost always negative.

If this assumption is wrong and your soil is considered to be in the realm of saturated soil mechanics, then loading and time do factor into the scenario since you're now dealing with positive pore pressures, which affect the soil strength.

 

[oldestguy]

It is seldom that I disagree with Slide Rule Era. First of all it is an unreinforced block wall. Has no real bending resistance and depends on the load from above to keep it together when loaded sideways.
The top is pretty much held from moving since it is loaded with a flooring system of joists AND FLOORING AND WALLS ON IT. That makes it a solid mass, not likely to distort horizntally in the middle, since the whole house, more or less is resisting the push. So the basement wall can't take its load from expanding soil that and the mid height zone bows some inward, LEAVING THE TOP UNMOVED.
Also if it was a reinforced concrete wall and was being shoved outward, the stuff that settled won't go back up as shown when pushed from inside. Instead there will be a whole mass sliding on an outside inclined slippage plane. This all assumes the earth outside is incompressible, most unlikely. I'd flag it and ask that it be removed.

 

[PEinc]

Someone is not listening to us. So, go ahead and buy lots of those proposed jacking support posts. I'm sure the salesman knows more about earth pressures and unreinforced block walls than do those of us who responded.

http://www.PeirceEngineering.com

 

[MTNClimber]

Eh, I'm not too concerned that they are or aren't listening. The discussion has been entertaining and I would like to hear how things go if they proceed.

 

[hemiv]

MTNClimber -

Rest assured I am listening. I am genuinely interested in learning about this subject.

I don't live in an arid location. We get about 46" per year. But I imagine that unless you're talking about soils below water table, it's all unsaturated mechanics?

 

[oldestguy]

Obviously its like talking to a stone wall when this is the kind of reaction we get. No point in trying to educate an engineer who knows it all, even basic soil mechanics. I won't try. But I imagine that unless you're talking about soils below water table, it's all unsaturated mechanics?

 

[MotorCity]

SRE makes an EXCELLENT point that I had not even considered, which greatly increases the force required to restore the wall to vertical.

I just see this unreinforced block wall collapsing like a house of cards if you push on it. Build a model of the wall out of Legos and push on it and you will see what I mean.

 

[hemiv]

oldestguy -

Been awhile since I learned my soil mechanics, just trying to ask questions and learn. Sorry to offend you. Even so, no need to ridicule me.

---

A few of you all have been helpful, so thanks. I appreciate the insight. It's probably time for me to stop, because I appear to be bringing out the worst in the rest.