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Basement Wall Pressures
5

Basement Wall Pressures

Basement Wall Pressures

(OP)
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.midamericabasementsystems.com/foundati...)

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.

RE: Basement Wall Pressures

As a geotech I am surprised you say nothing about what caused this and existing external soil pressures. Any frost involved? How about moisture, drainage? Any digging out of some earth first? How many jacks used?

RE: Basement Wall Pressures

(OP)
It's clay soil against unreinforced block wall. In my opinion, it's due to earth pressure with some swelling during wet seasons. I get a decent amount of cracks around the property when its dry. Frost line here is less than less than 30", and the bow is in the middle of the wall, about 42" down from the top. So I don't think freezing is giving me any issues. There is drainage on both sides of the wall., as the previous owners put in an interior perimeter tile at some point, and there are two sumps. I don't get any spouting or running water through the block, but it does get a bit damp if we get a lot of rain. There are no downspouts on the side of the house that has the worst deflection. I am not going to dig out, and I'm not using any jacks at this time. I'm going to use a similar system as in my link. There's another similar product called the Force Bracket which uses a spring, but in my opinion the spring is a little gimmicky and not needed. Just a 7' tall simple span, restrained at the bottom by an an anchored toe plate and at the top by another plate with a thru bolt which can touch off on the beam and apply pressure as it's torqued over time.

RE: Basement Wall Pressures

hemiv, you reference Ko = 0.8. That's a good value for at-rest pressure, but for this you'll need passive pressure to push the dirt back. You're looking at a Kp of closer to 2-4 for that if I'm not mistaken. So your loading could be even higher.

The connections do look questionable to me. It's an interesting concept, but not commensurate with the calculated loads. I'm guessing they've put their product through testing that has shown the smaller connections work - there's a lot going on that we don't account for in a simple trib area calc, after all.

Since it's unreinforced, you may want to consider including a girt at midspan. It could help reduce stress reversals and exaggerated cracking in the block.

RE: Basement Wall Pressures

(OP)
Well, I'm wondering if I actually do need pssive pressure. With 1.1% wall movement, that's not quite passive territory yet, right? I usually hear something more like 10% wall movement.

Forgive me, but what do you mean by stress reversals?

RE: Basement Wall Pressures

Before you go ahead with your plan, wait a while and see what shows up here with other thoughts. Being a structures guy think about putting some tension rods on inside between top and bottom. Might be more effective than a beam.

RE: Basement Wall Pressures

Depending on how the soil is interacting with the wall, you will likely have to overcome passive soil pressure. Further, depending on how hot that clay is, you may have to have to overcome the swell pressure too. I've never done something like this before but it does make me uneasy to use joists to try to push a foundation wall back. I know you said you don't want to dig out but I would seriously consider relieving some (or all) before trying to push the wall back. Either way, good luck and keep us posted!

RE: Basement Wall Pressures

(OP)
Thanks. This is not something I'm going to go out and do today. And I get why there may be some anxiety about this (because we're all engineers......) but the fact that there are systems out there that foundation contractors will come and install to incrementally move a wall back makes me feel like there's something I'm missing; that I'm over-estimating these loads.

RE: Basement Wall Pressures

Try to get some references from previous, satisfied (and hopefully unsatisfied) customers. I agree with MTNClimber.

www.PeirceEngineering.com

RE: Basement Wall Pressures

hemiv: Just because a contractor is using or recommend a certain procedure, etc., doesn't necessarily mean it is good for all situations, etc. This wall "problem" is very common and many different techniques have been used and tried with varying success. From my standpoint with clays that expand and contract, usually as moisture content changes is very difficult to deal in the long term if it is not just removed and replaced by something that doesn't do that. Off hand your approach, using assumed soil pressures, etc. and no outside work likely is too minimal an approach. My bet is that you will be disappointed from the start. It isn't just solving a simple soil mechanics calculation even if you made correct assumptions. If I was asked by a client to comment on his (this) plan, I'd not do any encouraging and maybe say "It ain't gonna work".

RE: Basement Wall Pressures

Re: my stress reversal comment. For a long basement wall that is bowing in, it is essentially behaving with one-way action spanning from base to joists (especially near the center). When you add these beams and start jacking, you'll end up with continuous two way action the wall. You'll introduce tension in the exterior face at the beams where it is trying to span continuously and where it is currently only on compression. The more you can limit the flexural deflections (or strain, and therefore limiting the stress in the masonry system itself), the less cracking you'll get.

RE: Basement Wall Pressures

(OP)
Alright, so it seems there is a consensus that I'm going to have to overcome passive pressures. Just for my own geotech knowledge, can anyone comment about why I would be seeing passive pressures in the 1% range of movement? It could be lower at 0.5%, because I don't need to bring the wall exactly plumb. Again, I've seen more like 10% movement touted around here on the forums. I'm willing to believe, just would like someone to comment on that specifically. I would also like someone to comment on the fact that I would be attempting to move the wall over a longer period of time in small increments during dry seasons when the ground has shrunk, as opposed to immediately via a jack.

phamENG, ok I gotcha - turning the simple span into a two-span continous via the point loading. My preliminary design idea would prevent this because I wouldn't be jacking in the middle of the beam, I would be doing it via turn of a bolt at the top of the beam (maybe bottom, too), and I wouldn't be doing it all at one time.

Here's another picture for sake of clarity.
https://files.engineering.com/getfile.aspx?folder=...

RE: Basement Wall Pressures

That wall didn't get to its present position with active or at rest pressures. Come up with pressures caused by moisture changes in expansive clays and maybe you will be in the ball park.

RE: Basement Wall Pressures

I agree with OG, you need to understand how the wall got to it's current state.

If the movement is just due to at-rest pressure, meaning that the wall and/or connections to the floor were under designed (which is certainly possible), then the new load you impose by jacking the wall back plumb will be somewhat greater than at-rest but less than full passive. I expect that your at-rest of 0.8 isn't going to be far off, maybe a little low...

If the movement was due to swelling, freezing, or something else, then who knows what the pressure will be.

One option to consider is to excavate along the outside of the wall to relieve all or a portion of the load, then plumb and reinforce the wall before backfilling.

Mike Lambert

RE: Basement Wall Pressures

hemiv - I disagree with your assessment that this situation would be prevented. Flip this system on its side. If I understand you correctly (which I may not, so I apologize if that's the case), it sounds like you're saying that a slab supported by a beam with a jack on one end wouldn't be lifted by the beam as you raise the jack.

You may not get a perfectly uniform load and, yes, it will be lower at the center, but it will tend toward being uniform as your column gets stiffer. Also, your reaction would be just below the center of the wall at first. Look at the attached image I just sketched up. It's to scale with a 7' CMU wall and a 1" out of plane deflection at the middle. When you first install your system, the uppermost point of contact will be just below the inflection point of the wall and it will slowly come into full contact as you tighten the bracket above.

RE: Basement Wall Pressures

(OP)
phamENG -

Ah, yes, I follow you now. Tension stresses developing on the outside of the wall perpendicular to existing tension stress on the inside of the wall? The CMU wall turns into a continuous span with moment reversals at the supports? Even so, the wall is already support a moment of wl^2/8, and the reversed moments would be less than that, as "l" decreases and the denominator increases.

Your picture is basically the case, jacking at the top in small increments via a bolt. I would probably not incline the beam as you've shown; I would plumb it or split the difference, and possibly grout in front it to help distribute loads.

GeoPaveTraffic -

How does a geotech go about an exact diagnosis? It seems to me the answer is a little bit of everything. Downspouts aren't the issue, we get 46" of rain annually, and frost line is 30". Other than that, I know I have clay soil and I can see the wall deflection characteristics.

Also, since swell and frost are seasonal things, my plan is to jack during favorable times of the year. Jan, Feb, Aug, Sept, Oct, when the soil is really dry. At those times, it will be shrunken away from the wall and will probably be entirely self-supporting, so it seems the jacking load would be much lower. The wall has been supporting these loads for 50+ years, so once the ground swells back up or freezes, the wall will just keep doing what it's been doing, except now it will be in better shape and have some extra beam reinforcement. Loads will be higher because I'm restraining movement, but also I will have reduce the span quite a bit.

RE: Basement Wall Pressures

I wouldn't grout it unless it's a semi-compressible material. Otherwise your jacking will just translate the wall rather than remove the deflection.

And although the resulting moments will be less, the tension stresses will be higher than what it's seeing now, and I think you'll get some pretty nice cracks in your wall. Perhaps it will be less than I'm thinking - I usually go too far at first and then talk myself back with analysis.

RE: Basement Wall Pressures

We can't fully diagnose the cause with limited information. We don't know anything about the soil (consistency, density, moisture content, Atterberg limits, swell pressure, ect.). We don't know how the wall is actually moving throughout the seasons. Your question of: Why is this happening?, can only be answered by guesses.

It seems like you're taking a practical approach by trying to doing it when the soils shrinks. If the soils doesn't shrink far enough it may take awhile for you to get the wall back to being plumb or you will have to overcome a pressure somewhere between active and passive or the swell pressure, whichever is greater. Also remember that the whole 7' of soil will not likely shrink in one season.

RE: Basement Wall Pressures

If you are going to jack against the joists at the top of the verticals that you install against the bowed wall, you better be sure that the opposite ends of the floor joists don't push backward and off their foundation wall. Your jacking forces may be greater than the first floor's ability to resist backward movement. That is, how well are the joists attached (nailed?) to something (wood plate on top of block foundation wall?) immovable at their opposite ends. And, as someone mentioned above, your joists would need to be checked for combines bending, axial load, and possibly eccentric jack loading. If you try to push your bowed foundation wall to the north, you might just push the entire framed superstructure to the south - especially when there probably isn't any earth against the far ends of the joists, above the top of the block foundation wall.

Is the bowed wall on the north side of the basement? Is there a drainage problem along the bowed wall?

I suggest that you consider tieback anchors, replacement of expansive soils, and/or improvement of drainage to prevent both hydrostatic pressure and freezing as others have suggested above.

www.PeirceEngineering.com

RE: Basement Wall Pressures

Hey guys: We have been talking to a stone wall that has no ears. Our years of experience vs no experience in this field have been ignored. Might as well spend our efforts with those that pay attention. A rare thing here, but now and then these cases do come up.

RE: Basement Wall Pressures

(OP)
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.

RE: Basement Wall Pressures

2
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.

www.PeirceEngineering.com

RE: Basement Wall Pressures

(OP)
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.

RE: Basement Wall Pressures

Quote (hemiv)

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.

RE: Basement Wall Pressures

"...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.

www.PeirceEngineering.com

RE: Basement Wall Pressures

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.

RE: Basement Wall Pressures

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.

RE: Basement Wall Pressures

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.

www.PeirceEngineering.com

RE: Basement Wall Pressures

(OP)
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.

RE: Basement Wall Pressures

Quote (hemiv)

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.





www.SlideRuleEra.net idea

RE: Basement Wall Pressures

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.

RE: Basement Wall Pressures

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.

RE: Basement Wall Pressures

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. deadhorse

www.PeirceEngineering.com

RE: Basement Wall Pressures

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.

RE: Basement Wall Pressures

(OP)
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?

RE: Basement Wall Pressures

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?

RE: Basement Wall Pressures

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.

RE: Basement Wall Pressures

(OP)
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.

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