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.

 

[oldestguy]

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?

 

[hemiv]

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.

 

[phamENG]

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.

 

[hemiv]

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?

 

[oldestguy]

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.

 

[MTNClimber]

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!

 

[hemiv]

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.

 

[PEinc]

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

http://www.PeirceEngineering.com

 

[oldestguy]

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".

 

[phamENG]

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.

 

[hemiv]

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/getfi...5-88d424bd6008&file=Basement_Beam_Picture.JPG

 

[oldestguy]

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.

 

[GeoPaveTraffic]

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

 

[phamENG]

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.

 

[hemiv]

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.

 

[phamENG]

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.

 

[MTNClimber]

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.

 

[PEinc]

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.

http://www.PeirceEngineering.com

 

[oldestguy]

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.