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Considering Vertical Pressure of House Structure on Foundation Wall

Considering Vertical Pressure of House Structure on Foundation Wall

Considering Vertical Pressure of House Structure on Foundation Wall

(OP)
I apologize if this is the wrong forum for this question. I am not a civil or geo engineer.

We have a 9' foundation wall which is 33' feet long, 7' below grade and is bowing about 1" to 2" horizontally in the middle. We have gotten various foundation repair folks to look at it and they have recommended either steel columns or carbon fiber to reinforce the wall to prevent further inward movement.

I was discussing the solutions with a friend who has a physics - but not engineering - background and he is curious why none of these solutions consider the weight and pressure of the house, and focus entirely on the lateral soil pressure. I looked at both commercial and engineering resources on the web, and almost all them fail to even mention the weight of the house as a consideration. Why is that so?

RE: Considering Vertical Pressure of House Structure on Foundation Wall

Obviously the weight of the house is not helping anything. But engineers tend to address the big gorillas in the room first. Bowing of a wall implies a moment load issue. These are a function of beam length and loads acting perpendicular to it. The lateral soil pressure will act as a fluid load of about 35 pounds per cubic foot. The house probably weighs about 300 lb. per foot. But it hopefully acts as a vertical load, bearing on the wall and not contributing to the moment. This actually helps things (concrete or block in compression is in their wheelhouse), until the deflection gets pretty large. Then these are called secondary effects. Most foundation repair firms don't have the sophistication to consider them. But they're rarely an issue. And if the deflection was large enough to impact the wall, you'd wouldn't be living in the house.

RE: Considering Vertical Pressure of House Structure on Foundation Wall

since you said it is bowing...
The downward pressure of the house would help explain a part of why the top is restrained. Simply speaking... Walls prefer to freely topple over than bow in the middle and only bow when something is pushing the opposite way from above (and below at the slab or footing restraint point)

There is a debate about whether you pin the top of wall with the floor diaphragm or get to count down-pressure/friction on the sill plate.... but that is a different thing all together. most egrs i know design the top of wall to be restrained by the joists/blocking of the floor and then enjoy the downpressure as additional uncomputated safety factor.

i don't see how "steel columns" you mention would help unless they are actually being used as Buttresses, while looking like steel columns. i'm skeptical about the carbon fiber you mentioned too, but give it the benefit of the doubt, and i can't trash the idea without understanding how it would be used here.

There are probably a bunch more things that could be done, but the real answers require site observations to pick an optimal & functional approach that satisfies Owner needs in exchange for money.

RE: Considering Vertical Pressure of House Structure on Foundation Wall

(OP)
Thank you both for your respective responses. I think both warrant some further details from my end:

  • The wall is cinder block and the bow/crack is around the frost line. We acquired the house recently and unfortunately do not know how and when the bow occurred. It definitely hasn't gotten any worst in the last 12 months based on naked eye observations.
  • - The "goal" is to simply prevent the wall from bowing any further and be able to support the house going forward. It seems to be doing the latter for the last 60 years.
  • Multiple companies offer both i-beam and carbon fiber solutions, essentially as follows:
    • Steel Columns / I-Beams - These are placed 4 to 6 feet apart and bolted to the foundation slab at the bottom and one or two floor joists at the top. The beams sit right on the wall and are supposed to push against it. Some companies claim that these beams can overtime fix the bow, with periodic tightening of the top bolts.
    • Carbon Fiber - Strips of Carbon Fiber 6 to 8 inches in width are epoxied to the entire height of the wall and placed 4 to 6 feet apart. Depending on the provider, they are bolted at either or both ends (the top end to the rim joist framing). Most providers only do Carbon Fiber applications in cases where maximum bowing is 1" to 2". Carbon Fiber is touted by all providers to be "10 times stronger than steel i-beams."

RE: Considering Vertical Pressure of House Structure on Foundation Wall

CMU? It may not be reinforced sufficiently, if at all. I recommend engaging a structural engineer. It may have never been designed by an engineer, and if it retains soil, it should have been.

Mike McCann, PE, SE (WA)


RE: Considering Vertical Pressure of House Structure on Foundation Wall

Any pictures? I would be interested to see what this looks like, visuals are always helpful.

RE: Considering Vertical Pressure of House Structure on Foundation Wall

(OP)
Here are pictures of the wall in question: http://imgur.com/a/btVQf

RE: Considering Vertical Pressure of House Structure on Foundation Wall

(OP)
And no it is no reinforced as far as I can tell. It is below grade between 8' and 6.5'.

RE: Considering Vertical Pressure of House Structure on Foundation Wall

I see this all the time in our area. The steel columns (or soldier beams) are the most common solution. They work better than the carbon as they attach directly to the floor joists instead of counting on the existing wall to floor connection (if one exists other than friction). Do your floor joists run parallel or perpendicular to the CMU wall?

FWIW, I recently was on a job where the contractor installed the carbon strips over a painted CMU wall. Needless to say, they did not stick.

RE: Considering Vertical Pressure of House Structure on Foundation Wall

(OP)
Thanks ExcelEngineering.

I have been leaning towards Carbon Fiber myself because of it's disappearing profile. And yes,any reputable provider should be grinding the entire height of the wall before epoxy'ing the carbon fiber strips (or so I've learned). The provider that I am likely to go with does actually bolted at the top to the rim joist.

And the floor joists run perpendicular to this wall.

Do you prefer the steel columns because they support the house up in addition to supporting the wall against lateral pressure? While the carbon fibers only add support to the wall?

RE: Considering Vertical Pressure of House Structure on Foundation Wall

Crack gauges and observation at a minimum... Better would be to follow Mike's advice and get a local engineer's help.

RE: Considering Vertical Pressure of House Structure on Foundation Wall

Steel columns are generally preferred because they are cheaper. Almost any contractor can install them without any specialized training.

Perpendicular is ideal. Generally how it works is they break out a portion of the slab adjacent to the wall directly below a floor joist. The steel column (I generally use either HSS tubes or angles, prefer tubes but some contractors fight for angles) is then placed in this hole on top of the existing footing and is bolted to the floor joist above. The hole in the slab is then filled with concrete. Any gaps between the wall and the steel should be grouted with some form of "non-shrink" cementitious grout.

The size of the steel columns and connection to the floor joists should be sized by an engineer.

I have never attempted to removed the bowing from the wall but rather just stop any further movement. If the movement is so bad that they feel they need to attempt to push the wall back out then in my mind it's usually time for replacement.

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