Settlement - New 2 story Home & Analysis of Wall Foundation 3

[PT999]

I am looking at new home, with cracks in gypsum wall board, windows and door binding up, cracks in basement concrete floor slab, but no crack visible in 8" concrete foundation wall.

Still, I am concluding settlement of the wall foundation. How can I prove it conclusively. How can there be settlement without a crack in a foundation wall?

Secondly, regarding the unreinforced concrete wall foundation. I suspect the wall is to thin for the wall height which is 1 foot higher than the usual 8 or 8.5 ft (Total Height is 9.5 ft, concrete is supposed to be 3000 psi, will be tested shortly because of the cracked floor slab)

Exactly how is unreinforced concrete analyzed. Naturally all my texts and meager skills are for reinforced concrete design.

 

[Focht3]

Hmmm,

Where is this home located? Is it common practice to use unreinforced concrete for basement walls? (Seems like a very bad idea to me.)

Aside from the cracks, why do you suspect settlement? Are the floors uneven, or are the exterior walls cracked?

Other construction defects can cause drywall cracks; you have to systematically evaluate the structure, eliminating the possible causes as you go -



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[PEinc]

PT999,

Since you do not know how level the footings and top of basement walls were when originally constructed, you probably can't determine how much the house has settled from looking at the footings or basement walls. You might get a better idea of settlement if you check how horizontal the window sills are, how plumb the the door and window jambs are, or how level your ceiling is. These things are usually installed level or plumb.

Binding of doors and windows is usually a very good indication that a building has settled differentially. The doors and windows bind because they want to stay rectangular while their openings become racked into non-rectangular parallelograms.

I would have expected the concrete walls to have cracked also. Is it possible you didn't notice cracks in some obstructed corners? Is the basement made up of individual, separate, but abutting areas and that one area has settled differentially but the house walls above the settled area are still framed back to and restrained by the walls above a not settling basement? Something has to be restraining the upper house horizontally while the basement settles vertically, causing a racking action.

 

[pmkPE]

1) potential causes for cracks in gypsum board, other than settlement, could possibly be due to improper spacing of boards followed by thermal expansion, or even very slight movement in the structure.
2) Is the home built partially or completely on backfill? Is there cracking developing in the soil surrounding the residence? Could the structure possibly be sliding, or could a nearby hill be moving towards the structure? Movement does not have to be only in the vertical direction.
3) Are the widows binding in the out-of-plane direction with brick facade? I have found that when this occurs the brick was not adequately fastened to the framing as required by code. It tends to bow out and pull the window frames with it, causing binding;
4) Unreinforced concrete walls - do you mean unreinforced masonry (concrete) block walls (URM)? Often times residential construction utilizing CMU does not use vertical reinforcement, but should have horizontal reinforcement. Solid concrete walls have negligible tension capacity, but it does have some, and being solid has more than URM. If it is unreinforced concrete you can perform an analysis using the limiting cracking resistance of the concrete; ask questions - talk to the contractor, ask to see the building plans, talk to the building inspector(s) the county, municipality, city or town that is responsible for inspecting the residence during the progressing stages of construction....etc.
5) Since you strongly suspect foundation settlement, but do not have cracks for which you could install crack monitors over you may want to set a benchmark nearby and monitor select locations of the residence for continuing settlement over time - say three to six months. Do not feel pressed to come up with a conclusion too soon; you are not obligated to give an opinion without having reasonable evidence to back up your opinion.

 

[PT999]

Thanks all for your advice.

Location is Long Island, NY, where all the homes I have seen have plain concrete (unreinforced) foundation walls (even my own). (This home is not cmu foundation wall which is relatively rare here).

Are you all telling me that elsewhere in the world (lets just say the US) , foundation walls for single family homes actually use reinforced concrete.

Since my first posting I have reviewed unreinforced brick and cmu masony construction design (ASD) to see how sizing is figured, and for an allowable stress for tension in the plain concrete wall, used 1.6 x (sqrt (compressive strength fc')), which equals 88 psi for 3000 psi. This is from a text referring to ACI 318-63 (is this still correct or did it change in 40 years? - at least the code is still called ACI 318)

Turns out for the 88 psi allowable tension stress,8 inches thick just makes it for 8 ft high wall. Higher walls are overstressed.

I have seen a few wall blow-outs over the years in areas where the hydrostatic pressure we all design for theoretically was really actually there, along with most low strength concrete!

I am going to have a second look for where the window, door and wall problems are in relation to the geometry of the house, and see if there are some cracks in foundation walls at corners or other areas.

 

[Focht3]

We use reinforcing in our basement walls. But then again, we don't have many basements in central Texas...


What are you assuming for earth pressures? Do you really use eight inch thick unreinforced concrete walls for your basements? I don't even allow grade beams that are that narrow...



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[DaveAtkins]

Chapter 22 of ACI 318-95 (and probably newer versions as well) covers the design of unreinforced concrete, which is generally perfectly fine for basement walls. The ultimate tensile strength works out to be 178 psi for 3000 psi concrete. A 1.7 load factor is appropriate for lateral soil pressure, resulting in an allowable stress of 105 psi. I recommend using just 30 pcf for the lateral soil pressure, because the wall, as it deflects, will allow the soil pressure to become active.

DaveAtkins

 

[Focht3]

I recommend using just 30 pcf for the lateral soil pressure, because the wall, as it deflects, will allow the soil pressure to become active.

I strongly disagree!



It is very foolish to use a soil equivalent fluid weight that is less than half of the fluid weight of water - unless you are damn sure that a perched water condition cannot develop at that site and the soil is a clean, coarse, angular rock. That usually means the home must be built using wall and subfloor drains - which are a hell of a lot more expensive than a little rebar and labor.

(In areas where the water table is regularly above the basement floor level, these details should always be employed.)

The amount of movement that can result can be very disconcerting to a homeowner. And aren't the basement walls loadbearing units?



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[pmkPE]

Once unreinforced walls start to deflect and continue to bend, then they crack. Then what happens? After the cracking the tensile srength is greatly reduced and what little tensile strength there originally was is gone; the original section is gone and only a small area is left to offer resistance to the tension. This can and has led to failure. Soil types and potential ground water conditions determine what the lateral soil pressures are and could be for active and passive conditions.

Just as soil boring taken 50 feet apart can reveal totally different soils and conditions it is not prudent to make assumptions that contradict what we have learned from not only our eduction but also trial and error over the ages. Pick up any book on construction failures and you will find several case examlpes of failures involving soils and foundations.

It is easy to make assumptions. BUT It can be very costly and can and BE extremely dangerous to make assumptions that can not be backed up with evidence that the engineer of record had examined the specific conditions for their designs. This can be as little as talking with the local building inspector or the local governing authority having jurisdiction for construction. Review the soil survey for that area, the (formerly) Soil Conservation Service publishes surveys of soil commonly found throught the US by County. It will give a good indication of potential for high ground water elevations, the type of soils, permeability, suitability for construction of basements, vs. slab on grades, etc. Taking these (pro)active measures are known as the (minimum) standards of care that we as engineerrs are required to follow. You don't want to find out during trial when the plaintiff's attorney questions you on the stand as to what facts you base your design on. Pulling things out of the air will not satisfy anyone.

 

[JAE]

Focht3 - I would guess that the 8" wall is situated on a concrete footing, perhaps 16" wide at least -this is what's usually done "north of the Red River".

 

[Focht3]

It gives me the "willies"...



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[DaveAtkins]

The Wisconsin Uniform Dwelling Code (and I assume other State Codes for residential construction are similar) specifically allows an unreinforced 8" concrete wall to span 8'-0" vertically. I dare say thousands of single family homes are built this way, with only shrinkage cracks, not flexural cracks, occurring in the wall. I once did the math. If one checks an unreinforced 8" concrete wall for 8'-0" of backfill, using ACI 318-95, Chapter 22, the equivalent fluid pressure MUST be 34 pcf.

DaveAtkins

 

[Focht3]

Okay, in glacial tills you have a reasonably free draining soil that can be subangular - 34 pcf may well work for that condition. But not every part of the U.S. is covered by glacial till, with sustained "deep" groundwater - so this unreinforced wall approach has limited applicability. It isn't appropriate everywhere...

Long Island is subject to Nor'easters and an occasional hurricane, as well as hard rains. High tides can be a problem - so assuming that a basement will always be above the water level isn't appropriate for all of Long Island -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[DaveAtkins]

Good point, Focht3. I wonder if residential basement walls are designed differently in different regions of the country.

DaveAtkins

 

[pmkPE]

Yes, they are.

 

[craigory28]

PT999 as far as cracking in the basement slab were there any control joints? My house has few to none and there are shrinkage cracks in several locations. I know houses here (WI) only usually get a 3" slab. (That is what called out on plans and they might not even get that.) Our firm typically puts joints at 3 times the slab thickness to prevent cracking. I also know it is commmon to go up to 4 times. That would mean at most you should have them every 12'. I haven't seen too many houses in my day that have that.

You think an 8" plain concrete basement wall is scary? Many houses in this area have the basement walls out of 8" unreinforced block. (It seems newer construction is going to cast.) My parents house which is over 40 years old is block and was back filled with clay. Ten years ago they had to hire someone to push the walls back plum. Which involved excavating the backfill and replacing it with a granualar backfill.

Friends of mine experienced severe settlement in a corner of their house this past summer. They had a large tree at the corner of their house. It was a very dry summer and the tree sucked any and all moisture out of the ground it could. The drop in the water table caused the corner of the house to settle.

 

[denoid]

I just happened to be looking in the IBC 2000, ran across Section 1805.5, and Table 1805.5(1), and remembered this thread.

Check it out.

 

[pmkPE]

denoid
You may want to turn the page and take a look at the reinforcement requirements for the same thickness walls for 30 PSF, 45 PSF and 60 PSF of unbalanced backfills.

Q: As an example - Why does the code allow for unreinforced masonry/concrete to be 8" nominal thickness (Plain masonry and Plain concrete foundation walls) for a wall height of 7 feet having unbalanced backfill for 30, 45 and 60 PSF active soil loadings as shown in Tables 1805.1 and then on the following page has Table 1805.3 for 8-inch reinforced concrete and masonry foundation walls (where d >= 5 inches) for the same wall heights and active soil pressures?

 

[denoid]

pmkPE:

I agree - it doesn't really make sense. Good example of why to do your own calculations to satisfy yourself.

I was once asked to be on an "Ad Hoc" committee of several structural PE's to review a proposed change to our state code to add some similar tables for reinforced masonry in residential applications. We all did our calc's for various wall heights, reinforcing, and various types of backfill with varying lateral pressures, and then compared notes before making a formal recommendation of a table to the state code committee.

The meeting before the state code committee was also attended by lobbyists for the concrete industry, who felt threatened that inclusion of such a table in the code would take away from their business, since, in this location, concrete was the preferred method of constructing foundation walls. Bottom line was that the state code committee "comprimised" by taking our recommendations and beefing up the reinforcement to get the concrete people off their backs.

Therefore, I am now usually sceptical when I see such generic tables in any code, and I always check them myself on jobs of any importance.

 

[PT999]

Thanks everyone, this is a record number of postings.

I had the basement concrete slab core drilled and tested or compressive strength. Results came in at 4000 to 4100 psi. This was supposed to be 3000 psi concrete. How did that happen? Not enough water I suppose. No way did this contractor order a better mix. Turns out, both the vapor barrier and WWF called for on plans were not there.

So now that the concrete compressive strength turned out OK (at least for the slab), I am figuring the concrete wall foundation compressive strength is OK too, at least 3000 psi or better (It's just too difficult to core horizontally, unless there is a compelling reason to.)

Now the final question is what equivalent fluid value to use. Soil conditions, who knows, grass looks lovely. One post suggested 34 pcf for Long Island soil, Anybody else have a comment on this. Someone else's post said to considered perched water, thus 62.4. If any one can advise on values for various soils around here and in New York City, that would be great.