How Do Residential Basement Walls Stand? 4

[BSVBD]

For years i've been designing restrained basement walls for commercial construction with rebar at 12" oc EW as an example of the typical norm.

So why do residential basement walls not require rebar like commercial structures do? How is this possible?

I've heard a residential contractor say, "We don't do that in residential construction." So how do natural forces know whether or not to act dependent upon building occupancy?

 

[msquared48]

Smart earth technology.

Actually, the IRC is much more lenient than the IBC. And, yes, considering the quality of construction I have seen recently, I too do have a problem with that.

Mike McCann, PE, SE (WA)

 

[Archie264]

I think it becomes an issue of keeping the resultant force within the kern. It's not something I'd want to have to rely on but that might be what's going on with some of them.

 

[Guastavino]

The residential structures "knows" that it is following the IRC. Meanwhile, the commercial building "knows" it's a commercial building so it acts like one. Soil pressures realign and concrete refuses to follow 0.0018 temp/shrinkage steel requirements.

But...in reality, I think when you add the "general public" to the commercial equation, your Factors of safety CYA start to jump in. And not just life safety, but getting sued and the like. Business people realize there is liability in the bare minimum, albeit not all of them.

As an aside, I'm designing my own house right now and I just got into looking at basement wall criteria recently from the IRC and I was floored. I dared to show a 3'-0" wide footing on my drawings for a 2-story with a basement and brick veneer and they contractor said "I don't do that"...and then accused me of being a commercial engineer. He did it nicely, but he was right.

Now, with that said, I didn't ask him how many houses he has built that he visited 30 years later, but I'm betting that if he did, he might see things differently.

So back to the OP question, I think it just reveals that house contractors can lobby in the codes more effectively than commercial ones. It's not surprising because the IRC is people's personal money. The IBC is business money which isn't always quite as personal, so building owners are usually more lenient to spend the money and won't nickel and dime as much. I'm guessing that means the code gets implemented more for commercial.

One last thing to close this rant. Speaking as a commercial engineer, I can't get myself to do residential work because I feel like I'm divorcing my reason and engineering judgment for prescriptive magical load paths through "structural air" (not being cynical here, I just literally can't get myself past the lack of load paths, I just feel like a liar when designing that way). Meanwhile, similarly, I feel like residential contractors can't divorce their bottom line. I had a contractor tell me this week that was quoting my house "I don't even know where to get that". The "can't do" attitude of residential contractors seems universal in my limited experience. I think it's because most of the public is ignorant and so they can do what they want. So when you get a commercial engineer building a house, they don't understand you, and you don't understand them. It's not a negative, it's just something that communication skills are required to work through. Because it's like we speak two different languages.

 

[allgoodnamestaken]

I've heard a residential contractor say, "We don't do that in residential construction." So how do natural forces know whether or not to act dependent upon building occupancy?

Residential concrete contractors are a circus. If you get the minimum its a best case scenario. They just don't need the precisiou and care commercial concrete contractors do, because residential guys don't have an owner's agent checking their work and nobody asks them why they put in less reinforcement than the drawings asked, or why nothing is flat or there isn't a 90 angle to be found on anything.

Because their work is in general garbage.

 

[KootK]

I can't get myself to do residential work because I feel like I'm divorcing my reason and engineering judgment for prescriptive magical load paths through "structural air" (not being cynical here, I just literally can't get myself past the lack of load paths, I just feel like a liar when designing that way).

Yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes. Can I come work for you? I'm cheap. But then I screw around on eng-tips all day.

So why do residential basement walls not require rebar like commercial structures do? How is this possible?

1) Soil loads rarely actually come to fruition in my opinion. I've seen walls and shoring removed where 30' of "fluid" soil just sits there with a 110 degree angle of repose somehow.

2) Residential basement walls typically have a lot of returns in plan which allows them to span horizontally. This seems to be becoming less true over time somehow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

Kootk,

Until it rains a lot and then it slides to the bottom. The idiot builder excavated on a boundary on my new house for only a 6' retaining wall and left it for about 3 weeks while he got organised to build the retaining wall, then it rained! Neighbour was not impressed but got a revamped paved area and garden and fence out of it! Looks much nicer now.

 

[JAE]

I've seen dozens of homes where basement walls are cracked and water leaking in. The builders "may not do it that way" but single family residences do get easily damaged, cracked, settled, etc. much more often than commercial structures.

Check out Eng-Tips Forum's Policies here:
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[KootK]

If somebody who knows what they're doing runs "by the book" numbers on the wall to floor diaphragm connection, you'll be lucky to find that you've got as much as 1/4 the capacity of that required. Often there's a perp to grain issue in the load path somewhere. It's ugly stuff.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[oldestguy]

So far no one has mentioned the work of house basement repair contractors. They would be out of work if all basements were engineered and there never was any saturated ground or frost action from the side.

 

[KootK]

Do some basement residential basement walls become distressed? Sure. Is that a large proportion of all residential basement walls, particularly those built in modern times? Not at all. OP's question was why residential walls, in general, do not fall down. I think it's an important question that deserves a real answer.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

I've seen some residential foundations poured with more water than concrete... So the concrete would level out. Yes, being level is the highest priority here...

Mike McCann, PE, SE (WA)

 

[Ron]

If you look at an engineer and a contractor with "20 years of experience", you quickly note that the contractor has 1 year of experience, 20 times. They rarely see the long term problems because they skate after their 1 year warranty. We deal with the resulting problems.

 

[TLHS]

We're conservative on the soil side and we're conservative on the concrete side. Concrete codes are (rightfully) incredibly conservative on the rupture strength of plain concrete. We also tend to (still rightfully) assume some very bad worst case scenarios for soil loads. This leads to scenarios where things that don't calc out in the conventional way are still plenty strong.

Plus, our safety factors limit collapse to being such a rare event that even things less safe that that will still rarely end catastrophically.

 

[XR250]

If somebody who knows what they're doing runs "by the book" numbers on the wall to floor diaphragm connection, you'll be lucky to find that you've got as much as 1/4 the capacity of that required. Often there's a perp to grain issue in the load path somewhere. It's ugly stuff.

Certainly the 6 ft. O.C. anchor bolt capacity has to be adequate

 

[Guastavino]

Can I come work for you? I'm cheap. But then I screw around on eng-tips all day.

Haha, but then I'd have to pay for your insight instead of getting it free on Eng-tips!

And back to the OP, again, I just think it shows how conservative things are. I posted something in another thread recently about a 100 year old building I'm helping renovate. Previous contractor took out a column YEARS ago. It was supporting two girders and the transfer girder supporting them. It had only sagged down about 1.75 inches and was still standing after years of service. Literally, the plaster ceiling was holding up the transfer girder. The plaster was bridging to adjacent spans, etc. (as was the flooring above too). Point is, alternate load paths that are complex beyond our wildest dreams occur as well, same holds true for basement walls in houses. But, I'm still betting if houses were tested to the design loads in labs, we would see the code requirements change when issues started popping up. Luckily, the loads are conservative, and then again, so are our assumptions (like assuming plaster isn't load bearing...when I have proof it is!)

 

[bookowski]

This is a good question. I don't do this type of work but was recently fooling around with trying to get into it, thought it might be interesting. I found 2 things: the fees were really low and using irc felt like it required suspending logic and becoming more of a bookkeeper than an engineer.

In general soil loads seem very conservative. Ever design something with 3 or more basements. The slabs are resisting 40k+ axial load per foot in addition to bending??

This brings up a broader question about how much accumulated conservatism there is in what we do. I think structural engineering is pretty unique in the 'engineering' sense in that it's still very rough. If you had a time machine and grabbed an engineer from 1960 and plopped him at a desk now he would recognize basically all the materials and principles and could roughly design the same thing that you would, the major difference would be using computers to come up with a bunch of simplifications rather than by hand. Not sure that would hold true for most other fields - take an automotive guy from 1960 in your time machine and see what he comes up with.

 

[racookpe1978]

If (when!) a residential basement cinder block wall cracks, then "bends" then has a few joists "sag" the owner either doesn't notice, doesn't care, or "I'll do it later" ...

Commercial owner needs customers to come back regularly. Feel safe inside, or have no choice about coming back (the "old hardware store in a small town that hasn't been fixed since 1890". The commercial owner IS a target for regular inspectors and hygiene (restaurants, food stores, groceries) and fire inspections (hotels, stores, public access buildings.) Lawsuits and business impressions probably biggest threat commercially.

We "do" overdesign though - just like OSHA over-protects. Up to the point where some idiot is more stupid than the design.

 

[FixedEarth]

A bit of cohesion in the soil, some wall jogs, and less than 8 ft backfill height works in many cases. But when the temperature drops 30 Deg., or you backfill with soft cohesive soils or you have no wall jogs, then the stem wall will crack. It is a chance many spec. builders take.

http://www.soilstructure.com/

 

[HouseBoy]

I look at these conditions often. A couple of things come to mind;
Plain concrete in bending DOES have an allowable capacity per ACI Fb=.65 x 5 x sq. rt. f'c
Even so, it's tough to get things to "figure" on paper.
I agree that the 1/2" AB at 6 ft o.c. has very little capacity.
Probably the biggest factor contributing to residential basement walls "working" is the offset of the walls. I say this because one of the most common conditions where we see failure is in long straight walls.
I think soil pressures generally take time to materialize and cohesive soils have a great ability to stand up on their own (once the foundation wall moves away slightly).