How Do Residential Basement Walls Stand?
How Do Residential Basement Walls Stand?
(OP)
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?
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?






RE: How Do Residential Basement Walls Stand?
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)
RE: How Do Residential Basement Walls Stand?
RE: How Do Residential Basement Walls Stand?
But...in reality, I think when you add the "general public" to the commercial equation, your Factors of
safetyCYA 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.
RE: How Do Residential Basement Walls Stand?
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.
RE: How Do Residential Basement Walls Stand?
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.
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.
RE: How Do Residential Basement Walls Stand?
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.
RE: How Do Residential Basement Walls Stand?
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RE: How Do Residential Basement Walls Stand?
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.
RE: How Do Residential Basement Walls Stand?
RE: How Do Residential Basement Walls Stand?
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.
RE: How Do Residential Basement Walls Stand?
Mike McCann, PE, SE (WA)
RE: How Do Residential Basement Walls Stand?
RE: How Do Residential Basement Walls Stand?
Plus, our safety factors limit collapse to being such a rare event that even things less safe that that will still rarely end catastrophically.
RE: How Do Residential Basement Walls Stand?
Certainly the 6 ft. O.C. anchor bolt capacity has to be adequate
RE: How Do Residential Basement Walls Stand?
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!)
RE: How Do Residential Basement Walls Stand?
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.
RE: How Do Residential Basement Walls Stand?
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.
RE: How Do Residential Basement Walls Stand?
http://www.soilstructure.com/
RE: How Do Residential Basement Walls Stand?
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).
RE: How Do Residential Basement Walls Stand?
Oh, and I do not design with i-joists or floor trusses (thanks to all my forensic work) unless the client begs for them. Most of my clients dislike them and prefer 2x10's and steel beams.
RE: How Do Residential Basement Walls Stand?
They completely back filled against the wall without have the first floor installed. This is a 10 ft high retaining with 0 vertical rebar and they have the nerve to drive trucks very close to the wall.
I was watching the show in amazement that wall still worked link can be found here.
Episode of the wall being poured: http://www.pbs.org/video/2365707644/
Episode of the backfill and driving the trucks very close: http://www.pbs.org/video/2365713348/
RE: How Do Residential Basement Walls Stand?
Mike McCann, PE, SE (WA)
RE: How Do Residential Basement Walls Stand?
I see that alot in articles from JLC and Fine Homebuilding. Ironically, they are usually written about jobs that occur up north as well. Must be a cultural thing
RE: How Do Residential Basement Walls Stand?
RE: How Do Residential Basement Walls Stand?
Looks like 10 inch walls and 9 feet tall to me (counting lines on forms and guessing it's a 2x6 sill plate with about 4 inches of concrete showing).
I saw a couple of bars in there. Maybe they put 2 bars top, middle and bottom. Not enough to qualify as "reinforced concrete" but still, it is something. Still... looks like they would have just stopped the top bars in (along with "wet sticking" the ABs).
Looks like longest walls are 24 feet between offsets (counting forms and guessing based on AB spacing). We usually don't see problems until we get over 30 ft long and 40 ft and 5 ft walls are def bad actors.
Rock "ledge" at the site seems like it will help prevent much affect from the trucks onto the basement walls.
I'd prefer deeper gravel backfill and rigid pipe instead of corrugated though.
Just some thoughts
RE: How Do Residential Basement Walls Stand?
* Use ACTIVE, not AT REST soil pressure (I know, it doesn't fit our usual assumptions, but even the commercial IBC allows this for short basement walls).
* Use 7.5 times the square root of f'c for the modulus of rupture of concrete. And don't use the 0.6 phi factor for plain concrete design.
My point is I totally agree with TLHS--our conservative assumptions make it look like an unreinforced concrete basement wall won't work.
DaveAtkins