IRC Foundation Wall Reinforcing 3

[SteelPE]

In the 2009 IRC table R404.1.2(1) has requirements for horizontal reinforcing for poured in place concrete foundation walls. The requirements are as follows:

wall height less than or equal to 8'-0" 1-#4 at the top of the wall and at mid height
Wall height greater than 8'-0" 1-#4 at the top of the wall and at the 1/3 points of the wall height.

What is the purpose of this reinforcing? Is it to limit foundation cracking that tends to occur in foundation walls?

 

[KootK]

I wonder about this too. My guess is that it's something to do with constructability. Holding the verticals in place during pouring or giving workers something to grab on to. I'm skeptical that such a small quantity of reinforcing would do anything meaningful for crack control.

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http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[jayrod12]

It may slightly aid in keeping shrinkage in the longitudinal direction in check. It's not enough to stop it, but maybe keep it manageable? that's the only other explanation I could ever come up with.

 

[JAE]

It's meaningless in terms of shrinkage in my view.

We've seen all sorts of residential basement walls crack wide open with foundation settlements despite the bars.

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[SteelPE]

Well, this table is located in the residential section of the code. It appears to be somewhat new (maybe added in 2009?). This is a table you use when you have no vertical reinforcing in a basement wall. We were alerted to the requirements, and have tried to comply with it (we typically provided 2-#5't top and bottom as a standard detail). I was just wondering what the specific purpose is.

 

[hokie66]

No vertical reinforcement in a basement wall is silly, and horizontal reinforcement does nothing if there are vertical control joints.

 

[bones206]

Maybe it’s to provide some nominal catenary steel so the wall can span horizontally if it has to. None of the foundation walls being constructed in my neighborhood have vertical bars. But they do have the horizontal #4’s.

 

[jayrod12]

No verts? That sounds ludicrous.

We get a ton of calls to fix walls with no verts. And our repair detail is vertically oriented angles connected to the basement slab and the floor joists.

I've attached our local codes foundation wall spec. Even though it's almost nominal reinforcing, i.e. #3 @ 36" o/c, but it's better than absolutely nothing.

 

[SteelPE]

Don't talk to me about no verticals in basement walls, talk to the IRC. It's allowed under certain conditions which are routinely met with our clients. Typical basement wall construction here is 10" thick unreinforced concrete. Don't believe me, dig into the IRC and see for yourself.

Do I like it, no. Do I use it, yes, because it's allowed and if I don't then the clients will find someone else who will. We are talking about something that is specifically allowed in the code.

 

[jayrod12]

Oh I can appreciate that. You unfortunately have to do what everyone else is doing within reason to stay competitive.

 

[XR250]

I normally call out #4 @ 18" O.C. each way (assuming the numbers work out). if they complain, I change it to "basement wall per code" and let them take the heat.

 

[bones206]

I'm just now branching out into residential design after years of mostly heavy industrial, so I need to start getting familiar and comfortable with bare minimums. Not an easy adjustment.

Jayrod, I noticed the diagram you posted has a maximum wall length of 12 meters. Is that an indication that the code is considering two-way action to some extent?

 

[jayrod12]

I'm not entirely sure, I believe around here if you exceed 12m you have to have a P.Eng design your wall. However if you meet the prescriptive requirements of the code, you only need an engineer to seal the foundation elements in contact with the ground (i.e. footing, pad or pile sizing).

 

[dhengr]

Bones206:
‘12 metres in length’ and 12 metres in height, are two completely different animals.

 

[bones206]

I'll try to clarify: Basement walls are typically designed to behave as vertical cantilevers or propped cantilevers, correct? If so, why limit the length of the wall when you are designing a vertical span?

My guess was that the code is baking in a secondary mechanism (wall spanning horizontally), for general robustness of prescriptively-designed walls. If that's true, then it might be one reason for the horizontal bar requirement. But again, the residential code is not familiar territory for me and these are just my theories and guesses.

 

[SteelPE]

bones,

I believe we are talking about two completely different codes. The original post was dealing with the International Residential Code used in the US. The detail jayrod posted would be used somewhere else. Also, I am not a concrete expert, but I doubt at with an aspect ration of 5:1 +/- they are relying on 2 way action.

 

[GWoodPE]

The commentary for the horizontal reinforcing section of ACI332 (Residential Code requirements for structural concrete) mentions the horizontal reinforcing is used to reduce cracking for temp and shrinkage basically. They are similar to the IRC horizontal reinforcement requirements. My opinion is that it will help hold the wall together after it cracks and provide some minimal additional ability for the wall to span horizontally both like a grade beam and a 2 way slab.

You also have to remember the IRC is not always based on actual physics and engineering. (Though with each revision that changes a little) Many of the prescriptive requirements are just based on what has seemed to work in the past. Sort of "that's the way we've always built it before" That is also one potential reason for limiting the length of walls and overall size of the houses. Another is floor diaphragm loads, if the wall is spanning vertically, then the floor diaphragm has to take more load the longer the wall is.
There are a lot of factors of safety on the loads and strengths that the residential code sometimes seems to say, we don't need.

I'm also curious how others interpret that table. The table specifically says basement walls, but it is referenced in section R404.1.3.2 which is reinforcement for foundation walls. I typically would require the horizontal reinforcing in all foundation walls, including garage foundations, slab on grade, and crawlspace foundations because they are foundation walls also, but the table does say basement walls.

 

[WARose]

One thing to remember about the IRC when it comes to basement walls is: it's built around certain soil types. I've also suspected that it may be built around a at-rest pressure (with the first floor diaphragm "engaged" and getting it to shear walls). But, by it's own prescriptive nailing pattern, I checked that once and found myself questioning that theory.

 

[hokie66]

Seems to me the best thing about the International Residential Code is that it is not International. For those who are bound by it, commiserations.

 

[dhengr]

Hokie66:
Mostly it is the DIY’ers., lumber yard designers and small builders who deal with IRC on a regular basis. But, then sometimes we engineers are called on to justify their craziness, when they get in trouble with it. More often than not, I’m called in by the AJH, the Courts, etc. to show them the errors of their ways. The IRC is prescriptive, this has worked for years, so why change it, except where it doesn’t work any longer, then leavened slightly by some real engineering judgement.