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Rebar Question


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By Marty in Boston, MA on 11/20/2007


Can someone please explain to me how rebar works? I know why it is used (i.e. the lack of concrete tensile strength), I just don’t understand why it works. Is there a big difference between #4 rebar and #5 rebar for increasing concrete tensile strength? Is there a formula that is used? 

 

My footings are going to be 24”x12” (foundation walls are 10” thick). Per my request, the foundation plan calls for three runs of #5 rebar. I did this based on the assumption that more rebar and thicker rebar is better. The foundation companies I have called for estimates have told me that this is overkill. They are suggesting that the footers be decreased to 20”x10” with two runs of #4 rebar. I called a local steel company and learned that increasing from #4 to #5 rebar and from two runs to three runs increases my steel cost by approximately $300. Assuming that the footers will be “much stronger,” I would be willing to spend the extra money. On the other hand, if there is no (or very minimal) difference, then I would rather spend the money elsewhere.

 

Any help will be greatly appreciated.

 

Thank you

Marty


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By Kenneth in Lees Summit, MO on 11/21/2007


Marty,

Good question, I don't think I have seen anything similar asked here before. For rebar smaller than #8, the number signifies the diameter of the rebar. To illustrate this, a #4 bar is 4/8" diameter, or 1/2" diameter, a #5 bar would be 5/8" diameter, a #6 would be 6/8" diameter, and so on. Once you get larger than #8, there is a different sizing rule.

Exactly what are you hoping the rebar in your footings is accomplishing for you? Your footings are poured directly on a bearing surface, your walls are poured on your footings, so technically you will never have any concrete in your footings in tension (for a typical footing, which I am assuming you have please clarify if you do not). The intent of your footings is to take your roof and floor loads (including dead, live, snow, and wind loads), and spread these loads over a sufficient amount bearing surface.

Have you done any geotechnical testing to determine the size your footings should be? I poured my footings oversize (at least 24" wide, 13-15" thick to ensure a nice level footing) because it was cheap insurance, but I also used only two #4 because the steel really doesn't do much for you here.


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By Jon in Ellicott City, MD on 11/21/2007


Hey Marty,

It sounds to me like you need to do a LOT more homework. 

Don't get me wrong - I'm not posing as some kind of expert.  I regularly realize that I could have done something better "if I'd only known xxxx at the time."  We O-B's educate ourselves the best we can, but none of us know it all.

BUT... You can't begin the foundation work on your house with these kinds of basic questions lingering.  And you're going to cost yourself a fortune if you simply try to upsize every element of your project without understanding how each element functions.

My advice to you is to pay for an online subscription to JLC (The Journal of Light Construction) and read, read, read!!!!  They have extremely valuable articles on nearly every subject you can imagine, including several on footings. 

(I was going to copy and post one here, but I wasn't sure that was kosher since it's a pay-for-view site. If you subscribe, search for the article called "footing fundamentals."  It'll answer your rebar questions, plus a bunch of stuff you didn't know to ask.  I attached a couple figures from that article to this post.)

The online subscription is $30 a year, which is a bargain for the value get. 

Obviously, they aren't the only place to find this kind of info.  But, the quality, quantity, and diversity of information in this one place is impressive.  It's the first place I look for "how-to" answers.  IMO, this site should be required reading for O-B's.   

jlconline.com.storefront

P.S.  If your basement walls are going to be poured concrete, put your money in extra rebar in the walls.  A couple #4's in the footings will be fine.  Footing width should be determined from the bearing capacity of the soil and the size/weight of your house.  Footing depth should be approximately equal to the projection of the footing beyond the wall.  (For a 10" wall centered on a 24" footing, the footing will project 7" from each side of the wall.  So, the footing needs to be at least 7" deep.  Most people round that up to 10".)


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By Dave in Coarsegold, CA on 11/21/2007


Upping from #4 to #5 increases the strength by quite a bit.  Whether you need it or not and how wide your footings should be depend on your soil quality and the bearing weight of the structure.  These are questions for soils and structural engineers to answer for you.  Although it's clear that in many states people can do pretty much whatever they want, out here in California it's strictly regulated because of seismic issues.  Because my house is ICF (i.e., very heavy), I had the structure engineered and had a soils engineer test the ground.  The structural engineering was required, while the soils engineering was mostly for my own peace of mind.  Like you, I want to have a sturdy foundation that will never give me a problem. 

If you want to go with #5 for peace of mind, it's a small increase in cost.  Why not?  It's your house, so you get to make the calls.  I had my excavation contractor overdig the foundation in one area because it was softer soil than I was comfortable with.  It cost me a little more in concrete and steel but it was worth it to me. 

By the way, horizontal bottom reinforcement in the footers does indeed do something.  Imagine your continuous footing under the structure wall.  When bearing on homogeneous soils with unvarying bearing capacity it acts to simply hold the weight of the structure in compression.  But what if the soils underneath vary, and you have a weak point in the middle of the wall and settling occurs?  Now the footing acts as a beam, and that's why you need the horizontal bottom reinforcement. 


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By Kenneth in Lees Summit, MO on 11/21/2007


Good point on the beam on nonhomogeneous bearing surface. However this is not a typical scenario, at least in my area. A geotech investigation would show when and where that might be necessary.
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By Jon in Ellicott City, MD on 11/21/2007


Bridging across varying bearing capacity soil is why you want rebar in the wall

You're talking about putting your foundation (wall and footer) in bending.  Bending strength varies by the cube of the height, but only linearly by the material strength.  For that reason, your footers (being only a few inches thick) are nearly useless in this capacity. 

Put the same rebar in the wall and it will be 1,700 times as effective as it would be in the footer. 


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By Marty in Boston, MA on 11/21/2007


I want to thank everyone for their posts thus far – this has been very helpful.

 

Jon, I will agree with you that I am no expert when it comes to building homes and I obviously need to learn more about “how rebar works” – which is why I posted the question.  At some point I hope to give back to this forum by providing input from personal experience.  In the meantime, I will probably be mostly on the receiving end.

 

I’m not sure what you mean by needing to do a “LOT” more homework.  I have met with the building department and provided them with preliminary plans (which I drew using a CAD software).  Even though I am not building until May, 2008, I met with the building department to find out if I was on the right track.  I was informed that the plans were sufficient enough to obtain a permit.  I should also add that the roof trusses and floor plans were designed by the lumber company that will be supplying the lumber – I would not have been able to calculate loads, deflection and etc.  It also helped that the state where I will be building does not require an architect stamp.  So as far as the building department is concerned, I am good to go.

 

With that being said, in my original post I explained what the building department wants (which I assume is what meets “minimum” building standards).  What I want to know is does more and bigger rebar make the foundation better?  I tried to get input from the foundation companies, but all they can tell me is that what I am trying to do is overkill, without being able to explain why – so I posted the question here.  Now, if overkill means that I am adding no value, then why spend the extra money; however, if overkill means making something better, even by just a little, then why not weigh the cost vs. value options?  By the way, it’s not just rebar that I am doing this with.  For example, I upgraded the concrete from 3000 psi to 4000.  The lumberyard’s original plans called for TJI 360  - I upgraded to 560.  All of my upgrades amount to under $8,000.  This would probably be a foolish move if I were going to sell the house – but I am not, at least not for many years to come.

 

I guess my question should have been more specific.  I know what the various rebar sizes mean and I know what they weigh – I just don’t know how the different sizes differ in strength.  In other words, if #4 rebar improves tensile strength by “X” what does #5 improve it by?  What causes concrete to bond to the rebar and can it come apart (for example if the rebar rusts)?  Does it bond better with #5 as opposed to #4?  I am sure tests have been done to demonstrate this; I just don’t know where to look.  Will the JLC tell me this? 

In your last post you stated “Put the same rebar in the wall and it will be 1,700 times as effective as it would be in the footer.”  This is exactly the information I was looking for!  How did you come up with 1,700 times more effective?  Was it derived from a formula?  How many runs and what number rebar did you use for this calculation?  Are the runs top & bottom?

Lastly, I have been meaning to subscribe to the JLC for a while.  It seems like a no-brainer for the money - I will make a point to subscribe to it later today.

 

Thanks again.


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By Kenneth in Lees Summit, MO on 11/21/2007


Using the basic pi-r-squared forumla to figure a cross section of rebar, a #5 rebar will give you over 50% more steel in the wall.  Rebar in tension has a strength of about 35,000 lbs per square inch. However the location of the steel is critical to overall wall strength.

In Jon's drawing, it shows both pieces of rebar at the center of the footing at different heights looking at the footing down from the top. When I poured my footing, I put the rebar closer to the edges of the footing looking from the top down.  In the horizontal dimension, my rebar was about 3-4" above the bearing surface (maybe not quite that high), so it would be in the bottom-third of my footing. Generally you want the steel at the bottom of a reinforced concrete beam for strength, however if your intention is to reduce shrinkage cracking you might put it in a different location.

The bond, at least in part, is due to the deformed nature of rebar. If you tried to use smooth steel of equal diameter in place of rebar, it would contribute very little strength.

I also agree that codes are minimums, and you can substantially increase quality with little additional investment, and unless you are trying to maximize your profit from spec building is really a waste to not take advantage of this opportunity. You get to a point of diminishing returns though too. And just using better materials is really secondary to better installation. Here is a link to a very detailed build, there is a lot to learn at this site - imageevent.com/okoboji_images/deloreshouse.


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By Jon in Ellicott City, MD on 11/21/2007


I didn't sleep much last night.  Sorry if I came across harshly... I didn't mean it that way.  I only meant to paraphrase a saying from The O-B Book... "a thousand hours of research..." 

The bending stiffness and strength of a beam is linearly related to it's moment of inertia (I).  Double the "I" and you are twice as stiff and strong.  The moment of inertia in your concrete and rebar beam is basically linear with the area of the rebar. 

So.  Let's say you have a 12" thick footer with two runs of #4 rebar.  Going to #5 rebar will increase your steel area, and your overall stiffness by approximately a factor of 1.5  (0.625^2 / 0.5^2)  There are some assumptions there that a PE might nitpick, but it's close enough in this context.

Now look at spreading the rebar further apart.  (Here's where the apparently cocky/preachy guy has to admit he made an error.)  Increasing the distance from the neutral axis of the beam increases "I" by a factor of d^2  (NOT d^3!!!)

Let's say you put your extra steel in the base of the wall.  One extra #4 rebar in the bottom of an 8' wall.  That's 50% more steel, which is the same amount you would add by going to #5 in the footer, but it's approximately 4' from the neutral axis instead of 4".  48^2/4^2 is a factor of 144, times the area factor of 1.5, gives you an increased stiffness/strength of 216 times the original.

In other words... Put 1.5x the steel in the footer and you increase your bending strength by 1.5x.  Put the same amount of extra steel in the base of the wall and you increase stiffness by 216x. 

Footers are there to spread the weight out over a larger bearing area, not to bridge over soft spots.  If you want bending stiffness, put your rebar in the wall. 

Hey, I put a bunch of rebar in my footers too!!  I understand the conservative/"feel good" factor.  I widened the footers to lower the bearing stresses.  To support the wider footers, I put in lateral and longitudinal rebar.  My footers are ridiculous, and they only have 2 runs of #4 rebar. 

I still highly recommend JLC.


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By Marty in Boston, MA on 11/21/2007


I read the article from JLC titled footing fundamentals.  The article was very educational and worth reading; however, I also want to say that as informative as that article was, I still learned more from the various posts on this forum.

 

Of all the reading I have done on rebar, this was the first time I have seen rebar in footing run on top of one another – I have always seen them run vertically next to each other (spaced) – like Kenneth has posted on his site.  The rebar in the foundation walls also make sense.

 

Jon, I sent you a PM.

 

Kenneth, the images from the link you posted are fantastic – thank you.


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By Dave in Coarsegold, CA on 11/21/2007


I think we are arguing about two different things here.  Bottom reinforcement in the footing is primarily meant to give the footing tensile strength along the bottom.  All reinforcement in the footing, vertical and horizontal, contributes to the shrinkage and temperature cracking resistance.  Jon's wall vs. footing scenario does not support an argument against bottom reinforcement.  All his diagram was showing is that a deep footing (or "wall") is stronger than a shallow footing given the same reinforcement.  This is true.  However, as you place rebar closer to the vertical center of the wall or footing, it is less effective in resisting bending.  The horizontal rebar you have in the middle of your wall isn't for resisting up and down bending of the wall, it's for other purposes such as resistance to spreading, shear, and shrinkage and temperature.  It helps keep the wall from "bowing out" from soil or other lateral pressures. 

In reality, one should consider the footing and wall (if concrete) as one big beam.  One of the main functions of the continuous footing is to resist forces that put it in tension.  Adding bottom reinforcement improves its performance in this function.  Now, modifying Jon's example, rather than putting three #5s on the bottom AND the top of the footing, you would only put them at the bottom and then put extra reinforcement at the top of the concrete wall.  The top reinforcement would then resist forces putting the top of the wall in tension, such as if one of the corners of your house was settling.  You would definitely NOT want to reduce your bottom reinforcement and stick that amount somewhere up in the wall.  And you are not doing yourself a favor by putting more rebar in the top of the wall than you have in the bottom of the footing.

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By Jon in Ellicott City, MD on 11/23/2007


Hi Marty,

  I wouldn't take that JLC illustration too literally.  It was only intended to make the point that increasing the distance between the compression and tension-carrying material is an efficient means to increase stiffness.  (Just as it's more efficient to increase the height of an I-beam than it is to simply add more thickness to the webs and flanges.)

As far as I know, there's no good reason for putting rebar in the top half of the footing.

Thanks for the PM.  I'll read it when I get home (slow ISP here.)

Jon


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By Jennifer in Pueblo West, CO on 11/24/2007


Have you looked into Post-tension? That would kill two birds with one stone. You would be limited in it's application with a basement structure (best in slab foundation), but it's amazing the strength you can get.

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By Justin in Chandler, AZ on 11/24/2007


Jon,

Thanks for the tip on JLC. It is an awesome site and I read several articles last night. I will be signing up as soon as I can determine the best membership to purchase.

I love to read the forum posts from other O-Bs, but when I need technical advice on things I think it best to go to the pros and run what I learned by people here.

BTW, I was reading in the financing section on markup and margin. I was amazed that some charge up to 40% on jobs! I am glad I am an O-B and not paying that premium.


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By Jon in Ellicott City, MD on 11/26/2007


Dave,

  You can only consider the footing and wall to act as one big beam if you have a sufficient shear connection between them.  That's possible, I suppose, but it would take a heck of a lot of vertical rebar across that footing/wall joint.  I've never seen that done.

  I don't think there's any need to make this more complicated.  The footing's job is to spread the weight of the building over a wider area.  That's not a difficult thing to do.  If you were building in well-drained soil, you could use compacted gravel as a footing.  Poured concrete has the advantage of providing a more even surface for your wall forms and helps keep water out. 

  As far as rebar in the footings, well, I'll quote the engineer from JLC...

" Even though a lot of contractors do it, one thing that will not help you span a soft spot in the soil is to add more steel along the long dimension of the footing. Throwing more longitudinal steel into a footing in this situation is just a waste of time and money.  If you’re going to add lengthwise steel, put it where it will do some good: in the wall, not the footing. Just as a 2x12 on edge is way stronger than a 2x4 on the flat, steel at the top and bottom of an 8-foot or 9-foot wall does much more work than steel placed into a skinny little footing (Figure 7). A wall with two #4 bars at the top and two at the bottom can span over a small soft area with no problem."

  A couple #4's in the footings will be fine.  If there are any concerns about soft spots or pressure from unbalanced fill, throw a bunch of extra rebar in the walls.

       


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By Jon in Ellicott City, MD on 11/26/2007


Justin,

  Glad you found the site helpful.  I'm hesitant to pay for online content, because there's so much information available free.  But in the case of JLC, I consider the money well-spent.

  I know what you mean about markup.  I expect these guys to make their money - they have families to feed too - but the amount of markup they charge is difficult to justify.

 

 


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By Tom in Stroudsburg, PA on 11/26/2007


Marty,

Let's start at the beginning, because you unequivocally state that your footer is going to be 24", which may be fine. However, if you're building a three-story home with 8" solid or fully grouted walls on silty soil or sandy clay the minimum width per the IRC (International Residential Building Code) is 42". If you're building a one-story home on solid bedrock, 12" is all that's required.

I'm just trying to show you that your 24" footing is relative to factors other than size of the rebar. I noticed in one of your previous posts that you have several "ledges". Are these rock or clay that defines the limiting zone for your perc test? The recommended spacing on rebar in a footer is generally 12" o/c placed in the bottom 1/3 of the footer, minimum concrete below rebar is 1 1/2" per ACI. On a wider footing, transverse bars are also put in, usually at 18" o/c making a pattern that looks like a ladder lying on the ground. Vertical bars are required in seismic areas and are generally tied to bars in the footer.

If you are doing a block wall it is easy to push dowels in as you pour the footer, generally every 24". If you're going with a concrete wall and don't want to use rebar, then you should key the footer. Back to the point, your basic mild steel rebar has a 40,000 lb tensile strength - a #4 bar is more than adequate for residential construction. If you have some outrageous loading requirements an engineer should be involved. Personally I would be more than satisfied with 3 runs of #4 bar @ 9" o/c if 24" is indeed your footer width. Just one more note, minimum concrete depth is 6" and minimum concrete strength is 2500 psi. Hope this helps.


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By Mark in Seattle, WA on 11/29/2007


I'm very sceptical about putting less rebar in the footer and putting it in the wall where, allegedly, it will do more good. 

Two number 4s in a footer is a basic standard that applies to a basic house.  The county inspector won't say anything (assuming it meets the tables in the IRC).  If it's more than a basic house, or you want better than a basic footer, spend the $300.

5 has almost 60% more tensile strength than 4.  However, there are different tensile grades.  40K is used in many areas.  Here, in seismic D country, 60K is generally the requirement.  Be careful.  Suppliers sometimes have 40K #4 in stock for patios, driveways, etc.  Sometimes that's what the MegaStores have in stock.  Since 60K is 30% stonger, when going from 40K #4 to 60K #5, you've more than doubled your tensile strength.  Not bad for a couple hundred bucks.

Of course, placement is also an issue.  Because it's purpose is only to provide tensile strength, you want it as close to the bottom as possible.  Two inches off the bottom is standard, but only because those two inches of concrete protect it from corrosion.  When the contractor trips on it during the pour, it lays on the bottom, rusts away, an you've got nothing.  Same is true with many standard bracing system (wire ties, wood, moisture wicking dobies, etc.).  Anything that allows moisture to get in means that the footer is losing strength from day one.

The wall load on your footer spreads out laterally at about a 45 degree angle.  The reason that you have three rebar in a wider footer is because you generally have a wider wall.  In order to make sure that the entire tensile load plane has rebar, it takes three (or more).  Since rebar is cheap, I even put some laterals in mine to make sure that I'm getting the benefit of the 24" width. 

Once a footing is cracked, it's cracked.  A wall that cracks might be a moisture problem (or not, depending on how it's built), but it generally doesn't effect the upper structure.  A footer problem can cause problems all the way to the roof by also cracking the concrete wall above.  You could prevent an 8 foot wall above the footer from cracking by putting in 4 more laterals of rebar, but why not put one more #5 in the footer?

I also "over-rebarred" my project.  I've never actually calculated the amount it cost me to exceed code.  But now that I've had to have a loaded dump truck pass within a few feet of my 9' basement wall, I glad I put in so much 60K #5.

Instead of extra rebar in a wall, spend the money on better drainage.  More gravel, a second tiling halfway up the wall, filter fabric, dimple material, etc.  It's not difficult to reduce the hydrostatic pressure on the wall.  Also note that the wall rebar schedules in the IRC for solid walls calls for placement towards the inside wall surface to increase the tensile strength.  I was surprised when neither my sub or the county inspector was aware of this.

Most structures don't really challenge the compressive strength of 2,500 psi concrete.  Again, 4,000 psi is fine and costs little more.  Beware of the contractor who crows about the strength of the batch he requested and then pours it over a light schedule of rusty rebar that's poorly tied and barely standing.

Moderate Mark

 


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By Jon in Ellicott City, MD on 11/30/2007


Mark,

 

Luckily, most of what we’re disagreeing about is a matter of degrees of overkill.  There’s nothing wrong with 6 runs of #8 bar in your footers (more is better, right?), once you get past the money spent.  For that reason, there’s probably little point in hashing it out further. 

 

The one area I have to take issue with is the wall rebar.  That dump truck put virtually no load at all into your footers, but posed a very real threat to your basement walls. 

 

Trading wall rebar for drainage – or anything else, for that matter – is not a good idea.  The basement walls hold and distribute the weight of your house, hold the pressure from unbalance fill (and dump trucks), and provide the “foundation plane” for everything that sits atop them.  They are the backbone of your house.  This is the wrong place to skimp on rebar.

 

Anyone in the planning stages of their home, who might be reading this discussion, should talk these issues over with their engineer. 

 

If they are interested in the opinions of this engineer and the JLC engineer, it’s this:

 

1. Use the resources you have at your disposal (hopefully to include a soils analysis and structural assessment) to determine the required rebar in your foundation.

 

2. Place all the extra rebar you want in your footers, but only after you put all the extra rebar you can in the walls.

 

 


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By Mark in Seattle, WA on 11/30/2007


Jon,

You got it right except for #2.

"2. Place all the extra rebar you want in your footers, but only after you put all the extra rebar you can in the walls."

 

Wall rebar is primarily intended for lateral loads.  The lateral load calculation for unbalance backfill in a wall is simple.  A lateral load traffic surcharge on a wall (I used a DOT standard) is also simple.  Since the DOT standard is for highway traffic, it includes eighteen wheelers (or loaded dump trucks). 

 

Hydrostatic pressure on the wall is the easiest force to reduce and ensure that your rebar schedule is overkill.  Some engineers allow reduced rebar schedules where hydrostatic issues are addressed, even though Jon disagrees.  The lateral forces are why the rebar schedules are tighter on the verticals than the horizontal.  My verticals are 60K #5 20" o.c. in a 10" thick 9' tall wall.

 

As a side note, my county inspector red-tagged my project right before the scheduled pour.  I showed the inspector how it met the IRC, but he insisted that I hire an engineer to calculate the rebar schedule under ACI 318.  The engineer stamped my drawings that day and told me that it was overbuilt.  The county actually called the engineer and demanded that he produce his calculations.  Weird.  Since when is an engineer's stamp not good enough? Maybe the county was offended that I returned the same day with the stamp?  Another inspector told me that she would still red-tag the project even with the stamped drawings, calculations, and cover letter from the engineer explaining why the county was wrong.  Again, weird.  The engineer refused to charge me and said I'd been screwed by imbeciles.  As it was, the delay cost me $800.

 

Back to rebar.  The compressive load calculation is simple.  2,500, 3,000 or 4,000# psi concrete times the bearing surface.  Basically, the wall doesn't need much compressive reinforcement ASSUMING THAT THE WALL IS ON AN ADEQUATE FOOTER.  That's why #4 rebar 4' o.c. is a common horizontal schedule for a wall.

 

I don't remember what I spec'd for my laterals.  They were never an issue.  I know that they are only #4 and are wider spaced than the verticals, but spaced less than the standard 4'o.c.  There is also an "extra" around the first few inches of the wall bottom.  In other words, there is a lateral, and then the schedule starts, rather than nothing for the first X feet and then the lateral schedule starts.

 

The footer, which takes the brunt of the compressive load, is a different can of worms and the calculations are "iffier."  Will there be soil pumping under your footer caused by your drainage system?  Uneven settlement?  Are there undetected organics under the footer?  Are the soil pressure estimates correct and consistent around the entire perimeter?  Is there a compaction problem where the plumbing runs under the footer?  Did the site prep disturb the native soil?

 

Want to sleep soundly?  Put an extra rebar in the footer.  Any rebar left over can be thrown in the wall, or better yet, used to make lawn ornaments.  Trying to make up for less rebar in the footer by putting more in the wall only works if the lateral wall schedule meets or exceeds the footer schedule (i.e., at least 2x #4 every 12 inches.  Even then, you may still have a settling issue when your uncracked wall (with its reduced bearing area) bears on your broken, under-reinforced footer.  Better to just use the footer as the footer.

 

"Code" means the minimum strength that the county will allow.  For a few hundred bucks, you can exceed code by 50 to 100%.  I vote for putting extra rebar in your footer, especially if its width exceeds your wall thickness by 100%.

 

Moderate Mark

 

Wall was backfilled for a road.  Prior to backfilling, an extra drain was placed 1/3 up the wall to reduce hydrostatic pressure.


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By Jon in Ellicott City, MD on 11/30/2007


Mark,

 

Ok.  Tell me where you disagree and maybe we can get to the bottom of this. 

 

I’m assuming a simple example of a 9’ tall x 10” thick concrete wall on a 12” x 24” concrete footer.  If you have large openings in your basement wall, such as multiple garage doors, then the wall is clearly no longer bending member and all the load will go to the footer.  Assuming you have a "normal" basement wall....

 

 

  1. Rebar’s primary function, in the context of this discussion, is to add tensile capacity to concrete.
  2. Tensile stress in either a basement wall or a footer comes from a bending.load.
  3. Unevenly applied load and/or uneven soil bearing can give rise to such a bending load.
  4. The basement walls and the footer can each be considered as a beam, each of which resists this bending load.
  5. These two beams form a parallel load path, with each bridging over the low-bearing areas.***
  6. The sharing of load between the two beams is in proportion to the ratio of bending stiffness.  If either of them were infinitely stiff, then zero bending load would go into the other.  If they were equally stiff, they would each react half the load.
  7. Bending stiffness is linearly proportional to the product of the beam’s moment of inertia and the material’s Young’s Modulus. (EI)
  8. The moment of inertia of a 10” x 9’ concrete wall is over 300 times as large as that of a 12” x 24” footer. 
  9. Rebar increases the beam’s stiffness by increasing the Young’s modulus of a portion of the total area.  Adding 3 #5 bars to a 12” x 24” footer changes the EI of the beam from 1.3824E+10 to 1.3896E+10.  In other words, a “normal” amount of rebar adds essentially zero to the beam stiffness. 
  10. If the footer were made of solid steel, you increase the EI of the footer by a factor of 7.5.  The EI of the solid concrete wall would still be over 40 times greater than the footer.
  11. Using the two extremes listed above, the footer will react between 0.3% and 2.5% of the bending load.  The other 97.5% to 99.7% will be reacted by the wall. 
  12. No matter how much steel you put in the footer, the overwhelming portion of the bending load goes into the wall.
  13. The peak resultant stress is proportional to the distance from the neutral axis.  That distance is at least 10 times as great in the wall as it is in the footer.
  14. Combine #11 and #13 and you get tensile stresses in the wall that are between 400 and 3000 times higher than in the footer.
  15. Bending due to lateral pressure also contributes to tensile stress in the wall.  This is addition to stress disparity in #14.
  16. Lateral pressure is a guaranteed load, whereas the bending load described above is only a possibility.
  17. Therefore, it’s wise to plan for bending due to vertical load, but mandatory to plan for bending due to lateral load.
  18. Since the wall is likely much shorter than it is long, it makes sense to place rebar vertically.
  19. #14-#16 is why the vertical schedule is more tightly spaced and spec'd than the horizontal.
  20. Reducing the hydrostatic pressure is a very smart thing to do, and will decrease the rebar necessary to react this portion of the load.
  21. In other words, good drainage reduces hydro stress, which reduces the amount of vertical rebar necessary.
  22. Hydrostatic pressure is not the only source of lateral pressure.
  23. Some horizontal rebar in the walls will do double duty – helping to ensure margin for stresses due to either lateral or vertical loads.  Double sleep-insurance for the same dollar.
  24. From here, it’s simply a matter of calculating the resultant tensile stresses, deciding on a factor of safety, and spec’ing the rebar required.  If you agree to this point, I’ll run those numbers Monday.  I'm sure you don't.  Point out where you disagree and we can talk about them in detail.

 

*** This is a matter of the shear connection between the two.  In order for them to be considered a pure composite beam, the shear strength across that joint would have to equal that of the wall.  This would take approximately 16 square inches of vertical rebar every foot along the wall.  That’s 25 vertical #5’s every foot.  Granted, that’s a simplified approach.  We can trade calcs here if you like.  

 

 .   


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By Mark in Seattle, WA on 11/30/2007


"4.  The basement walls and the footer can each be considered as a beam, each of which resists this bending load."

Not really.  Under ideal conditions, there's no span.  The wall becomes a beam only if the footer fails.  Depending on how and why the footer fails, the wall may get tensile loads higher than the footer. 

"14.  Combine #11 and #13 and you get tensile stresses in the wall that are between 400 and 3000 times higher than in the footer."

Or, keep the footer from failing and you won't have to put between 400 and 3000 times more rebar in the wall.  If you do put rebar in the wall to resist tensile load caused by a failed footer, it would be at the foot of the wall.  The foot of the wall should also be made wider to support the load because it's only a beam to the extent that it's inadequately supported by the earth.  Hey, your wall is starting to sound a lot like a footer, albeit a lot weaker.  Maybe that's why a reiniforced footer is specified in accordance to vertical loads and a reinforced wall is specified in accordance to lateral loads.  I still vote for putting the increased rebar in the footer for vertical loads.

Keep the footer from becoming a beam by proper soil compaction.  Reduce any tensile load by making it wide, thus increasing the support.  As it gets wider, it should have more rebar.  Using your calculations, add between 400 and 3000 times less rebar to it than simply using a wall without a footer.  Sounds about right.  No reason for further calculations.

Moderate Mark

 


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By Jon in Ellicott City, MD on 12/1/2007


We've been discussing less than ideal conditions, such as the non-homogenous soil you described earlier, where the load is not applied and/or reacted evenly.  That gives rise to a bending load.

"The wall becomes a beam only if the footing fails" ?!?!? 

Now we're getting somewhere. 

Put two 2x4's between two saw horses (one atop the other) and press down on them at mid-span. 

They both deflect, right?  (if not, press harder!)

One board does not have to fail for the other to see load.

They both deflect the same amount, right?  

F=kx for each beam.  If "x" is the same, which it is, then
F1 = k1x and F2 = k2x, 

F1/k1 = x and F2/k2 = x

F1/k1 = F2/k2. 

Rearrange and you get F1 = F2*(k1/k2)   

So, the amount of load carried by each board is proportional to the ratio of stiffnesses. 

For your 2x4's, the stiffnesses are equal, so they each bear 50% of the load.

But, if one board were 300 times as stiff as the other, the load being carried by that board would be 300 times greater than the load carried by the other.  

Same is true for the wall and footer.

If there is a bending load applied to the foundation, even if it is only 1 inch-pound, the max resulting tensile stress in the wall will be orders of magnitude higher than the max resulting tensile stress in the footer, because if their relative stiffnesses.

As long as the wall is 40 to 300 times stiffer than the footer, it will carry 97% to 99% of the bending load. You cannot change this fact by placing rebar in the footer. 

What you can do is put the rebar where the tensile stress is, which is in the wall.

 

BTW,

Under ideal conditions, there is no HORIZONTAL span.  Vertical loads pass uniformly through the wall and footer and are uniformly reacted by the soil, resulting in a pure compression. 

There is a VERTICAL span.  Lateral pressure will always be present when there is significant unbalanced fill.  And that lateral pressure will put the wall in bending.

Therefore, even in ideal conditions, you need rebar in the wall, but not in the footer.

 

You know, just because I've been doing structural analysis for 25 years does not mean that I can't make a mistake.  Show me my mistake, in a analytical way, and I'm all ears.   

 


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By Jon in Ellicott City, MD on 12/7/2007


I finally got a chance to analyze a basement wall. 

The attachment shows the stress contours in the wall, footer, and soil.  I assumed a 10"x9' wall, 40' long, with a 10' segment in the middle that was unsupported.  The footer is 12" x 24".  I included 4' of soil in the model. 

I applied a load to the top of the wall that corresponds to 2000 lbs per linear foot.  I guessed that each linear foot of wall supported 20 sq ft of house, and that the total load on that space was 100 psf.  It's linear.  So, if you think the load should be different, you can just scale the results.

Difference in stress between wall and footer is actually more like a factor of 30, since stress is also inversely proportional to moment of inertia... an effect I failed to consider in previous posts.  My model predicts a max of 36 psi in the wall and 1 psi in the footer.

This is a pretty drastic model - your entire house bridging a 10' gap!!!  Even so, the tensile stress in the footer is very, very small. 

BTW, Mark: 30x greater stress in the wall does NOT mean you have a choice between putting a certain amount of rebar in the footer OR putting 30x that much in the wall.  It means that 30x more rebar in the wall is REQUIRED than in the footer.  Luckily, the amount of rebar required in the footer is really, really small.   So, 30x that amount is still a reasonable amount of steel. 

Jon


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By Jon in Ellicott City, MD on 12/7/2007


Here's another view of the model, showing the distribution of vertical stress. 
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By Avram in Raleigh, NC on 12/29/2007


Hey - first post here:

      I just wanted to muddle the issue some and give some food for thought.  Here in NC, the standard footing has no rebar in it. (This is standard residential, commercial uses rebar, high wind zone calls for it; etc., etc.)

Also, Superior Wall basements do not even have concrete footings, just gravel.

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By Marty in Boston, MA on 4/30/2008


I know it’s been a while since anyone added to this post, but I want to share some additional information and hopefully get some input.  I am thinking of installing three runs of #5 rebar in my 12” x 24” footers (3” from the bottom and starting 3” from the sides).  I will also run 18” #5 rebar on top of the three runs every 24” (so they will look like train tracks).  I should note that the footings will be placed on solid ledge – some of which I will need to blast.

 

As for the 10” foundation wall, I am planning on installing vertical rebar, which will be placed into the footing’s keyway before it dries every 24”.  I will then install three runs of #5 rebar in the wall starting 1’ from the bottom, 1’ from the top and one run right in the middle of the wall.  I am hoping that the vertical rebar will help with soil pressure from the outside wall.

 

Does anyone disagree that this would be proper installation of rebar or that this would be a total waste of money? I know of too many people that have discovered foundation cracks in their walls and I don’t want to be one of them.  Also, should the vertical rebar be placed in the keyway or should it be closer to the inside wall to resist bowing?

 

Thank you for your assistance.

Marty


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By Mark in Seattle, WA on 5/1/2008


Marty,

Sounds overbuilt, but I don't know what's going on top of it and, there's nothing wrong with being overbuilt. 

Your code probably does not require both vertical rebar and a keyway.  The keyway idea is kind of old school where it was used in walls that didn't even have rebar in them and were unevenly backfilled or didn't have a slab on the inside.  It keeps the wall from sliding off the footing.  Good idea, but not necessary in all situations.  The IRC allows for "dowels" (rebar) instead of a keyway. 

Think about it.  If you have two feet of soil on the outside of the wall, and two feet of compacted material on the inside, topped by a 6" slab, the keyway doesn't do much.  If there were sufficient earth movement to knock the wall off of a buried footer locked in place with backfill and a slab, you wouldn't notice because the rest of the entire house would have already fallen on top of it.

Check the code on your horizontal runs in the wall.  I think the top bar might have to be closer than 1'.  It needs to provide support for your toe plate anchor bolts, so it may need to be within a few inches.

Mark


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By Mike in Dover, DE on 12/6/2008


I am under the impression that footers are designed to reduce the pressure placed onto the earth from the house.  Code specifies the width based on the load presented, and the height is dependent on the width.  Without the footer spreading the load, the soil may be overloaded, and the house may begin to sink; likely not in an even fashion.

If there are soft spots beneath the footer, such that the soil compacts more easily than surrounding areas, then we would like the footer to remain rigid and not sink or crack across these areas.  It seems code specifies footer rebar to this end.  If a wall atop the footer provides much greater tensile strength than the footer itself, then I agree this rebar does little as Jon illustrated.  Perhaps the code is written to standardize footers regardless of what lies above them.

To 'overbuild' the footer in this situation, I would think it best to make the footer wider than code specifies (and in turn, taller).  If soft spots are beneath the footer, then as the load bridges those gaps, it will introduce additional pressure onto solid-bearing portions of the footer.  A wider footer will ensure the foundation doesn't sink under these conditions.

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By Roger in Beverly Hills, CA on 6/22/2019


Bridging across varying bearing capacity soil is why you want rebar in the wall.
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