Rebar in footing

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  #1  
Old 11-18-05, 04:16 PM
ollie
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Rebar in footing

I've searched but could not find the answer.
When using rebar in a footing where do you position the rebar.
1 In the bottom third of the footing?
2 In the middle of the footing?
3 In the top third of the footing?
On a 12" deep by 24" wide footing are 3 rebars enough?
Thanks for the advice.
Ollie
 
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  #2  
Old 11-18-05, 07:08 PM
Join Date: Mar 2005
Location: USA
Posts: 6,130
Rebar in footing

I assume you are talking about a footing for continuous wall, like a house foundation. Usually the minimum wall thickness is 8".

The minimum width would be the wall thickness plus 8" (4" on each side). If the soil is very poor, the footing may be wider as dictated by the allowable soil pressure.

If the footing must be wider than the thickness plus 8", then the footing should be thicker (10" or 12").

These dimensions are from any specific code requirements . They are some MINIMUMS based on common practice and some basic priciples.

If the concrete does not have any flexural stress on it, the steel is only needed for continuity and prevening differential settlement in the case of a loss of support. Since the steel is not stressed, the location is not critical and you just have to maintain 2" over cover over the steel. It is normally easiest to support the steel with a few brick placed periodically along the length.

Your 12 x 24 footing certainly fits this criteria. The 3 rebars (I assume #4 or #5) should be adequate. I would place one on the center line and the other 2 about 8" away. I would put them in center or lower portion on the wall. Always maintain 2" cover on the rebars.

The footings are not really stressed. If it was not for the need of a footing edge to pour a basement slab on, the soil pressures are so low that a narrower footing (or none) could be used from an engineering standpoint. The steel is only for continuity.

Good luck.

Dick
 
  #3  
Old 11-24-05, 07:21 AM
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Location: Chesapeake, VA
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Regarding the P<=Thickness requirement in the code books, I saw that (R403.1.1) but it didn't give any exceptions. My loads are far from equal all along my sill wall and I wanted to size the footings to try to get all my ground pressures similar, but I'm running into issues. Some of my exterior walls have virtually no loading and with the bare minimum width footing (12" for 1500psf soil per IRC), I get a ground loading of just 330psf. The interior wall supporting the second floor has over twice the loading, and thus needs over twice the footer area- 36" to get within 25%. Now, I know 330psf is nowhere near the 1500psf limit and thus is neither stressing the footer OR the concrete, but IRC makes no exceptions that I can see. One of the books I have (Ching's Building Construction Illustrated) states "Tensile reinforcement is required when a spread footing projects more than half of the foundation wall thickness and is subject to bending" so I imagine it's allowed *somewhere* in the codes, but I can't find it in IRC.

If I can't make the lightest footer any smaller, I can't increase the loading in the light walls to make everything settle evenly, and my only option is to increase the width. Thickening the heaviest-loaded footer to 16" to allow 36" width not only costs more, it exacerbates the problem by adding a LOT of additional weight, forcing me to a 48" wide x 22" thick footer! Are there exceptions in IBC that allow me to reinforce a standard 8" thick footer with lateral #4 rebar or 6" WWM and widen it?

Realistically, what sort of settling should I expect? I'm assuming no matter what I do, it will settle 1/2" but if my loading is so light that it will only be 1/8 across the 28' width of my house, I might be fretting over nothing! I just don't want it to settle unevenely and have my addition pull away from the existing house and cause cracks at the junction or racking in the doors and windows.
 

Last edited by grover; 11-24-05 at 07:35 AM.
  #4  
Old 11-24-05, 08:08 AM
Join Date: Mar 2005
Location: USA
Posts: 6,130
Rebar in footing

You are worrying to much about things that are really insignificant. Your bearing soil pressures are now about the level of a person of a person standing. This is if you actually get those loads applied. Down a a few feet into the soil you will not have any appreciable loads. Your settlement come from the deep soil, not the surface if you prepare it properly. With micro loads, you do not even attempt to theoretically balance soil pressures.

You are tackling the micro world of civil engineering only visited by academics in ivory towers. They teach the theory and leave design to those that are registered and more qualified.

Until you have had classes in soil mechanics and classes in the different basic methods of structural concrete design, you would be better off concentrating on other things. You did say you had an engineer didn't you?

Dick
 
  #5  
Old 11-24-05, 08:32 AM
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Well, in a manner of speaking. I'm an engineer myself but in more of the "ivory tower" sense with regards to this as I have no practical engineering experience in residential construction. (I've been in the real world almost a decade now, but my practical experience lies in other areas. I'm also a pretty good woodworker, so I have confidence to get the trim details right.) I'm working towards my PE, but part of being a PE is recognizing my limitations, and although I can run numbers all day long, I know I don't have the background to stamp a foundation design, just to regurgitate code. I do have contacts, however! One of my coworkers at my last job was a PE (civil engineer) who does commercial building design work on the side and he gave me some pointers in footer and slab design- basic stuff, really, as I found them all in the code book later, plus he some other suggestions like 6" slab with 6"WWM and rebar 3" off the bottom, cutting expansion lines in the garage where cracks would be visible, etc. He was completely unconcerned about the actual design of the footer since it's just residential and the stressing and loading are so low. At the time, I was unconcerned, too. But that was then, and now I'm faced with taking this to the city on monday and contracting it out, maybe as soon as 2 weeks! He also gave one final piece of advice: never stamp ANYTHING to take legal liability for the design unless I absolutely have to

It may just be a house, but it's MY house

Also, half the year a person standing in my yard will sink a good deal into the soil which is what has me worried! For sh'ts and giggles I decided to run a half-assed ground pressure experiment on undisturbed soil with 2x4s and concrete blocks.

Edit: with my test rig, it *should* have failed early given the high permiter:area ratio, but didn't- I got absolutely 0 displacement from 330psf through 4000psf but the rig was getting so tall by 6000psf that it would fail at one side or the other and topple. But did leave a 1/4" depression on the cornet it toppled toward. Which indicates a failure somewhere between 3-6 tons per square foot, with a safety factor of 2, is 3000psf. This correlates fairly well with the thumb test (fingernail penetrates the clay easily, but I can't leave a thumbl depression without great effort). So, this is only quasi-scientific since I only did it at one spot, but at least I feel better about my soil- if this rig didn't sink so much as 1/16" in 8 hours at 4000psf so ground pressure at 300-500psf shouldn't settle much at all, certainly not enough to worry about.
 

Last edited by grover; 11-25-05 at 03:13 PM.
  #6  
Old 11-25-05, 05:02 PM
ollie
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holley molley what am I getting myself into.
Were building a garage/shop 28' x 36'. The footing will be below the frost line 48" foundation walls will be 8" block with four block above grade. The slab will be floating 6" thick with a 12 x12 border. Lower floor will have 10' ceilings with a storage loft above.The building will be one and a half stories high with a gambrel roof. I just thourgh that a building this size could us a larger footing with rebar. I just wasn't sure where to place the rebar in the footing.
Are we wasteing concret with the larger footing. Is it necessary to make the foot any bigger then normal for 8" block,(16" wide 12" deep) for this size building.
Thanks
Ollie
 
  #7  
Old 11-25-05, 06:34 PM
Join Date: Mar 2005
Location: USA
Posts: 6,130
Rebar in footing

Sorry, but I guess grover cut in on your thread, thinking it was another.

Your foundation is the size of a small house. Your footings and the reinforcement certainly fit into the average for a building of your size.

The 16" width is the bare minimum. Most areas have a practice of using 20" or 24" wide for all size walls. If there is any question of the soil quality something like 24" would be needed, if not thicker.

You only build the foundation once and should do it right to eliminate the possibility of settlement and footing cracking.

Your 6" slab would be adequate for any type of vehicle.

Your proposed design is good construction and I would not consider it wasteful.

Good luck with the shop. Maybe there will be room for a TV in the loft.

Dick
 
  #8  
Old 11-26-05, 03:08 AM
ollie
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Dick
Thanks for the reply. One other question. Some of the machinery in the shop will be large. The lathe will be about 2 Tons. Do you think a 6" slab will be sronge enouth for this. Do you have any suggestions. As you say it's only going to be done once and I would like to get it right the first time.
Thanks again.
Ollie
 
  #9  
Old 11-26-05, 12:07 PM
Join Date: Mar 2005
Location: USA
Posts: 6,130
Rebar in footing

The lathe should be large enough to spread the load enough for a 6" slab. Probably 2 or base beams or four pads. That is only the weight of a car whick has 4 concentrated loads.

If it was a large, dedicated piece of equipment that had strong vibration you could pour a separate foundation separated from the slab and building. Some equipment requires this. I don't think a smooth running lathe fits in this category.

Dick
 
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