Strip Footing Required Reinforcement

[pghsteelers]

To All,

I am designing a concrete strip footing. The thickness of the concrete is roughly 2.25'. This is set to match an existing railroad foundation nearby). I designed the main bottom reinforcing to carry the moment applied (#6 @ 6"). Also, I designed shrinkage and temperature steel in the other direction (top and bottom) by simply using 0.0018bh. My boss mentioned to place reinforcement in the top direction (same plane at the #6 @ 6") and he said there is a note in the ACI 318 code about using a percentage of the area of steel required for the main reinforcement. This is due to the high footing thickness and some reinforcement is needed at the top of the footing, however even if it is minimal. I cannot find this section and wanted to know if anyone out there has a direction to lead me. Thanks.

 

[bjb]

I am not aware of this requirement in ACI 318 either.

You only need reinforcement where there is tension. As long as the top surface never goes into tension you should be ok without any reinforcing there as long as you can meet all of your shrinkage and temperature requirements in the bottom mat. If your footing thickness was so large that your shrinkage and temperature steel became unreasonably large at the bottom, it would make sense to have bars at the top face too.

With that said, for a thick section I could see having bars at the top surface as a matter of engineering judgement, but I'm just not aware of that being required by ACI 318.

 

[msquared48]

He may be thinking of the practice for walls 10" and thicker for 50% of what is required for the primary reinforcing to be installed on the non-tension face of the wall. So if #5 @6 vertical is required for bending on one face, the opposite face will see #5 @12 vertical.

Then again, he may not...

Mike McCann
MMC Engineering

 

[Lion06]

There is no requirement for top bars if they're not needed.

 

[msquared48]

No requirement, but if there is any chance for localized settlerment, the installation is a good practice.

Mike McCann
MMC Engineering

 

[CTSeng]

Unless specifically needed stick with bottom reinf only. Additional steel for chairs, tie off and increased lap lengths seem like a waste of steel.

Why not check as plain concrete with this thickness? The majority of strip footings I see have only nominal longitudinal bars and no transverse bars.

 

[kslee1000]

I guess your boss was talking about distribute the T&S steel in the longitudinal direction. It is an old practice, I don't remember code requirement, to distribute 50% on top, the remaining goes to the bottom. The argument was T&S steel is proportioned based on Ag, without the top steel, wide-undesirable cracks might occur.

 

[pghsteelers]

Thanks all for the help. There is not exactly anything in ACI 318 regarding this issue. However, in ACI 360R-21 it mentions that S&T steel "should be at or above middepth of the slab on grade, never below middepth". That is why we decided to put some S&T steel in the top...only a portion of the required area of steel.

We wee able to come with a solution though... thanks again for all the help.

 

[bjb]

A slab-on-grade is a different animal. The steel in a sog is placed closer to the top to control how much the cracks open up. It's also usually a non-structural element, and the amount of crack control steel is usually very little as compared to the shrinkage and temperature steel in a structural member like your footing.

 

[dik]

What is the cantilever distance on the strip footing... if 2.25' thick then with wall thickness the footing can be about 4'-5' wide without reinforcing... as plain concrete.

Dik

 

[swivel63]

if you don't need top bars, don't put them.

 

[DonPhillips]

I try to avoid placing reinforcing in footings unless needed as well, otherwise beginning with the 2005 National Electrical Code, you have to bond this reinforcing to the building's grounding system.


Don Phillips

http://worthingtonengineering.com

 

[msquared48]

Why?

Mike McCann
MMC Engineering

 

[dik]

I think it's a code requirement... we often do that here...

Dik

 

[msquared48]

To all:

Except for residences that usually have integral stem walls with somewhat effective "top" steel, I still feel, regardless of any code issues, that it is good to have top steel in any footing, particularly grade beams, to span any soft spots in the underlying stratum. Without this steel, considering the placement of the steel, it is very hard to develop the negative moment to prevent cracking of the foundation with any settlement. Regardless of the cost, which is minimal with any project, it is very cheap insurance against being dragged into court over cracked foundations. I usually design the foundation footing to span at least 10 feet to account for any settlement, or outwash due to flooding. I realize this is not required in most circumstances by most geotechs, but I consider it good engineering practice.

Mike McCann
MMC Engineering