Concrete Footing Reinforcement

[craigmcg]

I am reviewing some old calculations/design of a 4' thick concrete footing. The design engineer placed their required (As.min) flexural reinforcement (tension/compression) on the top and bottom faces in each direction. This I understand. However, they placed an additional layer of reinforcement in the dead center of the footing, and this is where I'm questioning the logic. Based on the calculations, it appeared that the rebar is used simply as temperature reinforcement, but I know there is sufficient quantities in the top and bottom layers to account for this. Additionally, the location of the rebar is basically useless in my opinion because its situated on the neutral axis. Am I missing something here?

 

[Ron]

craig...I see no reason for it. I do a lot of 2' to 4' thick footings subject to high overturning loads and I use two mats...top and bottom.

 

[craigmcg]

Thanks Ron. Always a comfort to have a second opinion.

 

[PST09]

I would also agree. if the reinforcement is at the NA then in theory this is the area in the concrete where tension & compression is equal to zero.

 

[spats]

Your statement about the midheight steel being at the neutral axis is incorrect for a cracked section. However, I do agree with Ron's response.

 

[JoshPlumSE]

I've seen engineers do this as well for a very thick mat.... 6 ft thick or so. Though I remember the middle layer being included the total temperature / shrinkage reinforcement.

I've also been a young engineer on a job that did not require a 3rd layer where the EOR told me to add a middle layer only because it made him feel more comfortable with the design that I had come up with.

 

[msquared48]

Sometimes extra steel is placerd to support the steel at the top of the footing if there are two layers. This is non-structural steel.

Mike McCann
MMC Engineering

 

[JAE]

Or it is just a feel good rebar used, in a thick footing, for the engineer to feel that they are "holding together" the concrete....sort of a shear friction mentality.

Not saying they are needed, engineers just like to feel good, right?

 

[Teguci]

yeah - middle row of steel...thats good - sounds like a case of somebody else's money

 

[westheimer1234]

i remember i saw a drawing for a very thick footing for a vibrating compressor equipment..
around nine feet thick or so.. and there was like for 4 or 5 layers of rebars..

dont get the logic for that..

 

[hokie66]

Some people just don't understand. If an engineer can't think like concrete, he shouldn't be an engineer.

 

[spats]

I've had the opportunity to design several radiation vaults associated with medical facilities. They are basically a concrete bunker, with minimum concrete wall thickness of 3'-4' (depending on where in the vault), and up to 8' thick at the beam blocker. I used multiple layers of reinforcing, basically for minimum shrinkage & temperature... I believe up to 6 layers, if I remember correctly. The whole idea is to distribute the reinforcing throughout the section. Even though the coefficient of thermal expansion of concrete (0.00055 per 100 degrees F) is similar to that of steel (0.00065 per 100 degrees F), they are not the same. Layering creates a more "homogeneous" section throughout the member's thickness. I believe this is also important as the structure cures & shrinks.

Granted, I don't believe that I would do this for a spead footing, but the concept is valid.

 

[Teguci]

Shrinkage, bending, shear and other cracks start from the exterior of a concrete surface and then propogate inward. Wouldn't reinforcing always be the most effective near the surfaces?
If there was a choice between a top and bottom layer of #8s or 4 layers of #6s, wouldn't it be cheaper to stick with the fewer number of bars?
What would be an example of a failure mechanism that would be prevented by multiple layers of reinforcement over just the top and bottom mat?

 

[craigmcg]

Thanks for everyones input so far. One rule of thought is that multiple layers of reinforcing might be needed to allow sufficient spacing between rebar, else the rebar could get mighty crowded in one mat with a thick slab. However, with components such as footings, it is a good idea to limit your rebar spacing in order to minimize crack width. This will subsequently decrease permeability to groundwater intrusion.

In regards to the temperature reinforcement in footings, I'm not sure how needed it would be. The ground would have a significant moderating effect so as to prevent large temperature fluctuations. I'm sure shrinkage would be evident, but the relatively tight rebar spacing should compensate effectively.

I think the bottom line that as it was put above, this is pretty much "feel good" rebar in this application. I'm assuming there is no formal documentation or ACI references that really show otherwise.