Footing Rebar - 2 Matts?

[BSVBD]

The only time I typically specify (2) rebar matts for footings is when I'm dealing with pre-fab metal buildings with high uplift.

In this conventional building of joist and girders, I'm not anticipating uplift to be as much a requirement, if at all.

My 2-way punching shear forces me to go with 18" thick footing.

At what footing thickness might it be necessary to add a 2nd matt? And why?

 

[hokie66]

Uplift/overturning are the only reasons.

 

[BSVBD]

What if my footing gets up to 24" thick?

Wouldn't you put a top mat in for temperature steel?

 

[hokie66]

No, I would not. Some might, but you are just asking for plastic settlement cracking with a top mat.

 

[rapt]

I would put in a top mat if I wanted something to stand on while pouring/vibrating or to support/brace the column reinforcing cage. Otherwise only for the other load related reasons mentioned above!

 

[Leftwow]

Hokie66, plastic settlement, please elaborate. We've designed all our footings with a top and lower matt. I'm just curious to what your conclusion is

 

[KootK]

I've worked with a lot of engineers who would introduce top mats into footings in the 30-36" range. I don't think that it's necessary but I'll share their arguments anyhow:

1) with only bottom steel, you may get bottom side restrained, differential shrinkage curvature in the footing leading to cracking at the top. Flexure, when present, would tend to close those cracks. And I'm generally not too concerned about the cracks anyhow unless the footings are in an especially corrosive environment.

2) at some depth, perhaps the integrity of the sides of the footing becomes an issue and it would be best to have a continuous "basket" of reinforcing around the whole thing. This I don't buy at all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Leftwow]

I've heard people say "oh because the construction company could make a mistake and put the rebar at the top instead of the bottom", which if you think about this... it's stupid.

In a large raft foundation it seems like positive moments would incur if the reactions on the foundation is large. A typical footing is one thing but a massive thick raft foundation I would replicate the bottom flexural steel.

 

[dik]

Top fibre in compression, no need for compression steel or shrinkage reinforcing.

Dik

 

[hokie66]

Leftwow, if you have ever seen a footing, or most any element, where cracks follow the top bars, that is plastic settlement cracking. The fluid concrete settles after placement, and displaces itself around the bars. This can be prevented with adequate compaction, and in deep sections with revibration before the concrete hardens, but concrete contractors are rarely that diligent, so it doesn't get done unless you stand over them.

http://www.ccaa.com.au/imis_prod/documents/Library Documents/CCAA Datasheets/DS2005Settlement.pdf

 

[Leftwow]

Ok, this is pretty important for me to settle. If you have say, a 100 ft by 100 ft pile cap foundation 2 feet thick with 20 pedestals spaced evenly about, would you be opposed to applying a top level of rebar?

 

[KootK]

I normally would, yes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BSVBD]

Leftwow... the OP was intended to address isolated footings that experience tension in only the bottom half of the footing.

A pile cap as you describe is similar to a continuous concrete beam / floor system with tension forces alternating between bottom of cap/slab at "mid-span" and top of cap over the pedestal supports.

This is a different situation that i will defer to the apparently more experienced elaboration of hokie... please...

 

[BSVBD]

Koot... you normally WOULD be opposed?

Did you carefully read that Leftwow is inquiring of a 100' x 100' cap?

I must be misunderstanding something...

Will there not be tension in the top half of the 2 foot thick cap requiring rebar in the system of which Leftwow inquires?

 

[KootK]

Sorry. I normally would put in a top mat is what I'd intended to express.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

Generally 24" (and higher) is when I start using a second layer.

 

[hokie66]

Leftwow,

BSVBD has answered for me. Surely a pile supported mat slab, or an earth supported mat slab for that matter, of those dimensions, would have both top and bottom steel. My answer was about leaving out top steel where it has no benefit, and where I know plastic settlement cracking is probable.

rapt's suggestion of a mat for workers to stand on has merit, but there are other ways to do that. That is in the nature of constructability, what some here call "means and methods".

 

[Leftwow]

Ok, I was simply trying to understand, please do not take offence to my questions... I am just asking for learning purposes only.

 

[BSVBD]

Leftwow... on behalf of everyone... i hope... please forgive us if anyone has taken offense.

I don't see where any offense was implied.

It appears as though you were getting credible and considerate answers to a serious and legitimate question.