Reinforcement Spacing 7

[Shark4]

Hello All,

When deciding the reinforcement type and spacing per foot width in slabs or footings, should you go with larger rebars with larger spacing or smaller rebars with smaller spacing? and please explain why? for example, I need 0.77 in^2 of rebar per foot width in a footing, I have a couple of options, 1) I can use 1 # 8 bar at 12" or 2) 4 # 4 bars at spacing of 2" or similar combination of smaller bars with variable spacing to get it. What's the thought process when making these type of decisions? Thanks

 

[hokie66]

I would likely go with a spacing in between those. The less bars, the less tieing of the cage, so less labor. But 2" spacing is too close for placement in a footing.

Small bars give better crack control, but that is not normally important in an isolated footing.

Large bars are less labor intensive, but you need to be careful about development of the bars.

 

[BAretired]

#6 @ 8" seems like a reasonable compromise.

BA

 

[dik]

I'd run with #8@12... spacing is not too great, bars big enough they can be walked on, spacing is simple (beats '#?@13.75) and reduced number. If soil pressure is high, it's a matter of checking development length. Sometimes larger bars in small footings require a hook at the ends.

Dik

 

[BAretired]

Which all goes to show, there is no right answer. It's a matter of engineering judgment.

BA

 

[dik]

and with all the experience... judgement gets clouded sometimes... just try to catch everything all the time...

Dik

 

[TXStructural]

Don't aggregate "slabs" and "footings". Slab reinforcement usually has more things to consider, like crack control and the degree to which the reinforcement will be supported and walked on/over. Hokie covered the important parts for footings. In slabs, two of the biggest problems are 1) widely space reinf resulting in cracks over every bar but allowing workers to step between the bars, and 2) supporting closely-spaced smaller bars/wire adequately to allow workers to walk on the reinforcement and still end up with it in the right place after concrete placement.

Another thing: ACI 318 minimums for T&S are not adequate to control T&S cracks in slabs and walls. You need about twice the minimum (as required by ACI 350). It can be anywhere in the slab but the closer the reinforcement is to the surface, the tighter the cracks in that surface will be.

 

[dik]

TXS... yup, about 0.5% which I use for containment tanks... normal walls jointing and TS...

Dik

 

[msquared48]

Point of minor consideration...

In spread footings the structural depth to the reinforcing will vary 1" with #8 bars, but only 1/2" with #4 bars. Could make a difference in the As required in the opposite direction.

Not of consequence for retaining wall footings or very thick footings.

Mike McCann, PE, SE (WA)

 

[KootK]

I like:

- As few bars as possible for economy.
- spacing < 3h or 18" for the technical reasons (distribution/crack control).
- bars a lone human can carry for light residential and "detailing bars" in commercial.

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.

 

[dik]

I seem to recall 2 h for critical moment areas...

Dik

 

[Shark4]

Thank you! I really appreciate all the insightful info I have received, this was my first post ever on this forum, but now I have a feeling I'm going to use it more often. Much appreciated again!!

 

[Toby43]

Close enough that during inspection it is easy to walk on;-)

 

[dik]

Toby43... during 'review'... I avoid 'inspection' like the plague...

Dik

 

[BAretired]

Dik, perhaps you should explain why you avoid the word "inspection". Do you believe it has a different legal interpretation than the word "review"? Do you believe it may attract more liability? If so, perhaps the readers should be made aware of that.

BA

 

[jayrod12]

Most definitely that's how I feel as well. In fact we've been told that multiple times by out liability insurance providers. Apparently the court interprets inspection to mean you've taken detailed looks at all aspects. Review has a more general definition and attracts less reliability.

 

[dik]

Thanks Jay, my sentiments... and have been informed by numerous insurance underwriters of the same... Wouldn't have mentioned it if I didn't think it was useful...

There's a different standard of care between review and inspect.
Dik

 

[KootK]

A lot of responses coming in from Canadians. Certainly, here:

INSPECTION = Contractor Quality Control
REVIEW = Consultant Quality Assurance taken on behalf of the owner

Silly but apparently there were problems at some point. The part that gets me is that, in Alberta, we have to sign off on schedules for the building code and the language of those very much makes it sound as though I was out there tying the rebar myself, without friends even.



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.

 

[hokie66]

I would like to see that.

 

[rapt]

European and Australian codes are suggesting 12" maximum for crack control, not 18"!

They also suggest the smaller bar size the better for crack control. So I would be looking at maximum spacing of 12" and probably maximum bar size of #5 for slabs