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Reinforcement Spacing

Reinforcement Spacing

(OP)
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

RE: Reinforcement Spacing

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.

RE: Reinforcement Spacing

#6 @ 8" seems like a reasonable compromise.

BA

RE: Reinforcement Spacing

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

RE: Reinforcement Spacing

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

BA

RE: Reinforcement Spacing

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

Dik

RE: Reinforcement Spacing

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.

RE: Reinforcement Spacing

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

Dik

RE: Reinforcement Spacing

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)


RE: Reinforcement Spacing

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.

RE: Reinforcement Spacing

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

Dik

RE: Reinforcement Spacing

(OP)
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!!

RE: Reinforcement Spacing

Close enough that during inspection it is easy to walk onwinky smile

RE: Reinforcement Spacing

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

Dik

RE: Reinforcement Spacing

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

RE: Reinforcement Spacing

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.

RE: Reinforcement Spacing

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

RE: Reinforcement Spacing

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.

RE: Reinforcement Spacing

I would like to see that.

RE: Reinforcement Spacing

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

RE: Reinforcement Spacing

North American codes stipulate 3x slab thickness <= 18" or 2x slab thickness for critical areas... The closer the reinforcing spacing and the greater the %, the smaller and closer shrinkage cracking will be.

Dik

RE: Reinforcement Spacing

dik,

Other codes also used to use 18 - 20", but as reinforcement strength increases, this needs to reduce. ACI has not reduced this since the 70's but reinforcement strength has increased significantly.

These days in Europe/Asia/Australia, reinforcement is mostly 500Mpa, so about 72.5ksi (if I got the conversion right) and when we went to that strength the maximum spacing reduced from 18-20" to 12"!

RE: Reinforcement Spacing

One thing I like to check for is space for hooks, if your bars are hooked at the ends. For instance, a no. 8 has a 16 inch hook. It's going to be hard to fit in a 12 inch slab. No. 5's have 10 inch hooks. You can twist them a bit and fit them in a 12 inch slab.
So my vote is more smaller bars (but not a 2 inch spacing).
Another thing is I've trained everyone in my office to use either 6 inch spacing or 12 inch spacing, except in footings supporting CMU. It's easier to put in add bars at corners and the like.

RE: Reinforcement Spacing

Jed... I wouldn't normally use hooked bars and to put a 16" hook into a 12" slab, tilt it a bit, until it fits...

I try to space rebar so it can be readily walked on, but, normally don't use 6" or thereabouts spacing. Way too close. There is no question that closer spacing is much better for performance and I typically try to space between 12 to 16 inches.

Dik

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