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Why Top Rebar in Footing where there is 0 stresses
4

Why Top Rebar in Footing where there is 0 stresses

Why Top Rebar in Footing where there is 0 stresses

(OP)
My etabs analysis shows the almost zero stresses in the top level of isolated footing.
Should I provide rebar in top level though it is not necessary?

RE: Why Top Rebar in Footing where there is 0 stresses

If there’s no need for it then why would you?

RE: Why Top Rebar in Footing where there is 0 stresses

What do you consider "almost zero"? Is there any negative moment in the footing?

BA

RE: Why Top Rebar in Footing where there is 0 stresses

If the footing has soil on it and there's moment on the column, there's some tension in the top of the footing.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Why Top Rebar in Footing where there is 0 stresses

Also consider the size and thickness of the footing. Maybe a little temp and shrinkage steel on the top is not a bad thing.

RE: Why Top Rebar in Footing where there is 0 stresses

If no uplift or bending, there is no need for top bars in an isolated footing. Providing bars for "temperature and shrinkage" purposes there can indeed be a bad thing, as it often leads to plastic settlement cracking over the bars.

RE: Why Top Rebar in Footing where there is 0 stresses

Not required, some engineers do provide crack control reo. As this is a compressive zone by strut-tie then it is not required

RE: Why Top Rebar in Footing where there is 0 stresses

Quote (hokie66)

plastic settlement cracking over the bars.
Can you describe in more detail this phenomenon? I am genuinely interested to know more about it.

RE: Why Top Rebar in Footing where there is 0 stresses

Logan82,

With a deep concrete element, which footings often are, the concrete will continue to consolidate/settle until it hardens. This is especially true when compactive effort is lacking, another thing which unfortunately is often true with footings without an appropriate degree of placement supervision. If there is a top layer of reinforcement, supported off the bottom layer, the top surface of the plastic concrete can move down relative to the bars, this creating cracks over the bars. Where the top reinforcement is structurally required to be there, this plastic cracking can be avoided by adequate compaction and/or recompaction with vibration. But if the reinforcement is just along for the ride in the compression zone, it is best omitted to avoid the issue altogether.

RE: Why Top Rebar in Footing where there is 0 stresses

We generally include some bars, if only to form up the cage

RE: Why Top Rebar in Footing where there is 0 stresses

Sometimes we are just lazy and write “top and bottom” without thinking.

RE: Why Top Rebar in Footing where there is 0 stresses

This is a common issue. Some like to put in top bars, others don't. Here's the rationale I use:
1) Look at a load combination that contains overturning moment that produces the LEAST about of bearing pressure on the far side of the footing.
2) Compare that to the moment created by the footing self weight and soil weight to see if you have legitimate tension in the top of the footing.
3) Based your decision for top bars on the following:
Option a: Always provide reinforcement in the top of the footing when the negative moment from 2) occurs.
Option b: Compare the negative moment demand from item 2) to the "plain concrete" strength of the footing. Only if if the plain concrete strength is insufficient to resist that moment do you add reinforcement into the top of the footing.

KootK and I had a great debate on this forum between item a and b. I started out strongly in favor of item b, but KootK convinced me that item a was probably better logic. But, I've seen good engineers do it both ways.

That being said, I might use some engineering judgment to soften the requirement a bit. Instead of providing FULL reinforcement in the top of the slab, I might go with 50% of the 0.0018 temp shrinkage or 4/3 of what would be required by analysis, whichever is greater. Sort of a hybrid between options a) and b).... Where I want to put some bars in the top, but want to argue that I don't need to follow the full code provisions for minimum rebar.

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