Top layer or reinforcement in column footings 2

[msdengineer]

What is the top layer of steel for in a reinforced concrete spread footing?

I have seen multiple structural designs where some engineers specify a layer of steel at top and bottom of footings and others where engineers only specify the bottom of the footing.

The only two reasons I see it being necessary is either for uplift changing the direction of bending in a footing or temperature and shrinkage min. steel requirements. Just looking for some guidance. Thanks.

 

[Lion06]

Could be for uplift, or for ftgs with moment. I've never been given good guidance on this, and opinions seem to vary, but I would probably provide nominal top steel in ftgs over 2'-0" thick.

 

[jike]

I use a top layer of reinforcing for single footings with uplift.

 

[msdengineer]

What ratio of steel do you place at the top? same as bottom? 0.0018bh?...

After posting I read in the RISAfoundation manual that their program will use a top layer when the footing goes into "partial uplift" designed for the negative moment induced by the uplift action.

It seems to me that for simplicity, a lot of engineers just place whatever amount of steel used in the bottom (based on design) in the top of the footing also. Just wanted to make sure I wasnt missing something. Thanks for the responses.

 

[Lion06]

Yes, I would use a min of 0.0018bh in the top.

 

[msquared48]

If it is major steel, it is definitely for uplift.

If is is smaller bars, then it could be minimum reinforcing or for uplift.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jike]

I generally use the same in the top and bottom so that it doesn't get mixed up in the field. Sometimes the footing can work as unreinforced for uplift but I just add the top layer of steel.

 

[hokie66]

If a footing is subject to uplift or overturning, it probably needs top flexural reinforcement. If only gravity loading, top reinforcement is detrimental, as plastic settlement cracking often occurs due to inadequate consolidation.

 

[COEngineeer]

If no uplift I usually use smaller bars and space them twice as long as the bottom rebars (easier installation because they are at the same locations every other bot. rebar)

Never, but never question engineer's judgment

 

[rowingengineer]

I agree with Hokie

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[asixth]

Use top bars when uplift occurs. I also think it is beneficial to place two smaller diameter bars in the corners of the hold-down bolt to help engage the entire footing for uplift conditions.
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\o_o/

 

[westheimer1234]

nah some engineers are just too conservative.. in my field whatever rebar we use for bottom we use the same for the top especially for small footings..

 

[steellion]

I specify top reinf to match bottom reinf for all footing subject to uplift/lateral forces.

 

[ToadJones]

Hokie-
Can you explain your above comment with regard to consolidation?

 

[rowingengineer]

Hokie may explain more but here is a good discussion paper.

http://www.concrete.net.au/publications/pdf/Settlement.pdf

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[hokie66]

Thanks, rower. That's what I meant. The first time I saw a footing with orthogonal cracking on the top, I scratched my head a bit before understanding what had happened. Now I have scratched it too much, and there is no hair left.

With top bars in deep sections, it is usually necessary to revibrate to prevent plastic settlement cracks.

Those settlement cracks illustrated in the link due to different thickness are a worse situation than the ones over the bars.

 

[ToadJones]

Hmmm-
I have never seen it. Makes good sense and it is good to know.
Our guys do run vibrators in the footing during the whole placement. Maybe we're getting lucky?

 

[hokie66]

Yes, it can certainly vary with the concrete mix, weather conditions, etc., but in deep elements it can occur even with reasonably thorough initial vibration. This type plastic settlement is the same reason you don't pour the floor until the column below hardens.