Spread footing design 4

[BAGW]

Hi,

If I have a gravity column with just downward loads, can I get away with just the bottom reinforcement as that's where the tension face is? Does ACI limit having just bottom reinforcement?

If I just have bottom reinforcement, can I still use 4 sqrt (f'c) for punching shear calculations?

 

[hokie66]

There is no need for top reinforcement in a spread footing not subject to uplift or overturning.

 

[IDS]

There is no need for top reinforcement in a spread footing not subject to uplift or overturning.

... unless working to a code that requires all concrete faces to be reinforced, which many codes do.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

In that case, the code is fundamentally wrong, and should be changed. We live in a world of diminishing resources, so they should not be wasted.

 

[EngDM]

You might be able to even get away with no rebar reinforcing if plain concrete works

 

[BAGW]

Thnaks hokie66, that makes sense. Also, I found ACI examples where they have only bottom reinf for gravity only foundation(36" thick).

 

[hokie66]

Regardless of codes or economy, there is another good reason for avoiding top reinforcement where it serves no purpose. A well supported mat of top bars will stay in place, while the plastic concrete will settle. Settlement cracks which track the bars are common when compaction is inadequate.

 

[BridgeSmith]

... unless working to a code that requires all concrete faces to be reinforced, which many codes do.

AASHTO requires shrinkage and temperature reinforcement for concrete faces exposed to daily temperature changes, but a buried footing wouldn't be considered subject to the temperature changes, so S&T steel wouldn't be required. There wouldn't be any other applicable reinforcing requirements in the case being considered.

 

[HTURKAK]

I totally agree with hokie66 (Structural) and a BPS for his respond. Apparently all the codes getting more stringent ,( e.g. concrete faces to be reinforced, provide shrinkage , temp. reinf . etc.)

If the footing fully buried , temperature will not be issue. If the spread or pedestal footing rests directly upon sound bedrock and if the bond to the rock is provided ( with grooves etc) so the bottom of ftg cannot elongate, you don't need reinf. ( This was old method , i shared this knowledge just for info.)

ACI 318 allows the use of plain concrete for limited cases ( CHAPTER 14).




Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[Sheer Force Engineer]

No top reinforcement required in spread footing with downward load only in Australian region over here.

For the latest insights into the art of Structural Engineering

http://www.sheerforceeng.com

 

[IDS]

From the Australian Bridge Code (AS 5100.5):

I haven't checked the Eurocode, but when I worked in the UK it was standard practice to reinforce the top face of all footings for bridge structures.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[bugbus]

Another reason for top reinforcement would be to control thermal cracking, especially in a large footing.

 

[Tstruct]

Even in isolated footings with moment, we do not provide top reinforcement. It is an old practice here and many buildings have been standing here for decades with this approach. I am in UBC zone 2B.

 

[JoshPlumSE]

In that case, the code is fundamentally wrong, and should be changed. We live in a world of diminishing resources, so they should not be wasted.

I'd have to see the code to determine if it were "fundamentally wrong". In my experience, there are clients (and companies) that have this requirement base on past experiences with exposed concrete. Meaning crack control. But, also if the top concrete is exposed to the client and it develops larger cracks at the top, this may not affect structural performance, but clients don't like when that happens.

 

[dik]

crack control in a compressive zone... we should sit down and have a beer together.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[WARose]

Generally I don't have a top layer unless moment requires it or it is more than about 32-36" thick.

 

[dik]

ya... 36" is my magic number. The footing is burried and you cannot see the cracks, anyway.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[hokie66]

As I indicated previously, reinforcement in the top of isolated thick footings is more likely to cause cracking than to prevent it.

 

[JoshPlumSE]

crack control in a compressive zone... we should sit down and have a beer together. pipe

Well, it could certainly start as temp / shrinkage cracks. Right? Maybe you have some soil issues underneath. Or, maybe you've got equipment driving over the footing.... All of a sudden, what you thought was a compression zone isn't ALWAYS a compression zone during the life of the structure. Regardless, if a client knows it's going to be exposed, they have ever right to formalize a requirement that exposed footings have top steel as well.

Granted, I am thinking back to when we could use what was required for flexure in the bottom of the footing, then the top footing would be just whatever we needed to get us to 0.0018 for the overall thickness..... Rather than the ridiculousness of using 0.0018 for the both top and bottom like we have now.