Skin Reinforcing 1

[dik]

I have a large transformer base that is roughly 10' W x 15' L x 4' D.

I intend to reinforce it for flexure only (shear not an issue). I plan to use top and bottom bars, with hooks to ensure the bars are developed. I often see small 'masses' of concrete detailed with 'skin reinforcing' all faces. This includes walls with 'U' bars at the top, providing a continuity of reinforcing from the inside to outside face.

The work is in a corrosive salt environment and I've of the opinion that added rebar is just going to cause problems.

I've not been able to any reference to using skin reinforcing for this type of application, that includes pile cap design. Any suggestions about why the small masses would be have skin reinforcing?

Dik

 

[JAE]

dik,

Here's on I did some time ago - about 12 feet thick and 26 feet wide - pentagon shaped on top of six drilled piers. Used vertical cages within the thickness.

 

[civilperson]

I question the statement,"shear not an issue..". All pile caps of my experience have shear calculations as necessary to develop the correct depth since no steel is used to resist shear forces. Piles which react against pile cap require shear investigation for the corner, side and interior condition as well as beam shear at the face of loads on the top of the cap.

 

[JAE]

civilperson - that is not exactly correct.

Shear can be a NON-issue if the pile cap is thick enough. See ACI 318 section 15.5.4.2.

It depends on the pile location, spacing and depth of cap.

 

[Ron]

JAE...did you take a vacation? Hope things are well.

Dik...I essentially use "skin" reinforcement in small block foundations (3x3x2 feet up to 5x5x2')just for convenience. I see no need for it, but it helps the tie and placement process. These are reinforced for bending only...shear is not an issue. Columns are embedded, no pedestal.

I put extra cover on these for the corrosion issue....at least 3 inches all sides.

 

[JAE]

Ron,
In this depression - I've been pretty busy lately.

 

[Qshake]

All - I work a lot with AASHTO and some with ACI. When working with ACI I usually have to read the specs over to remind myself what's required.

On this matter I would think that AASHTO and ACI are similar in that AASHTO requires temperature and shrinkage reinforcing on a face that is "insert depth requirement here" and shall be no less than 0.125 sq. inches per foot.

Footings are an exception, perhaps because they are underground and not exposed to large temperature flucuations.

Ron/JAE - It's good to see your contributions. I hope all is well in your neck of the woods.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[miecz]

Qshake-

AASHTO requires temperature and shrinkage reinforcing on a face that is "insert depth requirement here" and shall be no less than 0.125 sq. inches per foot.

I can't find this provision in AASHTO. Can you tell mer where it is?

 

[civilperson]

JAE,
ACI 318-08 (15.5.4) requires computation of shear, the section considered with piles inside a distance of d/2 can ignore the reaction of those specific piles, CAN NOT IGNORE ALL PILES OR NOT CALCULATE SHEAR VALUES.

 

[JAE]

Civilperson - granted yes. You are correct that the piles within the distance can be ignored but the piles outside that distance cannot be and do develop shear in the cap.

However, your statement of Feb 10th above was a very broad, general catch-all statement that implied that you always have to check shear in a cap.

You even said that "interior" piles create shear and I was just noting that in some cases that isn't true.

In the picture I posted above, the "column" on top of the pile cap was over 15 feet across (compared to a 23 foot cap) and 12 feet deep.

So ALL my piers (piles) were within the given limit...thus, section 15.5.4.2 indicated that none these piers develop shear in the cap.

Despite that, we still added vertical stirrups within the cap simply based on our gut feel for providing some reinforcing within the mass concrete.

 

[kslee1000]

For all parties interested in pile cap design, CRSI Handbook has very good picture and example on it.

 

[dik]

Thanks, gentlemen... I think I'll reinforce it with regular required reinforcing and provide hooked ends just to ensure bar development due to higher shears in the interior of the cap. In the long face, I'll put shrinkage reinforcing equivalent to a foot of concrete. (I was thinking of using teflon sliders on the pile end, but decided not to <G>.) I'll use 'Z' support bar chairs since I don't think they can get really high chairs.

I'm still not really convinced why the face reinforcing is necessary; I'm not sure, other than thermal and shrinkage issues, why there would sufficient tension in the face.

Dik

 

[kslee1000]

dik:

Sometimes engineers do not soly act by conviction, but to the lack of certainty. (Many times concrete deterioration/defect is caused by intrusion of foreign substances through fine, but wide enough, cracks.)

Strickly to your original question on the need, or not, to provide "skin reinforcing" on mass footing elements, to me, it is a question on "certainty" that is to be judged by the designer/engineer.

 

[Qshake]

miecz - it is in the concrete section in the back with reinforcing.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[miecz]

Qshake-

I just can't find it. Could you please reference the Code Revision and the article number. The closest thing I find in the LRFD 3rd Edition is Article 5.10.8.2, where A_s[&ge;]0.11A_g/f_y.

 

[dik]

kslee1000:
You're point's well taken; and I concur to a large extent. I like to have a real understanding of things and then usually act accordingly. I have greater problems with things that I don't really understand the reasoning. I have great problems, sometimes, with "it's always been done that way" without understanding why... I'm now delving into thermal stresses to gain a better understanding.

Dik

 

[miecz]

dik-

Well said.

 

[kslee1000]

dik:

Thanks for your response. As mentioned before, "certainty" is more of a judgement call (against potential odds which may or may not occur), as opposed to "conviction", which is based on known fact(s), and/or knowledge(s). I applaud you for going through that route.

Personally I think, skin reinforcing would not be that important if the problem is merely the concern over temp. & shrinkage effect, which can be minimized significantly by exert controls over concrete mix and placement, curing method and duration. However, if either water, or chemical, or both are present, without reinforcing, the fine surface cracks may widen and get deeper into the concrete mass, which could leads to spalling and splitting of significant amount of the concrete.

For your case - pile cap/foundation for transformer, here is one more odds to be watch out for. I recall transformer spill is a hot topic, for which the acidic fluid with high temperature is quite harmful to the concrete, and not easy to clean up. Please consult with a mechanical engineer, or the supplier on this matter.

 

[dik]

Modern transformers have a mineral oil coolant and dielectric and some jurisdictions do not require containment; I'm currently embroiled in a series of discussions where I want to provide containment... just in case. Engineers can be taken to task for environmental damage and I'm looking for some perpetual hold harmless clause from the client; they are re-considering their stance.

Earlier transformers used PCB's for coolant/dielectric which over a period of time converted into toxins. I'm not aware of any corrosive effect and corrosion is not an issue for this condition.

On a recent project used a material called Sorbweb; this is a product that allows water to pass, but in the event of a spill, it dissolves and forms a liquid tignt membrane.

Dik

 

[dik]

Just putting together a final procedure for this. Because of the mass concrete I intend to insulate the forms with 1" extruded polystyrene rigid insulation. The formwork can be stripped for reuse or whatever with the rigid insulation remaining in place for a week or so to prevent rapid cooling and evaporation of the moisture from the concrete. Same with the bottom of the mass. The top of the mass will have a liquid curing membrane with an insulated tarp applied for a week or so. I do not intend to use skin reinforcing.

Dik

 

[dik]

Forgot to add that I will use the highest water-cementatious material ratio that I can and still accommodate corrosion issues and aggregate that is as large as I can get without it being a special mix.

Dik