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dik (Structural) (OP)
9 Feb 09 12:42
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
jike (Structural)
9 Feb 09 12:44
dik:

I would still use "skin" reinforcing, but make them epoxy bars.  Or use a corrosion inhibtor.
Helpful Member!  dik (Structural) (OP)
9 Feb 09 13:04
Jike:  Any idea why?  I don't like using epoxy bars in general.

Dik
kslee1000 (Civil/Environmental)
9 Feb 09 18:26
The valid reason is to prevent surface(random) cracks to form and deeply penetrating the inner mass that would create paths for water and other substances to enter. You know the results from such attacks.
miecz (Structural)
9 Feb 09 19:53
Why should cracks form?
kslee1000 (Civil/Environmental)
9 Feb 09 21:09
Nature of concrete, it cracks.  
beton1 (Civil/Environmental)
9 Feb 09 21:52
Is the corrosion surface related (ie. salt to control icing
or freeze thaw action) or soil related? You would I believe
specify appropriate concrete (air-entrained, minimum cement
content etc.)and could use epoxy bars for the top/exposed areas, if it is a case of surface salts. Otherwise, you may want to consider sulphate resistant concrete and epoxy
throughout. What about a precast structure or is this impractical? Jike & Kslee1000 are right and some minimum steel is required; how is the soil bearing and uniformity?
jike (Structural)
10 Feb 09 9:10
dik:

If you are asking why use bars, I believe it is good prevention against the unpredictable nature of cracks especially in a transformer base. The recommendation to use epoxy bars and/or corrsion inhibtor is to counter the corrosive salt environment.
dik (Structural) (OP)
10 Feb 09 10:38
The mass concrete is supported on six friction piles spaced about 6' apart and the load is approx 160 Kips.  I was planning on reinforcing it for flexure only as I've done with numerous pile caps.  I haven't checked it, but the cap may work as plain concrete.

I couldn't think of any cause for tensile stresses that would approach the modulus of rupture and the size is small for shrinkage stresses.  I was thinking that thermal stresses from the size might be an issue, but it is likely small enough that the mass will only warm slightly.

I've seen this type of construction with face reinforcing all over and was wondering why.

Any other thoughts.

Dik
enginerding (Structural)
10 Feb 09 11:03
ACI requires skin reinforcement on the tension half of beams greater than 36" deep.  Maybe the designers are applying this requirement whether or not it is directly applicable - applying the principle.  See 10.6.7 in ACI 318-02.
jike (Structural)
10 Feb 09 11:08
I look at predicting stresses in foundations and pilecaps as potentially very inexact! One or more pile may not be seated properly (bearing pile) or in a softer layer (friction pile) which could produce stresses in the cap that you did not anticipate therefore I try to be always conservative in these cases.
kslee1000 (Civil/Environmental)
10 Feb 09 12:02
Agree with Jike.

I tended to treat pile cap as a deep beam, or slab, for which the state of stresses (non-linear) is not well understood, also localized stress concentration is likely to occur that may exceed typically assumed stress (average) used in the design. Given considerations to importance of its role in entire system, and consequences of something goes wrong, I always provide skin reinforcing to gain better sleep at night.  
jike (Structural)
10 Feb 09 12:25
Often, the actual location of the piles may be quite a bit different than the drawings indicate.
civilperson (Structural)
10 Feb 09 13:06
If the side face will be visible upon completion, use "skin" reinforcement to minimize the size and visibility of the cracks, (if buried then ignore).  The mass pour may be successful in producing no thermal cracks by using a temperature probe in the center of mass and keeping the exterior face near that same temperature by heating or using insulating blankets.  
miecz (Structural)
10 Feb 09 14:15
I don't see skin friction helping the situation.  If there is enough tension in the concrete to overcome the modulus of rupture, then that magnitude of tension will overwhelm any skin reinforcement.  The reinforcement will yield, the concrete will crack, the steel will rust and spall the concrete.  Are there any publications that call for skin reinforcement in pile caps?  I can't find it in the CRSI Handbook.  By the way, dik, you may want to take a look at slideruleera's foundation page.  http://www.slideruleera.net/FoundationsForCompressors.pdf
kslee1000 (Civil/Environmental)
10 Feb 09 14:27
what is the stress required to cause a bar to "yield", say #3 (M-10)?
enginerding (Structural)
10 Feb 09 14:32
miecz - "If there is enough tension in the concrete to overcome the modulus of rupture, then that magnitude of tension will overwhelm any skin reinforcement"

This seems to be an impossible statement to believe.  Would the same hold for tension reinforcement in the bottom of a concrete beam?  Obviously not.  Steel will take much more tension than concrete - which is the reason reinforced concrete exists at all.  

What the skin reinforcement does is limits the crack widths (and therefore crack depths) and distibutes what could be a few major cracks into a network of smaller micro cracks that will be less damaging to the structure by limiting moisture penetration.

According to ACI, "for relatively deep flexural members, some reinforcement should be placed near the vertical faces of the tension zone to control cracking in the web.  (See Fig. R10.6.7.)  Without such auxiliary steel, the width of the cracks in the web may exceed the crack widths at the level of the flexural tension reinforcement" (R10.6.7, ACI 318-02)

While this is not specified explicitly for pile caps, it is required for beams.  It does not seem to be such a stretch to believe that this would be helpful in a pile caps.  ACI also has minimum horizontal and vertical reinforcement in the side faces of "deep beams" for the same reasons that are noted by kslee1000.  

Depending on the exact layout of the pile cap, it might be a really good idea to have the skin reinforcement.  If the loads and load distribution are more predictable, they may not be necessary.
dik (Structural) (OP)
10 Feb 09 15:06
enginerding:
The Canadian code for reinforced concrete excludes skin reinforcing for foundations.  They do have a requirement for skin reinforcing for deeper beams.

civilperson:
The concrete is completely buried except for the top surface.  Might be that that surface could have skin reinforcing and/or flexural steel.

The other thought is, "What are the consequences of skin cracking?"  Although I can't envision cracking forming, I can't see minor cracking as a problem if water is excluded from the surface.

There's no question that skin reinforcing will help minimise any cracking that forms, or distribute them more uniformly.  I just don't grasp that this should be provided when considering that the mass of concrete will be exposed to chloride.  I'm already using 3" concrete cover min to all rebar.

Dik
miecz (Structural)
10 Feb 09 15:29
enginerding-

If one were to provide the same amount of "skin reinforcement" as one provides at the bottom of a beam, then I agree, it wouldn't crack.  But skin reinforcement  much lighter than the reinforcement at the bottom of a beam.

If ACI thought about skin reinforcement for beams and walls, then I have to believe that they considered it in regards to footings.  So, if ACI explicitly requires skin reinforcement in beams and walls, why not explicitly require it for footings and pile caps?

As I see it, the difference is this.  A footing is much wider than a beam, so skin reinforcement, while effective in a beam or a wall, can't possibly be effective for a footing.
beton1 (Civil/Environmental)
10 Feb 09 17:58
Sorry, it was not apparent to me from the initial question
that pile caps were involved. Still think some minimum
reinforcing is required.
JAE (Structural)
10 Feb 09 19:57
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 (Structural)
10 Feb 09 20:16
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 (Structural)
10 Feb 09 20:33
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 (Structural)
10 Feb 09 20:59
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 (Structural)
10 Feb 09 22:52
Ron,
In this depression - I've been pretty busy lately.   
Qshake (Structural)
10 Feb 09 23:56
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
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 

miecz (Structural)
11 Feb 09 11:17
Qshake-

Quote:

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 (Structural)
11 Feb 09 15:39
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 (Structural)
11 Feb 09 19:43
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 (Civil/Environmental)
11 Feb 09 20:59
For all parties interested in pile cap design, CRSI Handbook has very good picture and example on it.
dik (Structural) (OP)
12 Feb 09 9:11
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 (Civil/Environmental)
12 Feb 09 12:20
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 (Structural)
12 Feb 09 22:31
miecz - it is in the concrete section in the back with reinforcing.

Regards,
Qshake
pipe
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
 

miecz (Structural)
13 Feb 09 9:27
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 As≥0.11Ag/fy.

 
dik (Structural) (OP)
13 Feb 09 12:38
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 (Structural)
13 Feb 09 13:10
dik-

Well said.
kslee1000 (Civil/Environmental)
13 Feb 09 15:22
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 (Structural) (OP)
17 Feb 09 9:53
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 (Structural) (OP)
6 Apr 09 23:29
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 (Structural) (OP)
6 Apr 09 23:32
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

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