Coeff. of friction 1

[papaaj]

Hello,
I posted this in the structural engineering forum, but I thought I'd repeat it here. I apologize if it's against the rules to post the same questiong in two places.

We have a ductile iron pipe supported by concrete pipe cradles. The pipe was set in position first, then the concrete for the pipe cradles was placed under the pipe, so the top of the pipe cradle matched the OD of the pipe. At some pipe cradles a single layer of tar paper is present in the joint between the pipe and the pipe cradle.

My question is: Does anyone have any data or references for what the coefficient of friction is for a ductile iron/tar paper/concrete joint, in both a wet and dry condition?

I've seen references saying the coefficient of fricion of steel to concrete is about 0.4, and I would think cast iron to concrete would be about the same. My gut tells me the presence of tar paper would reduce this coefficient, but I can't find any documentation confirming this. Any help you can give me would be greatly appreciated.

 

[KENAT]

I suggest you put a link here to the structural post and ask people to post responses there.

You can do this by pasting the thread number (above just under thread title) from the structural post here.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at

http://eng-tips.com/market.cfm?

 

[unclesyd]

ACIPCO]
1. Principles presented in the DIPRA publication "Design of Ductile Iron Pipe on Supports" can be used to calculate the maximum localized stress due to support reaction and to approximate the maximum beam-bending stress near mid-span of these systems. This publication can be obtained from AMERICAN.

http://www.dipra.org/pdf/pipeOnSupports.pdf

http://www.dipra.org/pdf/thrustRestraint.pdf

 

[unclesyd]

A little trigger happy this morning.

The first site is the paper mentioned from ACIPCO. The second might have what you need in the section on "unit frictional force".

 

[papaaj]

KENAT - thank you for the suggestion. I put in a link over on the Stuctural post to here, being that there are already some responses here.

unclesyd - thank you for the links. After a quick look through those references I didn't see anything specifically related to what I'm trying to find out, but they do include a lot of other helpful info.

 

[NS4U]

See

Idun, E. K., and Darwin, D., 1999, “Bond of Epoxy-
Coated Reinforcement: Coefficient of Friction and Rib Face
Angle,” ACI Structural Journal, V. 96, No. 4, July-Aug.,
pp. 609-615.

I believe they found that the coefficient of friction between reinforcing steel and concrete cast around it is about 0.5.. if that's the case 0.4 is probably a good design value to use.

Also, how much concrete is case below the pipe? If there's a lot of concrete below it air pockets will form underneath the pipe and you will not develop the full frictional stress there.

 

[NS4U]

sorry, defintly didn't read your post all of the way...

the paper also talks about how the epoxy covering affects the coeff. of friction... it reduces it about 20% so i can only imagine what tar paper will do

 

[unclesyd]

I should have added one thing in my response is we used a bitumen saturated felt instead of tar paper on concrete supports.
Personally I've never seen a COF in any calculation even for large bullets supported by concrete piers where the felt was used, not saying I verified every calculation.

If you don't find the number you need I would give ACIPCO a call as they did have very knowledgeable and helpful people answering the phone. I would call the DI divison number.

 

[chicopee]

How much of the pipe is embedded into the concrete pipe craddle and why is the felt paper used?

 

[unclesyd]

We use saturated felt for several reasons.

On CS it acts as moisture barrier for the crevice created by the supports. This helps even for coated steel as they are inherently some vibrations or movement that will cause the coating to be compromised. We used it on bullets and piping where it is on concrete supports.

For SS our main reason again is as a moisture barrier for the crevice between the concrete and the SS. We have found that concrete + moisture + SS doesn't make a good sandwich for more reasons than one. The most prominent being the leaching of chlorides form the concrete. Vibration or Movement always creates an active surface on the SS.

For Al all the reasons stated above with the exception that it is for the Alkalinity of the concrete.

For all the above the saturated felt is a good disconnect between the rigid support and moving metal.
As stated above it improves the COF between the support and metal.

 

[chicopee]

If COF is one reason, why not place a cross bar at the end of the pipe embedded in the concrete, thereby preventing slippage?

 

[unclesyd]

Do you mean restraining the pipe from movement?

Pipe systems are normally anchored to produce movement in the desired direction there for the pipe has to move or bend or buckle.
On our bullets there is absolutely no restraint either by piping or mechanical device. We do not use any tank or vessel for an anchor point.

 

[MiketheEngineer]

And just how long will the tar paper last??

 

[unclesyd]

I don't about tar paper but some of the felts have last over 60 years. I would estimate that 20% have been replaced due to deterioration.
One point to add is we have two small SS flat bottom tanks that were to be place on a concrete pad until we found out the contractor had added Calcium Chloride to mix. We set these tank on 1" thick heavy saturated felt. The tanks were set around 1974 and are still in service today.

 

[papaaj]

unclesyd - thanks for your input. I did call the DI division of ACIPCO, and they didn't know the answer, and (not surprisingly) had never even heard this question asked.

Like you had said, our pipe cradles are not usually intended to provide any significant restraint to the pipe, and for our normal applications, I believe tar paper has been used in this type of joint with some regularity, with no issues that I am aware of.

But in this situation, a pipe cradle detail on our drawings for a small (24" dia max) pipe located no more than 2' above the bottom slab of a concrete liquid-containing tank was incorrectly adapted for the support of a 48" dia pipe located 8' above the concrete slab. The reinforcing in the pipe cradle is minimal, so the pipe cradle has little capacity to resist moments caused by longitudinal load transferred by the pipe, for example due to thermal expansion of the pipe. I'm not a piping expert by any means, but it is my understanding that as the pipe tries to expand, slippage at the pipe-pipe cradle joint will not occur until the frictional resistance is overcome. So the higher the coefficient of friction, the higher the force transferred to the pipe cradle. There are dozens of these pipe cradles, so at this point the cost of modifying either the pipe-pipe cradle joints or the pipe cradles themselves is prohibitive, not to mention the delay in a project that is already behind schedule. If I can get an accurate value for the COF for this joint, we will at lease have a better idea of where we stand.

One factor working in our favor is that in the normal condition, this pipe is submerged, so the presence of water in the joint will lower the COF. But there will occasionally be situations where the pipe is not submerged so we really need to check the dry condition.

 

[unclesyd]

As the felt isn't anchored to the concrete and held in place by compression wouldn't the shear modulus of the felt govern?
This is an equally hard number to find.
Hopefully some of the structural or civil people will join in and discuss this possibility.

It is done to allow for expansion in socket end pipe.

http://www.hyd.gov.hk/ENG/PUBLIC/PUBLICATIONS/stdraw/doc/section_3/pdf/h3131.pdf

 

[chicopee]

Looking at that last link, now I understand the setup. The felt between concrete craddle and pipe is to provide a cushion for irregularities in the concrete,to minimize abrasion of the pipe and to protect the pipe from chemical reaction with the concrete. Similar situations abound in the world such as resting a propane tank on concrete saddles.
As far as the COF, in my opinion it is really not a factor to consider.

 

[rconner]

I am likewise not aware of any reference that has necessarily applicable tests, at least if the desire is extremely accurate values for this coefficient. Also, you do not mention whether or not there are e.g. bolted tie-down straps (sometimes used to support piping along with saddles), that I would assume could add to the weight/normal force in friction calculations.
Is it however possible that there may not be much longitudinal/thermal movement to worry about in general with most ductile iron piping, particularly if there are not e.g. high pressure thrusts etc. on this system, and maybe particularly if this section of piping happens to be short and continually immersed in fluid, that might also minimize the range of thermal fluctuations? [In any case determinations of anticipated aggregate or segmental thermal movement in a given temperature range could be made with the information on Table 17-12 on page 17-24 at

http://www.acipco.com/adip/products/Sect17.pdf.

]
In any case, if by any chance the existing supports would be dependable for whatever weights and vertical accelerations might be applicable to this posthumous design, is there any chance that the tops of the supporting structures could be somehow supplementary braced to withstand any unintentional over-turning forces in other directions?

 

[Jordonlaw]

I guess you want to do some sort of calculation with the CoF value ? If so an approach could be to use the extreme range of values (maybe 0.4 to 0.5) and see what difference it makes. If a little then don't worry, if a lot maybe you need to do a specific test.