Why is it that the frictional force at a support is sometimes less than (coeffecient of friction * Normal force)?...What is the mathematical explanation for this?...Thanks in advance fellows...

I think CAESAR II models it as a spring of high stiffness until the breakaway force is reached. In that case, your model is telling you that there is a very small horizontal movement there.

This is consistent with the classical friction model of a high breakaway force followed by a lower coefficient of sliding friction.

http://www.intergraph.com/products/ppm/caesarii/friction.aspx


quote from this link
"...
Ideally, if there is motion at the node in question, the friction force is equal to Mu * Normal force. However, since we have a non-rigid stiffness at that location to resist the initial motion, the node can experience displacements. The force at the node will be the displacement * the stiffness. If this resultant force is less than the maximum friction force (Mu * Normal force), the node is assumed to be "not sliding," even though we see displacements in the output report.

..."

Also see the discussion on "friction" in Chapter 6 of the Technical Reference Manual.

Dear Mr. Richard Ay,

Is the friction coefficient for support shall be defined only once in the restraint option.

For example, if my support system consists of rest with guide and limit stop.

In Caesar-II input spread sheet under the restraint option, am i have to input the friction for each support elements such as

+y with 0.3,
Guide with 0.3 and
Lmt with 0.3

or

only once in +y with 0.3,
Guide (Leaving friction coeff. blank)
Lmt (Leaving friction coeff. blank)

Since there is a lot of contradiction between the contractors and clients..

Please clarify.

I will choose your second option for static analysis.
but guide and limits are considered when doing Dynamic analysis where reaction loads on guide and limits are considerably big.

Look for L.C.Peng docs on friction on static and google the dynamics.

Regards,

It is a stress engineer decision. I normally choose the second option except in front of the pumps (where I can have guides with no gaps)

Regards,

... and friction are to be considered transient load for equipment nozzle loads. "IT CANNOT BE AN AID IN REDUCING LOADS"

As mentioned above search "friction" in the forum.

Regards,

Dear Members,

As of all your reply is acceptable for the piping system inside plant, where the displacement are limited to max of 100 to 125 mm.

But in case of pipeline system (for example, Pumping station), for a bend of 20D elbow, normally the displacements are limited to 300 to 350 mm. To limit the displacement near the small bends (i.e.) cold bend of 40D, PGB's such as Pipe Guide near Bend is provided to limit the displacement.

Please refer following cases,

Case-1 - Supports with friction for all support elements,

+y with 0.3,
Guide with 0.3 and

The loads on supports are Fa = 7.8 kN, Fl = 12 kN and Fv = 13 kN,

My query is since there is no axial stop in the support to limit the pipe in axial direction, why there is a huge axial load on the support. We had a doubt, if there is no axial stop, then the max. axial load on support due to friction shall only be 0.3 * Fv = .3*13 = 3.9kN but why the result shows 7.8kN. After many checks with the same kind of friction effect on support elements, it is seems that the Caesar-II calculates the axial load due to friction for support elements without axial stop as

Fa = 0.3*Fv + 0.3*Fl = .3*13 + .3*12 = 7.8kN (if friction is specifed for all support elements).

Case-2:

If the friction is specified only once in +y with 0.3,
Guide (Leaving friction coeff. blank), the axial load is calculated as

Fa = 0.3*Fv + 0.3*Fl = .3*13 + 0*12 = 3.9kN.

From the above it is seen that there is huge variation in the force in axial direction, the change in force due to friction effect creates a huge variation in the cost of support.

So i conclude, to be conservative it is better to go for case-1 type friction effect on support elements. Since if the support is designed for Case-2 (i.e. for 3.9 kN), after many displacement effect there is a more chance for the support to fail.

Even in Mr.Peng articles, he is advised not to neglect the effect of friction.

If my conclusion is not acceptable, please clarify.....

sorry

Yes your correct! Sometimes guide are used as stoppers.
As an example to this is at pipe rack corners. Where loads due to friction appears very large and this is critical in loading data.

I don´t do this in my practice. But guides are design not to accept large loads. IMO. I rather use stops before hiting the corner.

Regards,

We are not living in a ideal world.

Ideal is to perform both ways (with / without friction). Sometimes results with friction are less conservative.

Remember that the forces are in equilibrium so if you have an extra load on this support you will have less load on other (on paper) and since friction is not something you can rely on, that support will eventually have that load.

Regards,

Your correct! Thats what experienced engineers are payed for. To know what is conservative and what is economical design.

And again you can search for "friction" in the forum.

Regards,

Dear Mr.Danb and Bom,

Thanks for your valuable reply, and whatever you replied is best suitable for the piping, but in pipeline it has some small variations than the piping. With your comments it seems that you people are well enough experience in piping than pipeline.

A person have much experience in pipeline paticular to cross-country or pumping station of many kilometer pipeline can able to understand it.

Thank you...