Hi guys. Question. I have searched the forum and not found the answer so please forgive me for posting if the answer is already out there.

Can credit be taken for friction when running OCC seismic? In other words, do I turn off friction, or leave it on, when evaluating OCC seismic stresses?

I am talking here about computed stress level in the pipe, not loads on the restraints.

B31.3 is silent on this issue, as far as I can tell. ASCE 7-05 says "Friction resulting from gravity loads shall not be considered to provide resistance to seismic forces". Note that that section of ASCE 7 is referring to the design loads to be used for the design of the restraints, not the evaluation of stresses in the piping. That tells me that, yes, I do turn off friction to calculate restraint loading, and I use those loadings to design my restraints. But it has nothing to do with the stress level in the piping itself.

In my opinion it is an out-of-context extension of ASCE 7 to assume this section tells me I have to turn off friction when evaluating OCC seismic pipe stresses. Td do that I believe will provide excessive conservatism in the OCC stresses. It is not a priori known whether a seismic event will lift the entire system off the restraints therefore eliminating horizontal loads on the restraints due to gravity seismic. That would be the most conservative approach however so perhaps that is why folks do it. But that approach also leads to overdesigned restraints.

I'm certain I'm not the first guy to ask this. What say ye? Thank you!!!! Pete

Pete,

First, read this article - "Treatment of Support Friction in Pipe Stress Analysis " at Peng's website(http://www.pipestress.com/Pages/PEpapers.html)
and read this FAQ(http://www.coade.com/FAQ/167), and obtain how CAESAR II applies friction in the analysis.
Then use 'Advanced' option in Search function to find post about 'friction'.
Here, Dear danb reply some posts related your query. Check it out

http://65.57.255.42/ubbthreads/ubbthread...=true#Post49888

As you have found there is little in the way of definitive statements regarding friction under seismic action. One of the few is in ALA:

Seismic Design and Retrofit of Piping Systems – American Lifelines Alliance – 2002
Section 8.2 The seismic design should not take credit for the friction force between pipe and support, which tends to reduce seismic motion of the pipe.

Consideration of the behaviour of a piping system in a seismic event must lead you including analysis using the zero friction case. Friction is commonly modelled as 0.3g. However the rolling wave nature of a seismic event causes the vertical acceleration to vary, and therefore the friction component to vary. And it is practically impossible to calculate. So we know that friction forces acting on the piping system lie somewhere between full friction and zero friction. It is recognised that zero friction seismic is generally "toughest" on the pipeline, therefore to not analyse zero friction seismic is to ignore a real pipe design case.

I have found that analysing for zero friction drives you to good pipe supporting solutions. Such solutions generally mean the piping is well restrained, and is therefore also able to resist other types of shock loadings such as water hammer, two-phase flow, slugging, pressure relief loads etc.

- Analysing with full friction tends to reduce the loads on fixed points such as anchors and line stops, and increase the load on other supports.

- Analysing with zero friction tends to put large loads on the fixed points and reduces the loads on other supports. It doesn’t increase the total loads, but it does tend to focus them more.

We design a lot of cross-country pipelines, and one of the consequences of using zero-friction seismic is that you are driven towards putting side guides or clips on almost all supports. This means that lateral seismic forces are shared amongst almost all supports, rather than being taken on say every 3rd or 4th support – and this is a good thing.

We actually run both friction and zero-friction analysis in one model and output both cases for pipe support design (using a different analysis software that has load sets – which makes the setup relatively trivial).

Ross - Thanks for the great comments. I appreciate your input.

Since apparently there is no direct Code guidance on this, I believe the question on this issue then becomes, at least as far as the lawyers would say, is 'what would another similarly-experienced individual, of equivalent skill level, exercising a reasonable level of care and custody, do in this situation?' Again I guess the conservative answer is as you stated - turn off the friction. Thank you!