| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | 2004, XM, V8i | ||
| Area: | Analysis | ||
| Original Author: | Bentley Technical Support Group | ||
| Date Logged & Current Version | Sept. 2015 09.06.02.06 |
Problem:
Why does AutoPIPE provide an option to “Ignore friction E” on the Non-linear analysis dialog?
Solution:
1. AutoPIPE online help:
"This option may be used to satisfy the analysis requirements of ASME B31.3-1996, para. 301.5.3 EARTHQUAKE in which induced friction forces at supports in the non-linear analysis should be ignored."
2. Response on Bentley Community from Karim Rinawi: “ASME B31 codes rely on ASCE 7, UBC and IBC codes for earthquake criteria.
UBC 1997, section 1632.1 states:
Friction resulting from gravity loads shall not be considered to provide resistance to seismic forces.
Same statement is repeated in ASCE 7-2005 section 15.5.2.1 (Pipe Racks).
Similar information can be seen in ASCE-7-2005 Chapter 13, e.g. Section 13.4 (nonstructural component anchorage) and 13.4.6 (Friction clips).
It is true that Section 13.6.8.1 does not refer to 13.4 or 15.5, but I strongly recommend that friction is ignored for static seismic cases. Usually friction (because it is nonlinear) is ignored in all dynamic cases in AutoPIPE unless modeled as an additional spring and with the stiffness based on an assumed displacement (Stiffness = Normal force / Assumed Displacement).
The reason behind ignoring friction in seismic cases is that the earthquake have both horizontal and vertical motions (usually vertical is about 2/3 of horizontal). The upward vertical acceleration will reduce the normal force which friction relies on. So unless you reduce the friction factors for seismic loads based on vertical acceleration, which cannot be done easily, it is best to ignore friction since that is likely to produce conservative results.
Reduced friction factor can be calculated from:
Seismic friction factor = (1-Ey) * static friction factor
Where:
Ey = maximum vertical seismic acceleration in g's, for Ey > 1g use Ey=1g”
3. "Should Friction Be Turned Off for OCC?" Ross Sinclair on Coade forums with some implications:
As you have found there is little in the way of definitive statements regarding friction under seismic action. One of the few is in "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 behavior 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 recognized that zero friction seismic is generally "toughest" on the pipeline; therefore to not analyze zero friction seismic is to ignore a real pipe design case.
I have found that analyzing 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.
Analyzing with full friction tends to reduce the loads on fixed points such as anchors and line stops, and increase the load on other supports.
-Analyzing 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).