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The following occurs when I run an analysis:
---------------------------Warning--------------------------- W801-121: V-stop liftoff(s) have occurred under operating load. See support output for details---------------------------OK ---------------------------
what shoudl I do?
This warning occurs when pipe moves in a +ve upward direction off of the support (AKA lift-off has occurred). Another words in the forces and moments report, for any combination where the total vertical +ve force is greater than or equal to the Gravity (-ve) force, lift-off has occured.
Please see the following AutoPIPE help section: Help > Contents> Contents Tab> Bentley AutoPIPE> Frequently Asked Questions> FAQ document>
Question 22: When I see the V-stop Lift-off warning message after the static analysis how can I find out where the lift-off is occurring?
Most programs (at least the popular ones like CAESARII and AutoPIPE) evaluate sustained stress under the cold condition.
AutoPIPE correctly considers the support lift-off effect on expansion stress by differencing two real operating cases (Hot i.e. GR+T1) - cold i.e GR. It also gives a warning when support lift-off is detected. The recommended practice at least by one B31 committee member, is to use cold sustained stress (one that ignores support lift-off) when the lift-off is less than about 1/4" as it would be too conservative otherwise and sustained displacements of that order would not constitute failure. The logic of this is that this 1/4" displacement is self limiting like the expansion case and the system would settle to this no-liftoff configuration before it fails.
For large support lift-off, many but not all, engineers tend to delete the supports that have the lift-off, and then rerun for hot sustained stress case. This is easy in AutoPIPE as you can list supports with lift-off using the results grid or support filter.
Results Grid Method:
On the Support Tab of the Results Grid, uncheck all load cases except combinations (i.e. GRT1, etc..) and doubel click on Global DY column to sort by displacement looking for any value >0.00. If your company procedure requires removeal and re-analyze, delete these supports where lift off has occurred and rerun the analysis and look only at sustained stress. You can then undo this support deletion after the check or save the model as another name if you wish.
Support Filter method :
Please see the following AutoPIPE help section: Help > Contents> Search Tab> enter "Show lift-off supports" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information.
Most designers try to avoid large lift-off because of this issue and so they move the supports or replace by hangers.
Many companies have their rules in dealing with support lift-off and we do not side with any approach.
always use cold sustained and ignore lift-off
always remove uplifted supports and rerun for hot sustained
ignore lift-off if less than a certain tolerance, like 1/4" or 1/16".
The ASME B31.3 piping code has a new alternate method (Appendix P) for evaluating operating stresses as well as expansion stresses. This may partially address the hot sustained issue as this sets an allowable for operating loads that include gravity, pressure and temperature. AutoPIPE evaluates these under the hot condition.
In conclusion, Bentley cannot provide you with specific details on how to handle support lift-off. Your company should have standards set in place for such occurrences.
****Note: Also see problem Id: 67425Title: CAE-CR-2815, v-stop lift off warning
In addtion, Please see the following AutoPIPE help section: Help > Contents> Search Tab> enter "VStop LiftOff" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information.
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