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 should 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 occurred.
a. 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?
b. In addition, 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
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 evaluating two real operating cases, Hot (i.e. GR+T1) - Cold (i.e GR). When a net +ve up movement is detected a warning for support lift-off displayed. 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-lift-off 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 double click on Global DY column to sort by displacement looking for any value >0.00. If your company procedure requires removal and re-analyze, delete these supports where lift off has occurred and rerun the analysis looking only at sustained stress. After the check, you can then undo this support deletion 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 with a spring hanger type support. Again, many companies have their own rules in dealing with support lift-off and therefore Bentley does not side with any approach. It is up to the user what to do when Lift-off occurs:
a. Always use cold sustained and ignore lift-off
b. Always remove uplifted supports and rerun for hot sustained
c. 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 addition, it may be a tolerance issue. Check the displacement at the support for that case and see vertical displacement is greater than zero. If it is very small displacement like +0.001 inch or zero that means it is a tolerance issue. You may reduce the nonlinear iteration tolerances if you wish to have this support removed from the list of supports where lift off is occurring.
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.