Problem:

AutoPIPE: E801-1: Unstable System

When running an analysis the following occurs:

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Error
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 E801-1:   FATAL ERROR : Unstable system. Need support at node XXX    Dir XX
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OK  
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Why and how to fix it?

TR Number: None
Product: AutoPIPE
Version: n/a
Area: General, analysis
Problem ID#: 75241

Solution:

There are many issues that can cause an unstable system. Insufficient supports, not enough guides for wind or earthquake loading, insufficient soil stiffness in the vertical direction for soil problems. Evaluate your support arrangement for your particular loading conditions to see where the instability lies.

Please see the following AutoPIPE help section:

Help > Contents> Contents Tab> Bentley AutoPIPE> Frequently Asked Questions> FAQ document> FAQ #19.

In addition, Please see the following AutoPIPE help section:

Help > Contents> Search Tab> enter "E801" (include the quotes), press List Topics button, double click on the selected topic from the list provided to see more information.

The error message mentions a possible solution (i.e.Need support at node XXX    Dir XX), this is just a one possible solution to the satisfy the stiffness matrix. After inserting a support at this location, the error message, E800-1,  may reappear suggesting another support at another location. This will continue until the stiffness matrix is satisfied and the analysis is completed without error. Another approach would be to understand how the model is moving due to load cases, friction, gap settings, etc.. Sometimes adding a support at one specific location or reducing gaps / friction at a few key support(s) are enough to enable the analysis to run without error. 

Again, there could be numerous reasons for the "Error "E801-1: FATAL ERROR: Unstable System" message to appear while running a stress analysis, see the following for scenarios and respective fixes to better understand how to resolve issues related to this error message:

Scenario #1:

Model has multiple connected segments groups in the model and the error message above appears.

Fix

Check that for each group of connected segments (Select> Connected Segments) in a model has adequate stiffness to prevent free rotation about any axis or displacement along any axis.

Typically adequate stiffness is in the form of at least 1 anchor on each group of connected segments. 

See Video here:

Scenario #2:

Back to back expansion joints where Torsional stiffness = 0.00.

Fix:

All AutoPIPE components must be connected to an object (i.e. anchor) to resist being rotated freely in any one of the 6 axis. In this case the expansion joint may be connected to an anchor through some piping but due to the "Torsional stiffness" = 0.00 the piping between the expansion joints would freely spin. To stop the warning from appearing, set Torsional stiffness = +ve value >0 (ex. 0.01).

Scenario #3:

With the following model and anchor settings:

Fix:

The problem with the model above is that there are no supports / anchors to stop the entire model from rotating about the X-axis as all the anchors are set with Rx stiffness = 0.00 and a Line Stop support only provides axial restraint. There must be some small resistance in All directions for the analysis to run. In addition, all the line stops have a large gap. 

Simply change an anchor to have some small amount of resistance (i.e. 0.10 Ft-lb/deg) for Rx and change 1 line stop to have a 0.00 gap or change 1 anchor X-Stiff setting to a small value (i.e..0.10 lbs/in)   

Scenario #4:

The following warning message was found in the Model consistency check Report:

* * * W A R N I N G - MODEL * * *
W726-14: No anchors in current system

Scenario #5:

Model contains one or more Nozzle elements. If the nozzle element stiffnesses are too low the model is able to move such that the E801-1 error message is displayed. To prove this is the problem, save the model as a different name and delete all the Nozzle elements in the model and re-run the analysis. If the E801-1 does not appear then the problem is directly related to the Nozzle Stiffness values.

Scenario #1:

After importing in the frame model through Pipelink. Error "E801-1: FATAL ERROR: Unstable System. Need support at node XX" is encountered. The AutoPIPE model (without the frame model) runs with no issues. I'm unsure why AutoPIPE would think the frame model is unsupported when the frame model runs with no problems in STAAD.

FIX:

While reviewing the AutoPIPE model, multiple imported beams were found to have their ends released in all direction, (i.e. on a beam dialog screen is the following option "End Releases: End-I/J Axial (translation), Y-Y and Z-Z".

Therefore the beams were able to freely rotate. AutoPIPE requires that all components are restrained to some small degree in at least one direction.

Checked one box for Axial (translation), Y-Y or Z-Z for all beams.

Note: The STAAD model failed when loads were applied, it was also fixed.

Scenario #2:

Using copy / paste command to create a 10' x 10' grid of beams / pipes over 100' x100' area. when running analysis the message appeared.

Fix:

When using copy / paste command, only the initial pasted to point is actually connected. All other connections need to be manually connected. Every intersection across the 100 x 100 ' area needed to be correctly connected. Confirm connection at each end point of beam / pipe

Scenario #3:

After importing in the frame model through Pipelink. Error "E801-1: FATAL ERROR: Unstable System. Need support at node XX" is encountered. The AutoPIPE model (without the frame model) runs with no issues. The model appears to be all correct and all the beam are were found to be all connected.

Fix:

1. Go to Modify – Properties of Beam Section.
2. Select Section ID 2 (UA90x90x8)
3. Toggle from Standard to Non Standard Section type
4. Do not alter any properties, just click Apply and OK.

Now the model is OK.

The development team is looking into this issue and hope to have it resolve in AutoPIPE 9.6 and higher.

Scenario #4:

If all the beams in the model have been double checked to confirm connectivity. Check the Beam properties,

a. Some values may be 0.000, as shown below:

Fix:

Increase the wall thickness and try to re-analyze the system. This may be an iterative procedure until the analysis is able to run.

b. Some values may be extremely large, as shown below:

Fix:

The problem seems to be with defining a very high modulus value for the Rigid material defined for the Rigid beams. This value is too high even for a rigid, and when several such high rigidity beams connect on to a point which also has a very low rigidy element connected results in an unstable solution.

Setting the modulus value for the rigid to a more reasonable value (say 1000 times steel modulus = 200 E6 N/mm2) resolves the problem.

Scenario #5:

The following is displayed: "E801-1: FATAL ERROR: Unstable System. Need support at node XX"

Fix:

The model Looks ok, however upon closer inspection, the beams are connected between the anchor (B0) and the piping (7750) using Incline supports, shown here:

This would cause the Fatal Error issue, WHY?

Because, if one would check the bearing spring direction of an incline support, one would clearly understand that it is in the axial direction between the 2 end points specified on the dialog (ex. 7750 - B1).. 

Therefore there is not enough supports resistance on the 6 different global axis to prevent the piping from rotating / moving laterally out of control, ultimately resulting in the Fatal error message being displayed. 

Workaround

Insert additional supports as needed to prevent Fatal Error message from being displayed when Analyzing the model 

See Also

Error Messages

Bentley AutoPIPE