User Area > Advice
Typical warning message from LUSAS Solver text output file (*.OUT):
***WARNING*** ELEMENT P, NODE Q, VARIABLE
R HAS SMALL, NEGATIVE PIVOT -0.1234E-25 AND DIAGONAL DECAY .1234E+15 > .10E+05
Most common causes & links to solutions
This message points to a specific
element P, node Q and degree of freedom R relative to global axes
(eg variable 5 => THY rotational freedom) and is generally related to a difference in stiffness between the elements and/or boundary conditions at this node.
Modelling errors (like mesh disconnectivity) may cause this warning
too.
Negative pivots can be the result of
ill-conditioning. However, a well conditioned stiffness matrix can produce a negative pivot if:
- The system is unstable. An unstable structure could be passing through a bifurcation or limit point
- The system is not adequately restrained/supported
- A mechanism has been excited
The most common cause of such a message is that there are insufficient
supports assigned to the model. It is necessary to ensure that
there exist supports that restrain the entire model from rigid body
movements. The model should not be free to translate or rotate
freely. More common
causes on a page of related warnings
For further information see:
Help > Help Topics >
Contents > Modeller Reference
Manual > Appendix B : LUSAS Solver Trouble Shooting > LUSAS Solver
Errors > Diagonal Decay Warnings and Negative Pivot
How to find the target Node
or Element:
Solver error or warning messages are
also scanned from Solver .out file and reported in Modeller Text
Output window. By default the Text Output window
appears at the bottom of the graphics window just above the Status
Bar. This displays messages and warnings during a modelling session.
Specific objects referred to in error and diagnostic messages in the
text window by ID number in the form "Element P" may be
identified using the identify object dialog. The “Identify
Object" dialog is invoked by double clicking on the message
text. From this dialog the identified object may be selected, moved
to the centre of the screen, scaled to fill the screen, have its
properties displayed, or identified by an annotation arrow or a
temporary indicator.
Element P can also be located in the
model using Advanced Selection, activated by right-
clicking with the mouse in the graphics window, or via:
Edit > Advanced Selection
Continued difficulties? Try
the pivot checklist
More on cause & theory links
More
on Diagonal decay
More
on Negative pivots
Use of the option in Solution Control "Continue solution if more than one negative pivot occurs" should be used with care, as it is likely to hide more fundamental analysis problems. It invokes the LUSAS option number 62.
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