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Typical warning message from LUSAS Solver text output file (*.OUT):


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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