Small pivot error

[jesp]

Hello! I'm trying to model a 3D truss structure (electrical transmission tower) in ANSYS, using the LINK8 element, and after completion of all preprocessing (built geometry, defined materials properties, defined real constants, assigned them to elements, loaded and constrained the model) I keep getting the following error, when attempting a linear static analysis

*** ERROR ***
Large negative pivot value ( -4.434697936E-02 ) at node 316 UZ May be because a rigid body motion has occurred in the structure

In this model X and Z axis form the base plane of the tower and the Y axis represents its height (of approx. 40m)

I get the same message even when unapplying external loads and running the model just with gravity acceleration, to find out own weight of the whole thing.

I could also notice in the ANSYS Output window that the error repeats in several other nodes, and only in the Y and Z directions. i.e.: Small pivot at node 459 UY.

Am I doing anything wrong? I have re checked the geometry and properties and have also looked for any inconsistency in input data, constraints (over or under constraints), and as I said before, I have tried with different load scenarios; including no loads at all.

Hope you can give me some pointers. Thanks in advance,

jesp

 

[Drej]

> *** ERROR ***
Large negative pivot value ( -4.434697936E-02 ) at node 316 UZ May be because a rigid body motion has occurred in the structure

Check your boundary conditions. Rigid body modes and pivots are associated with an a poorly/un- constrained model. Remember also that there is a designated ANSYS forum on Eng-Tips.com to be used for all things ANSYS.

Cheers,

-- drej --


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[johnhors]

Run a modal (natural frequency) analysis, and display the first few mode shapes. These will be at zero frequency and are the rigid body modes, from inspection it should be obvious where your structure is improperly connected.