Interpretation of a common Error Message 1

[Cansand]

I keep getting this message which I cannot understand deeply its meaning

"Solver problem. Zero pivot when processing D.O.F. 8 of 45 nodes. The nodes have been identified in node set"

(as far as i know that this means that there are zeros diagonals in the stifness matrix which usually reult from rigid motion, instability ..)
Plesae if you know give details about the message

 

[xerf]

Rigid motion of the D.O.F. 8 ?
DOF8 typically is (pore) pressure.

 

[Cansand]

xerf Thanks alot for your reply. Physically what does
rigid motion of pore pressure DOF mean.? For displacement this is very clear , but I am trying to think for temperature or pore pressure when zero pivotal stiffness or negative eigenvalue occur
Physically, in heat (or pore pressure) analysis flow corresponds to force and temperature (or pore pressure) corresponds to displacement.

 

[gurmeet2003]

Cansand,

In solid mechanics, FEA program solves the equations of motion/equilibrium equations. For static analysis, the forces and reactions have to balance out to zero within some tolearance. The reactions are created at the points of constraint.
If a particular degree of freedom does not have any constraint defined, then if there is no load in that direction theoratically there should be no problem. However when the discretized problem is solved on computer, due to round off errors some residual forces are created. Since there is no constraint some very large artifical displacements are calculated.
You can interpret above for other branches of physics in a similar manner. Some equation which should thoeratically balance out to zero does not do so due to round off errors. As a result large values of corresponding variables are calculated.

Gurmeet

 

[Cansand]

This is what we call Ill-conditioning .
Thanks Gurmeet... I will set dowin quitely and project your interpretation on the difussion problem.