a question about elements death

[liushuhao]

dear all :
I use "remove" command to kill the solid elements by testing the Mise stresses .
while I have met a difficult problem： if I kill too many elements during the analyze , it will throw an error “ 0x7c931948 0x00000000 read （I don't know how to translate it to English） or !!!pfsfct(): The diagonal entry 46812 is zero !!!
In SymSparseLinSolver: error in factorization.
WARNING Linear::solveCurrentStep() -the LinearSOE failed in solve()
StaticAnalysis::analyze() - the Algorithm failed at iteration: 0 with domain at
load factor 0.75
OpenSees > analyze failed, returned: -3 error flag”
and cann't complete the analyzing successfully . can you tell me why？ about the stiffness matrix？thanks！

[vesna]

It looks like you removed too many elements so your system became unstable.

[fmk]

make sure you have no hanging nodes.

[liushuhao]

thanks vesna. I also think so .
thanks fmk , I make sure I have remove the invalid nodes because I have written some tcl commands to judge wether it is necessary to kill the nodes by using getEleTags and getNodeTags Commands . I have test a solid beam by killing elements during analyzing.it runs well .
so I want to know how I can fix the problems when I kill too many elements ？ another question ， it seems that the died elements release their stresses to the next step disposable， that makes the residual elements increase their stresses suddenly，so many elements reach the stress level. how can I fix it ?

[fmk]

you can't .. that what happens when you remove an element, the resisting forces that it supplied are gone, the nodes are unbalanced and equi must be restored on the next step.

[liushuhao]

[quote="fmk"]you can't .. that what happens when you remove an element, the resisting forces that it supplied are gone, the nodes are unbalanced and equi must be restored on the next step.[/quote]



deer fmk
I have a question about elements death（remove elements command）
as I know, other FEM softwares as ansys , abaqus and so on ，they kill elements by multiplya minimal coefficient to the stiffness matrix.what about opensees? does it delete the elements directly from the model? if it is ,how can I change the remove elements methods ？

[fmk]

OpenSees removed the elements from the model .. applying minimal coefficients to the stiffness terms may lead to numerical problems (conditioing of matrix if hanging nodes) and does not help your unbalanced force problem.