Issue with the force-based element

[OmerOdabasi]

Dear forum,

I would like ask a quick question about a particular error that the force-based element spits. The error message is "ForceBeamColumn3d::update() -- could not invert flexibility\n". You might be familiar with this too. This means that it cannot obtain the element stiffness matrix. I am not entirely sure what is the reason for this but I am usually running into this problem with elements that have a large sectional dimension proportions (h/b). And apparently it is quite sensitive to how the fiber section is created. In some cases I could avoid this problem by increasing the number of fibers along the short side axis of the section but not always. The strange thing is that I run into this problem even right before the structure is in free vibration after a strong shaking (meaning that the error does not arise during the actual loading)... Does the numbering + the solver scheme and how the ax=b is handled affect this? Does anyone have any consistent solution to this problem?

Many thanks.
Ömer

[fmk]

the numbering of the dof does not effect the answer (at least if numerical roundoff is not an issue) .. the error you see from the force beam column is coming from the formulation, it is force based and not displacement based .. as a consequence at each trail set of displacements the element does some iteartion to find equi which a displacement based ele does not have to do .. the iterations require that the section flexibilty be inverted .. if it cannot invert the flexibility matrix you get the error you see .. to avoid the error make sure that the matrix can be inverted (think about adjusting material properties, i.e. if elastic perfectly plastic give the material a small bit of hardening) .. the theory can be found in the references: http://opensees.berkeley.edu/wiki/index ... mn_Element

[OmerOdabasi]

That is exactly what I did, many thanks Frank for the clarification!

fmk wrote:
> the numbering of the dof does not effect the answer (at least if numerical
> roundoff is not an issue) .. the error you see from the force beam column
> is coming from the formulation, it is force based and not displacement
> based .. as a consequence at each trail set of displacements the element
> does some iteartion to find equi which a displacement based ele does not
> have to do .. the iterations require that the section flexibilty be
> inverted .. if it cannot invert the flexibility matrix you get the error
> you see .. to avoid the error make sure that the matrix can be inverted
> (think about adjusting material properties, i.e. if elastic perfectly
> plastic give the material a small bit of hardening) .. the theory can be
> found in the references:
> http://opensees.berkeley.edu/wiki/index ... mn_Element