"matrix is singular" - problem 1

[amorrison]

I am a ME "playing" with a truss freebe download.
I am constantly getting a "matrix is singular" error with some of the structures I create. What is "wrong" with the structures that I get this error. Thanks.

 

[mloew]

Is the model constrained?

Best regards,

Matthew Ian Loew
"I don't grow up. In me is the small child of my early days" -- M.C. Escher

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[amorrison]

Yes. A fixed node and a rolling node have to be applied.
The software is for designing bridges.

 

[GregLocock]

Just looking at the definition of a singular matrix perhaps the code is designed for non-redundant structures?

Cheers

Greg Locock

 

[GregLocock]

More from the wolfram site: Asingular matricx has a determinant of zero.

5. A determinant with a row or column of zeros has value 0.
6. Any determinant with two rows or columns equal has value 0.

Implication of 5 being you may have a mechanism, which would have a zero stiffness.


Cheers

Greg Locock

 

[jhardy1]

Is the program capable of doing frames (with shear and bending members) as well as true trusses (axial loads only, no shears or moments)?

If so, you can get a singular matrix if you put too many pins in your member ends. In a typical frame analysis program, every node must be rigidly attached to at least one member, even if all members are notionally pin-ended at both ends. Otherwise, the rotation at the node is undefined, and the matrix solution will fail.

Check the documentation for your program for more information.

 

[joekm]

A singular matrix often means an error is specifying the BC's.

BC errors could be surface to surface contact that is incorrectly specified, too many constraints, too few constraints, etc.

Alternatively, you may have transitioned from one mesh to another locally (say, manually specifying a tighter mesh near a bolt hole) and didn't merge the nodes between the regions. Another possibility is changing mesh styles outright and not specifying how the two regions interact (bonding, for example). Often those don't give singular matricies though, just non-sensical results.

Hard to troubleshoot without seeing the problem, but these are things that come to mind offhand.

--
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation