"Structure is unstable" -Recommendations to find the problem

[amorrison]

I am designing - for education- a 3D box truss of three similar(but not exactly the same) repeat sections.

If software is giving me the warning "Structure is unstable" .

Is there any systematic method of finding the problem?

The computer screen structure looks "good".

The software is "Dr. Frame".

Thanks

 

[corus]

Unstable means you haven't applied enough restraints or it's buckled under load, probably the former.

corus

 

[ThomasH]

One way, provided that the software can do it, is to run a eigenfrequency analysis. That will give you any rigid body modes that need to be constrained.

Good Luck

Thomas

 

[Stringmaker]

ThomasH and Corus are correct you have and underconstrained model from the sound of things. As ThomasH suggested, the best way to determine where the instabilities are occuring is to perform a modal analysis. You should only need to specify that that the first five (or less depending on how many places it's unstabile) modes be extracted. Review your results and the areas needing attention will be rather obvious by their large displacements.

Good luck!
-Brian

 

[crisb]

Truss members are pinned at each end so there is a possibility that you have inadvertently created a mechanism if the structure is not fully braced.
A systematic method of finding the problem would be to keep applying constraints until the model runs and then deduce the offending joint. Unless of course Dr Frame does frequency analysis when analysis of the mode shapes would be easier.

 

[GregLocock]

If you look in a statics book there is a theory that tells you whether you have enough constraints and joints and so on to form a determinate structure. Sorry, I can't remember what it is called.

Cheers

Greg Locock

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