Can a truss with released ends still form a mechanism?

[gabuchia]

Dear all! I have encountered a problem with modelling a truss.

When modelling on an analysis software, you must release the ends of the joints, but not all
or it will form a mechanism, therefore you release all but one of the joint.
But what I have here, is a truss that is released on all ends except for one for every joint,
yet a mechanism still seems to occur here are the pictures! They're attached!
Try modelling it! If it is possible, please do let me know what the problem is! This little experiment is driving me nuts =D. All I could pull out is that, one of the nodes will rotate, but I can't make sense out of it.

Thanks!

Edit: the centre is not a node

 

[PhanthomJay]

If the center is not a node, you have instability, since your triangles don't exist. Pin everything together at the center node.

 

[AlephZero]

To see what the mechanism is with one diagonal removed, suppose you remove the horizontal one.

Then, bars 2 and 5 form a mechanism so you can delete them, and the same for bars 4 and 7.

The 4 bars that are left form a "crossed quadrilateral" or "bowtie" which also has a mechanism.

Post #2 is wrong. The complete structure doesn't have any mechanisms without a center node, even though it can't be decomposed into triangles. But it is not statically determiate, so you can't analyse it with the "standard" methods for statically determinate structures. The forces in the different bars depend on their relative stiffness as well as on the applied loads.

The main reason why people used to design trusses from triangles only was so they WERE statically determinate and easy to analyse by hand, not because they were the most efficient structural designs.

 

[PhanthomJay]

Post #2 is wrong. The complete structure doesn't have any mechanisms without a center node, even though it can't be decomposed into triangles. But it is not statically determiate, so you can't analyse it with the "standard" methods for statically determinate structures.

Indeed, the structure is statically indeterminate, yet stable without the center node. Sorry about that, thanks for the correction.