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This question originates from this and that question. I have realized the concerns boil down to the following figure.

enter image description here

Simply put, is the truss above, with the displayed boundary/internal fixity conditions and distributed loading, structurally stable?

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    $\begingroup$ You've clearly created this in a structural analysis program. Why don't you simply see if it is successfully calculated? The program will warn you if there's anything wrong with the structure. And yes, this should be stable. $\endgroup$ – Wasabi Mar 17 '16 at 14:57
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    $\begingroup$ Also, you seem to have agreed with the answers given in the previous questions. If so, you should accept them (full disclosure, I answered one of them). $\endgroup$ – Wasabi Mar 17 '16 at 15:02
  • $\begingroup$ 1) I've accepted your answer to the last question, however it does not result in a stable structure given the current loading. The structure is not stable according to the program. Therefore I believe this needs to be taken a step further as far as understanding why it is not stable and refining the loading/boundary or internal fixity conditions such that it becomes stable. $\endgroup$ – user32882 Mar 17 '16 at 15:20
  • $\begingroup$ Wait, are you using a 3D analysis program? If so, then you can't simply use pinned and roller supports, since the structure won't be restrained from rotating around the x and y axes. One of your supports needs to restrain such rotations. I was assuming you were using a 2D program, which only considers rotations around the z-axis. $\endgroup$ – Wasabi Mar 17 '16 at 15:20
  • $\begingroup$ I'm using RISA3D and fixing all out of plane displacements. The boundary conditions are fine, the instability occurs at node 6. $\endgroup$ – user32882 Mar 17 '16 at 15:23
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As mentioned in the comments to the question, since you are using a 3D analysis tool, you need to fix rotations around the X and Y axes. If you had been using a 2D analysis tool, simple pinned and roller supports would be suficient since rotations are only allowed around the Z axis, but since this is a 3D model, rotations around the other axes must be restrained.

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@Wasabi is right. I fixed Z displacement and X and Y rotations at each and every node and it worked. The truss is structurally stable for given loading. However it is important to be careful with software assumptions and fix nodes where necessary to simulate a 2D model in a 3D program.

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