Please how can I eliminate moments from a space truss in staad so that I have only axial loads? I am trying to check a 90m high telecoms tower in staad for compliance to EN1993-1-1:2005 only. I tried to assign truss command to all members to eliminate moments but this gives me several instability. When I remove the truss command, I get a design failure of clause EC-6.2.9/3 and EC-6.2.9.1 which implies that staad is analyzing the tower members as beams
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Instability warnings by themselves do not indicate that the analysis results are unreliable. The warning is stating that at certain joints of the model, there is no path for a load along certain directions to reach the support. However, if a load is not acting along those directions, it may not have any adverse effect.
You should check the static equilibrium report in the Post processing - Reactions page to see if there is any mismatch between applied loads and reactions. If they do not match, that means that the instabilities need to be addressed. But if they match, examine the displacements to ensure that they are reasonable. This is because the program adds stiffness amounts by itself to avoid an illegal mathematical operation but they are usually of such small magnitude that though it will cause the equilibrium of forces and moments to be satisfied, they are not sufficient to produce reasonable displacements.
In a space truss, instability usually occurs at joints where all the members at those joints are in only one plane. For example, if you consider a transmission tower which has sloping faces, at every panel, you need members that can take forces in all 3 global planes. If you don't, then, a load such as selfweight which is acting along global Y will have a component orthogonal to the inclined plane of the tower and since there is no member to take that out-of-plane force, that load is "lost" and cannot reach the supports.
The suggestion to declare them as beams (instead of truss members) and then release the moments is to avoid the hassle of investigating the cause of the instability. You can apply a 99.5% partial moment release on all the members of the structure and it will produce just enough out-of-plane stiffness at every joint to avoid instabilities. But this method also requires you to check the displacements.
But if you can figure out the cause of the instability and resolve it using truss members instead of beams that can take bending and shear, that is perfectly OK.