Hi, I am a PhD student in math and in my current research I'm faced with the problem of analyzing the rigidity of a structure of rigid beams in 3D, that is if the structure can move. In particular, something like this: http://img525.imageshack.us/img525/1545/98322092.th.jpg [Broken] where all the lower and left lines vertices are fixed. It is thus clear that the row and column formed by little squares are rigid, and the same is true for the bottom-right and the upper-left triangles. However, it seems that this structure is not rigid in 3D since is possible to move the upper-right pair of triangles along the diagonal of the big square by "folding" the little square they form. Is there a way to choose the orientation of the triangles such that the structure cannot move? Is there a theory I can refer to in order to understand this? 2D is very easy, but the counting of degrees of freedom seems different in 3D, if possible at all when the structure has the topology of a surface. Thank you in advance, AQ PS I don't know if the homework area is more appropriate for this question, sorry.