Equal DOF incoherency in Eigenvalue Analysis
Posted: 19 Oct 2010, 19:37
Hi, I'm working on a model where I have pairs of nodes connected with equal DOF constraints (x+y+rx+ry+rz), in order to unite originally separated buildings.
When I run an eigenvalue analysis the deformed shape of the first 3 modes is very reasonable and one can see the expected effect of those constraints.
However, from the 4th mode on, I realise that the rotations of those pairs are not equal. In fact, the deformed shape shows almost independent behaviour of the buildings.
First I thought this could be just some sort of graphical bug. But then I checked the step output table and the rotations that are supposed to be equal on those pairs of nodes are actually different, confirming that it's not just a graphical problem. I've checked the constraints very carefully and I'm sure that they are connecting the right nodes. In fact, all the translations are equal. Only the rotations are different and only from the 4th mode on (vertical modes).
I apologise for the extension of the text. Thanks a lot once again for this forum.
When I run an eigenvalue analysis the deformed shape of the first 3 modes is very reasonable and one can see the expected effect of those constraints.
However, from the 4th mode on, I realise that the rotations of those pairs are not equal. In fact, the deformed shape shows almost independent behaviour of the buildings.
First I thought this could be just some sort of graphical bug. But then I checked the step output table and the rotations that are supposed to be equal on those pairs of nodes are actually different, confirming that it's not just a graphical problem. I've checked the constraints very carefully and I'm sure that they are connecting the right nodes. In fact, all the translations are equal. Only the rotations are different and only from the 4th mode on (vertical modes).
I apologise for the extension of the text. Thanks a lot once again for this forum.