peteroznewman
Subscriber
nYes, a rotational DOF is the angular rotation about the node.nDegree of Freedom (DOF) also means unknown quantities that will be determined by the solver. Some nodal DOF are removed from the solution by being specified by Boundary Conditions where those DOF are specified, such as X = 0.nSolid elements have mathematical equations built into them that use nodal displacements. There are no equations for nodal rotations. This is because that is not necessary. If the element as a whole needs to rotate, that is done by translating the nodes.nShell elements represent 3D objects with zero thickness geometry. In order to represent bending, there must be equations that include nodal rotation, because you can bend a single linear shell element without any nodal displacement by just rotating the nodes at each end. nYou can?t ?bend? a single linear solid element. If you have many elements through the thickness and along the length, you can represent bending by translation of the nodes of many elements to make a curved shape.nA solid cube with a fixed support on one edge will fail to solve, even if there is no moment about that edge. n *** ERROR *** CP = 1.125 TIME= 17:15:41n There is at least 1 small equation solver pivot term n(e.g., at the UX degree of freedom of node 26). nPlease check for an insufficiently constrained model. nThe mathematical steps the solver uses to compute the deformation of the nodes requires a matrix inversion. If the matrix is singular, which happens if there are not at least six constraints to ground, that causes an error much like dividing by zero. This is done before the forces acting on the structure are used in the solution. nANSYS has a feature called ?Weak Springs? that will put six weak springs to ground for every body in the model. If you turn that feature on, then instead of dividing by zero, the solver gets to divide by a small number. In the case of a single straight edge support, no force will go through the weak springs, but their stiffness will allow the matrix to be inverted.n