Definition of the Slopes and the Finite Element Absolute Nodal Coordinate Formulation

In the finite element literature, beams and platesare not treated as isoparametric elements becausethe displacement gradients at the nodes are used todescribe infinitesimal rotations. Such alinearization of the slopes leads to incorrect rigidbody equations of motion when the beams and platesrotate as rigid bodies. A simple and efficient absolute nodal coordinate formulation, in which noinfinitesimal or finite rotations are used as nodalcoordinates, can be systematically developed andefficiently used in many large deformationapplications as well as in the analysis of curvedstructures. This formulation leads to a constantmass matrix, and as a consequence, an efficientprocedure can be used for solving for the nodalaccelerations. In the absolute nodal coordinateformulation, the displacement gradients at the nodesare determined in the undeformed referenceconfiguration using simple rigid body kinematics.The problems that arise from the linearization ofthe slopes and the use of the finite rotations asnodal coordinates are discussed in this paper.