RBEs and MPCs in MSC.Nastran.ppt

RBEs and MPCs in MSC.Nastran A Rip-Roarin’ Review of Rigid Elements RBEs and MPCs Not necessarily “rigid” elements Working Definition: Motion at one GRID drives another Simple Translation Motion at one GRID drives another Simple Rotation RBEs and MPCs Small Displacement Theory Rotations Small displacement theory: sin(?) = tan(?) = ? cos(?) = 1 For Rz @ A RzB = RzA= ? TxB = (-?)*LAB TyB = 0 Typical “Rigid” Elements in MSC.Nastran Geometry-based RBAR RBE2 Geometry- User-input based RBE3 User-input based MPC Common Geometry-Based Rigid Elements RBAR Rigid Bar with six DOF at each end The RBAR The RBAR is a rigid link between two GRID points The RBAR Can mix/match dependent DOF between the GRIDs, but this is rare The independent DOFs must be capable of describing the rigid body motion of the element RBAR Example: Fastener Use of RBAR to “weld” two parts of a model together: RBAR Example: Pin-Joint Use of RBAR to form pin-jointed attachment The RBE2 One independent GRID (all 6 DOF) Multiple dependent GRID/DOFs RBE2 Example Rigidly “weld” multiple GRIDs to one other GRID: RBE2 Example Note: No relative motion between GRIDs 1-4 ! No deformation of element(s) between these GRIDs Common RBE2/RBAR Uses RBE2 or RBAR between 2 GRIDs “Weld” 2 different parts together 6DOF connection “Bolt” 2 different parts together 3DOF connection RBE2 “Spider” or “wagon wheel” connections Large mass/base-drive connection RBE3 Elements NOT a “rigid” element IS an interpolation element Does not add stiffness to the structure (if used correctly) RBE3 Description RBE3 Description By default, the reference grid DOF will be the dependent DOF Number of dependent DOF is equal to the number of DOF on the REFC field Dependent DOF cannot be SPC’d, OMITted, SUPORTed or be dependent on other RBE/MPC elements RBE3 Description UM fields can be used to move the dependent DOF away from the reference grid For Example (in 1-D): RBE3 Is Not Rigid! RBE3 vs. RBE2 RBE3 allows warpingand 3D effects In this e