The Beam Interface
The Beam (beam) interface (), found under the Structural Mechanics branch () when adding a physics interface, is used for modeling slender structural elements, having a significant bending stiffness. The formulation allows geometric nonlinearity, with large rotations and small strains, and beams can be modeled on 2D boundaries and 3D edges.
Two-noded straight elements with a Hermitian formulation are used. Two different assumptions about the physics can be used:
Among the computed results are displacements, rotations, stresses, strains, and section forces. In addition to giving the beam properties explicitly in terms of area, moment of inertia, and so on, several predefined common cross-section types are available. Cross-section data to be used in Cross-Section Data settings can be computed using The Beam Cross Section Interface.
The Linear Elastic Material node is the only available material model.
When this physics interface is added, these default nodes are also added to the Model Builder: Linear Elastic Material, Cross-Section Data, Free (a condition where points are free, with no loads or constraints), and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, loads and constraints. You can also right-click Beam to select physics features from the context menu.
Settings
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is beam.
Sketch
In the Sketch section, a conceptual sketch of the degrees of freedom in the Beam interface is shown.
Beam Formulation
Select Euler-Bernoulli or Timoshenko to use the appropriate beam theory. Timoshenko theory includes the effects of shear flexibility and rotary inertia, and is appropriate for beams with cross-section dimensions which are large relative to the length of the beam.
Structural Transient Behavior
From the Structural transient behavior list, select Include inertial terms (the default) or Quasistatic. Use Quasistatic to treat the dynamic behavior as quasi static (with no mass effects; that is, no second-order time derivatives). Selecting this option gives a more efficient solution for problems where the variation in time is slow when compared to the natural frequencies of the system. The default solver for the time stepping is changed from Generalized alpha to BDF when Quasistatic is selected.
This is often the case when the time dependence exists only in some other physics, like a transient heat transfer problem causing thermal strains.
Automated Model Setup
This section will only be displayed if a mesh on NASTRAN® format, containing RBE2 elements, has been imported in an Import node under Mesh. The purpose is to automatically create rigid connectors from RBE2 elements in the NASTRAN file.
An RBE2 element represents a rigid connection between a set of mesh nodes. This means that it can, and often does, connect elements from different physics interfaces.
In the menu in the section title, you can select Create Rigid Connectors from RBE2. The effect is that one rigid connector will be created for each RBE2 element in the imported file. This will happen for all physics interfaces in the Interfaces list. Supported interfaces are: Solid Mechanics, Shell, Beam, and Multibody Dynamics. If there are RBE2 elements spanning more than one physics interface, they will be automatically connected.
The created rigid connectors will have point, edge, and boundary selections as inferred from the nodes in the RBE2 element and the mesh connectivity. The ‘independent node’ of the RBE2 element is used as center of rotation for the rigid connector.
The Automated Model Setup section is present in the Solid Mechanics, Shell, and Beam interfaces. In a model that contains several physics interfaces, you should use the automated model setup from only one of them, and make sure that all the involved interfaces are selected in the Interfaces list.
Advanced Settings
You can choose how extra ODE variables added by some features are grouped in the Dependent Variables node of a generated solver sequence.
Select the Rigid materials check box to group variables added by Rigid Material nodes.
Select the Rigid connectors check box to group variables added by Rigid Connector nodes.
The selection made in the Advanced Settings section can be overridden by the settings in the Advanced section of the Rigid Material or Rigid Connector features.
Discretization
The discretization cannot be changed. The element has different shape functions for the axial and transversal degrees of freedom. The axial displacement and twist are represented by linear shape functions, while the bending is represented by a cubic shape function (“Hermitian element”).
Dependent Variables
The Beam interface has these dependent variables (fields):
The displacement field u, which has two components (u, v) in 2D and three components (u, v, and w) in 3D.
The rotation angle θ, which has one component in 2D (th) and three components in 3D (thx, thy, and thz).
The names can be changed but the names of fields and dependent variables must be unique within a model.
The dependent variable names remain same in both a geometrically linear and a geometrically nonlinear analysis. Under geometric nonlinearity, the dependent variables are however not defined though shape functions. The equivalent shape function variables are (beam.uLinx, beam.uLiny, beam.uLinz) and (beam.thLinx, beam.thLiny, beam.thLinz). In this case, you will see the latter names under Dependent Variables in the Solver Configurations tree.
Channel Beam: Application Library path Structural_Mechanics_Module/Verification_Examples/channel_beam
Instability of a Space Arc Frame: Application Library path Structural_Mechanics_Module/Verification_Examples/space_frame_instability