Name

ebc — Essential boundary conditions block

Synopsis

ebc id [system s] [title t]
  value v
  dof list
  nodes...
  value v
  dof list
  nodes...
  ...
end

Description

The ebc command specifies one or more essential boundary conditions (EBC), i.e. single-point constraints, to variables, such as displacements in case of solid mechanics, temperatures in case of heat transfer analysis, etc. A ebc set is identified by id, a non-negative integer which must be unique for the ebc conditions of the current model. id is the number which is referenced by the ebc option of the case definition. Sets with an id of 0 will be active for all analysis cases.

Essential boundary condition can be specified for nodes (nodes option) or, in case where elements have internal degrees-of-freedom, for elements (elements option).

Options

system branch | local

Specifies the reference frame in which the conditions are formulated. The default reference frame is local.

branch means that the constraints are formulated with respect to the branch-global reference frame, irrespective of node-local reference frames at any node.

local means that the constraints are applied to the node-local reference frame(s) where defined, and to the branch-global reference frame otherwise.

title "..."

Specifies an optional title for the ebc set.

Specifying Values and Degrees-of-Freedom

The directive value is used to specify the magnitude. This value is assigned to one or several degrees-of-freedom by means of the directive dof. The couple (value, degrees-of-freedom is then assigned to individual nodes or collections of nodes.

The directive value v takes a single floating-point number, for example:

value 0

The directive dof is followed by a list which identifies the degrees-of-freedom, for example:

value 0 dof [UX RY RZ]
  • In stress analysis, UX, UY, and UZ represent the displacement in the x-, y-, and z-direction, respectively.

  • In stress analysis, RX, RY, and RZ represent the rotation (in radians) about the x-, y-, and z-axes, respectively.

  • In heat analysis, T means the temperature.

  • When a single degree-of-freedom is locked or set rather than a list of degrees-of-freedom, the square brackets can be omitted:

    value 0 dof RZ
  • The degrees-of-freedom that are present at a given node depend on the elements to which this node is connected. Specifying a value for a degree-of-freedom that does not exist will have no effect.

The following directives are available to assign essential boundary conditions to individual nodes and to collections of nodes:

allnodes

Assign the essential boundary conditions to all defined nodes of the current branch.

branch br

For models that consist of several branches. Specifies the external branch number br. To be used in conjunction with the allnodes and nodes directives.

epatch id p1-p8|e1-e12|f1-f6|b

When the discretization of a part of the discretization was created by means of the epatch command, a number of pre-defined nodelists are available for use with the ebc command. The epatch is identified by id.

Individual patch vertex nodes are specified with p1 to p8.

The collection of nodes that are located at a patch edge are specified with e1 to e12.

The collection of nodes that are located at a patch face are specified with f1 to f6.

The collection of nodes of the whole patch body are specified with b.

nodes list

Specifies a list of nodes (of the current branch) to which the essential bounndary conditions will be assigned.

nodelist name

Specifies the name of the node list to which the essential boundary conditions will be assigned.

nodeset name

Specifies the name of the node set to which the essential boundary conditions will be assigned.

Example

Specify the set 123: For the mesh node 1, set all 6 degrees-of-freedom to 0. For the mesh node 9,10,11,12, set the z-rotation to 0, and for mesh node 9, the y-displacement 2 to 0.12.

ebc 123
  value 0.   dof [UX UY UZ RX RY RZ] nodes 1
  value 0.   dof RZ                  nodes 9
  value 0.12 dof UY                  nodes [9 10 11 12]
end

The ebc set must then be activated by referencing it in one of the cases to be computed. This is achieved in the case definition. Example:

case 20
  ebc 123
  ...
end

Element-internal Degrees-of-Freedom

Some element formulations involve internal degrees-of-freedom. With the ebc command it is possible to prescribe values to these degrees-of-freedom, e.g. for debugging purposes. With elements, the individual degrees-of-freedom are identified by numbers, starting from 1.

For example the shell element Q4.S.MITC.E4 is an enhanced-assumed-strain element with 4 internal degrees-of-freedom. Locking them for all elements is done as follows:

value 0 dof [1/4] allelements

The following directives are available to assign essential boundary conditions to collections of elements:

allelements

Assign the essential boundary conditions to all defined elements of the current branch.

branch br

For models that consist of several branches. Specifies the external branch number br. To be used with the allelements directive.

elementlist name

Specifies the name of the element list to which the essential boundary conditions will be assigned.

elementset name

Specifies the name of the element set to which the essential boundary conditions will be assigned.

Additional information

To activate essential boundary condition sets, add the required essential boundary condition set identifiers to the case definition.

Node-local coordinate systems can be defined with the transformations command. The B2000++ coordinate systems are described in the programming manual.

Natural boundary conditions can be defined with the nbc command.

Please read the additional notes in the description of the join command for information on branch connectivity.