**impose_nodal_reaction#

Description#

This option sets the “force” reactions which act on a node set. Note that applying a force to a node set is very different than applying a pressure (using the BC **pressure). The condition is general so it may be used for all types of problem. For the simple mechanical case, the nodal reactions are in units of force.

Syntax#

**impose_nodal_reaction nset dir val table

nset

Character name of the node set where the condition is applied.

dir

Direction in which the reaction is applied. This will be the DOF name whose associated reaction is affected. See the appendix for DOF names.

val

Real number for the multiplicative base value of the condition.

table

Character table name of a valid table, or a list of character table names.

Example#

The current example demonstrates the incorrect use of this option to impose a surface pressure. The difference is a common misconception of boundary conditions in the FEM. The problem uses a plane-stress eight node square element with fixed U1 on the left border and fixed U2 on the bottom. Material behavior is linear elastic.

The top node set is defined using the right top, middle top, and left top nodes in sequence. As the square has unit sides, one could suppose that the three forces applied should be \(1/3\) the stress. The BC is therefore applied as:

***bc
  **impose_nodal_reaction
    top U2  0.3333 tab
# **pressure
#   press  1. tab

which results in the following distorted deformation:

../../_images/impos.fig.svg

Proper application of a pressure requires knowledge of the surface integration along the boundary. The options **pressure and **shear must therefore be used.

**impose_nodal_reaction_rate#

Description#

This is an associated reaction version of **impose_nodal_dof_rate. The condition is useful when the initial reaction state is unknown, which can be generated with a mix of displacement and reaction BCs using the exp keyword.

Syntax#

The syntax is analogous to **impose_nodal_dof_rate.

**impose_nodal_reaction_rate \(~\,~\,\) nset_name  dof_name  value  table_name