FEA Boundary Conditions

Nodal and element face boundary conditions in FEA analyses are applied to nodes or elements by using the commands on this page.

Scripted Input Files

In scripted input files, FEA boundary conditions are applied with a series of FixLine commands and FixPoint commands, each of which is available in two versions. The FixLine command can either select nodes and elements along a previously defined path or can select all nodes close to that line:

FixLine (pathid)
  (boundary conditions commands)
EndFixLine

FixLine (x1),(y1),(x2),(y2),<(tolerance)>
  (boundary condition commands)
EndFixLine

The FixPoint command can either select one define keypoint or can select the one node closest to a point:

FixPoint (keyid)
  (boundary conditions commands)
EndFixPoint

FixPoint (x3),(y3)
  (boundary condition commands)
EndFixPoint

In these commands, the arguments are:

• (pathid) to select a previously defined path by id, which must have be used in an area.
• (x1,y1) to (x2,y2) define (x,y) coordinates (in mm) for the points at the beginning and end of the line (or (R,Z) coordinates in axisymmetric calculations). All nodes close to that line are selected.
• (tolerance) is an optional tolerance (in mm). When used it accepts nodes within that distance of the define line; when omitted, tolerance is set to small number bases on mesh dimensions.
• (keyid) to select a previously defined keypoint by id, which must be part of a path that is in an area.
• (x3,y3) define (x,y) coordinates (in mm) for a single point (or (R,Z) coordinates in axisymmetric calculations). The node closest to that point is selected.

All nodes along the selected path or along the selected line (for FixLine commands) or the single selected node (for FixPoint commands) can be assigned nodal displacement or load conditions, rotated for skewed boundary conditions, or designated for conditional output.

In addition, when using FixLine to select a defined path, all element faces along that path can be assigned stresses. You cannot, however, apply stress conditions along interior paths; they can only be assigned to paths on the boundary of the object. Also note that FixLine commands that use a line instead of a path cannot assign stresses to element faces).

XML Input Files

In XML input files, all FEA boundary conditions are applied within a <GridBC> block:

<GridBC>
  (boundary condition commands)
</GridBCs>

Within this block, most conditions are set with a series of <BCLine> commands and <BCPt> commands, each of which is available in two versions. The <BCLine> command can either select nodes and elements along a previously defined path or can select all nodes close to a line:

  <BCLine path='(pathid)'>
    (set conditions)
  </BCLine>

  <BCLine x1='(x1)' y1='(y1)' x2='(x2)' y2='(y2)' tolerance='(tolerance)'>
    (set conditions)
  </BCLine>

The <BCPt> command can either select one defined keypoint or can select the one node closest to a point:

  <BCPt keypt='(keyid)'>
    (set conditions)
  </BCPt>

  <BCPt x='(x3)' y='(y3)'>
    (set conditions)
  </BCPt>

These commands and attributes function exactly as defined above for the scripted FixLine and FixPoint commands.

An alternative to the above boundary conditions commands, XML files can alternative explicitly set boundary conditions on nodes and element faces, but this method is rarely used.

Displacement Conditions

Load Conditions

Stress Conditions

Skewed Conditions

Selecting Nodes

Explicit XML Boundary Conditions

An alternate way to assign FEA boundary conditions (which can only be done in XML files is to to explicit list each one. This method is rarely used because the generate conditions desribed above are much more flexible. If needed (e.g., when creating an XML file in custom software), the basic format is

<GridBCs>
   <DisplacementBCs>
     <fix node='24' dof='2'>0</fix>
     <rotate node='5' axis='3'>45</rotate>
   </DisplacementBCs>
   
   <LoadBCs>
     <load node='5' dof='1'>45</load>
     <rotate node='5' axis='3'>45</rotate>
   </LoadBCs>
   
   <EdgeBCs>
     <stress elem='3' face='1' dir='1'>0,100</load>
   </EdgeBCs>
</GridBCs>

Notes

1. When doing axisymmetric FEA, nodes at r=0 will automatically be set to zero velocity in the r directions. You therefore need not set those conditions.