Reaction forces are unexpected in a Shock/Response Spectrum analysis in Nastran

Autodesk Support

Oct 8, 2023


Products and versions covered


Issue:

The reaction forces in a shock/response spectrum analysis with Nastran are unexpected. For example:
  • The sum may be higher than anticipated.
  • If the constraints are changed so that more nodes are constrained, the sum of the reactions will increase. This would be unexpected for the same applied load.
Note: The total reaction force is shown using the SPC Summation in Inventor Nastran or Nastran In-CAD: right-click on a constraint and choose "SPC Summation". 

Causes:

The model is not constrained properly for the reaction force summation.

The shock/response spectrum analysis results shown in Inventor Nastran is a combination of the results due to each natural frequency. The combination is some type of square root sum of the squares (as chosen under the "Options > Summation Option" on the "Analysis > Edit" setup). Because of the square root sum of the squares (SRSS), all results are positive.

If multiple nodes are constrained, the reaction force at each node will be positive. Therefore, the sum of the reaction forces for the entire model will be higher than it should be. See Figure 1.
shock response spectrum total reaction SPC summation
Figure 1: A beam model is held at two nodes in a shock/response spectrum analysis. The probes show the X reaction force (SPC force) at each constraint. The sum of the X reaction forces is 6330. However, the deformed shape implies that one X reaction should be positive, and one X reaction should be negative. The reaction forces should not be summed.

Solution:

 
The proper way to constrain a model in order to get accurate reaction forces is to apply a rigid body connector to all of the nodes being constrained (that is, select all of the faces, edges, and/or vertices that need to be hel), and apply one constraint to the center of the rigid connector.
 
response spectrum analysis simulation nastran constrain reaction force
Figure 2: Two constraints are replaced with one rigid connector (red lines and arrows) and one constraint (blue arrows at node 54). The sum of the X reaction force is 52.7 which equals 3190-3140 (to the precision of the value shown by the probe).

Note: Other results such as displacement and stress are not changed by constraining one node (Figure 2) or multiple nodes (Figure 1), provided the same nodes are held. The main difference is that displacement and stress results are not summed, so the fact that they are all positive is not as critical.

Products:

Inventor Nastran;


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