Hello!,
There are two methods available for specifying an enforced displacement at grid points in any nastran flavour:
• The first method is to enter the value of the enforced displacement directly on an SPC entry.
• The alternate method to enforce a displacement at a component is to use the SPCD Bulk Data entry. The SPCD entry is actually a force, not a constraint, therefore, the SPCD is called out in the Case Control Section with the LOAD command, but it is used in conjunction with an SPC entry to enforce the displacement. When you use an SPCD entry, internal forces are computed that are applied to the structure to produce the desired enforced displacements.
The SPCD method of enforcing a nonzero constraint is more efficient than using an SPC entry alone when you're using multiple subcases that specify different constraint conditions. Note also that when you use an SPCD entry, the displacement values entered on the SPC entry are ignored. The software only uses the SPCD values.
The SPC/SPCD combination is more efficient. It is most advantageous to use this combination when the magnitudes but not the locations of the constraints change. The larger improvement seen by the magnitude only change is reasonable because only one SPC needs to be processed; the multiple boundary conditions are represented as multiple loads. In general, whenever you have multiple subcases with different constraint conditions, the SPC/SPCD combination is the preferred method for efficiency.
As a final comment, if the SPCD entry is the only loading in the model, then the NX NASTRAN software will issue a warning message stating that no loading is specified for your model. If you intend for the SPCD entry to be the only loading condition, you can ignore the message.
Best regards,
Blas.
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Blas Molero Hidalgo
Ingeniero Industrial
Director
