There are two methods available to you for specifying an enforced displacement at a component. 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, 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 format of the SPCD entry is as follows:

Grid points with an enforced displacement using the SPCD entry must also appear on an SPC or an SPC1 Bulk Data entry.

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.

As an example of enforced displacement, consider the flat plate shown in Figure 9-7. The plate is modeled with 2500 CQUAD4 elements.

The purpose of this example is to compare the efficiency of using the SPC/SPCD combination to that of using the SPC entry only. For the plate model, four runs are made, each having two subcases with different constraint conditions. For the input files “const1.dat” and “const2.dat”, the locations of the constraints are the same for both subcases, but the magnitudes of the constraints change. The file “const1.dat” uses a different SPC for both subcases. The file “const2.dat” specifies the same SPC for both subcases, but it has different SPCD entries.

The input files “const3.dat” and “const4.dat” are similar to the first two; however, the locations of the constraints in the two subcases are different. Table 9-1 summarizes the constraints and CPU time for all four analyses.

The SPC/SPCD combination is more efficient for both modeling situations. 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 mentioned previously, the SPCD is a force entry, not a constraint. Therefore, the SPCD is called out in the Case Control Section with the LOAD command. The input file for “const2.dat” is shown in Listing 9-1.

$ FILENAME - CONST2.DAT 

ID     LINEAR,CONST2 

SOL    101 

TIME   200

CEND 

TITLE = ENFORCED DISPLACEMENTS USING SPC1 ONLY 

DISP(PLOT) = ALL 

SPC = 1 

SUBCASE 1 

 LOAD = 11 

SUBCASE 2 

 LOAD = 12 

BEGIN BULK 

PARAM   POST    0 

$ 

SPC1    1       123456  1       51      2551      2601 

$ 

SPCD    11      1       3       0.0 

 SPCD    11      51      3       0.0 

 SPCD    11      2551    3       5.0 

 SPCD    11      2601    3       5.0 

 $ 

SPCD    12      1       3       0.0 

SPCD    12      51      3       5.0 

 SPCD    12      2551    3       5.0 

 SPCD    12      2601    3       0.0 

 $ 

INCLUDE 'const.bulk'

$ 

 ENDDATA

As a final comment, if the SPCD entry is the only loading in the model, the software issues 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.