3.2.4.      FEM Mode Shapes for a Modal or Hybrid TAM

The Modal and Hybrid reduction methods require the FEM mode shapes to create the TAM transformation matrix (see Sections 2.3.4 and 2.3.5). This is provided by using the write_modes alter in the FEM modes run. This alter provides three dif­ferent formats for the frequencies and mode shapes controlled by PARAM,OMODES. Setting OMODES to a positive number writes the FEM fre­quencies and mode shapes in OUTPUT4 format to the FORTRAN unit defined by OMODES[14]. Leaving OMODES at its default value of 0 writes the FEM frequen­cies and mode shapes in DMI and DMIG format to the .pch file. If OMODES is set to a negative number, the FEM frequencies and mode shapes are written in OUTPUT2 format to the FORTRAN unit defined by |OMODES|.

Note that in the case of the OUTPUT4 and DMIG options, a matrix called XLAMA is written with the FEM frequencies. This is a three-column matrix where the first column is the frequencies, the second is zero, and the third is the modal masses. This can easily be converted to a LAMA table using the LAMX DMAP module. In the case of the OUTPUT2 option, the LAMA table for the FEM mode shapes is written directly. In all cases the modes are resequenced to external sort, though internal sorting is rarely used.

Further discussion of the use of the write_modes alter is included in Section 3.1.