3.9.6.4. Mode Selection Based on DDAM Procedure
The mode selection is performed during a modal analysis restart. It is applicable to real eigensolvers
only. The intent of this mode selection method is to perform a Dynamic Design Analysis Method (DDAM)
Spectrum analysis following the modal analysis. It is based on the procedure described in NAVSEA [ 411 ].
In this procedure, the modes are selected using a significance threshold equal to 0.01.The significance
level of a mode is equal to the modal effective weight divided by the total weight. The threshold is the
same for all directions.
The sum of the modal effective weights of all calculated modes must be greater than 80% of the total
weight, otherwise you may need to extract more modes and run the procedure again.
After the significance calculation, the procedure can select additional modes based on the nodal accel-
eration check.This check accounts for the localized response of relatively light-weight sub-components
of critical areas. To apply the acceleration check, you need to select all the nodes of interest and create
a nodal component named “_DDAM_NODCHK” (see Creating Components)
The additional modes included correspond to those for which the nodal acceleration of any of the
nodes of interest exceeds by 10% the maximum nodal acceleration calculated from the selected modes.
If you want to calculate residual vectors, do so during the modal restart analysis. Note that the residual
vectors calculation is always based on all the extracted modes and not the selected modes.
A sample input listing of the procedure follows:
! Build the Model
/filnam, ...
/title, ...
/prep7
...
esel,s,,,78,89
esel,a,,,96
esel,a,,,112
nsle! Select nodes of interest
cm,_DDAM_NODCHK,node! Create a component for the
! nodal acceleration check
allsel,all,all
...
finish
! Obtain the modal solution
/solu
antype, MODAL
modopt, LANB, ...! Block Lanczos
solve
finish
! Obtain the DDAM Mode Coefficients
/solu
antype, SPECTRUM
spopt, DDAM! DDAM Spectrum analysis
addam,1.0,10.0,37.5,12.0,6.0! Input spectral acceleration
vddam,1.0,30.0,12.0,6.0! Input spectral velocity
sed,0,1,0! Input spectrum direction
solve
finish
! Perform the modal selection and expansion
/solu
antype, MODAL, RESTART
mxpand,,,, yes,,, DDAM! Mode selection is based on DDAMRelease 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationModal Analysis Tools for Subsequent Mode Superposition Analysis