exist between the residual vector and the modes depending on the mode combination
method.A frequency is associated with the residual vector. The input spectrum for this frequency
should correspond to the Zero Period Acceleration value. The frequency is also used for
the calculation of the velocity and acceleration solutions.- Damping (Dynamics Options)
If you specify more than one form of damping, the program calculates an effective damping ratio
at each frequency. The spectral value at this effective damping ratio is then calculated by log-log
interpolation of the spectral curves. If no damping is specified, the spectral curve with the lowest
damping is used. For more information about different forms of damping, see Damping (p. 2)
in Transient Dynamic Analysis (p. 107).The following forms of damping are available:- Beta (stiffness) Damping [BETAD]
This option results in a frequency-dependent damping ratio.- Alpha (mass) Damping [ALPHAD]
This option results in a frequency-dependent damping ratio.- Constant Damping Ratio [DMPRAT]
This option specifies a constant damping ratio to be used at all frequencies.- Modal Damping [MDAMP]
NoteMaterial-dependent damping ratio (MP,DMPR) is also available, but only if spe-
cified in the modal analysis where an effective damping ratio is calculated based
on the elements’ strain energies. You must request that element results be cal-
culated in the modal expansion (Elcalc = YES on the MXPAND command).- Start solution calculations.
Command(s):SOLVE
GUI: Main Menu> Solution> Solve> Current LS
The output from the solution includes the Response Spectrum Calculation Summary. This table,
which is part of the printed output, lists the participation factors, mode coefficients (based on lowest
damping ratio), and the mass distribution for each mode. To obtain the response of each mode
(modal response), multiply the mode shape by the mode coefficient (based on lowest damping ratio).
You do this by retrieving the mode coefficient with the *GET command (Entity = MODE) and using
it as a scale factor in the SET command.The mode coefficients based on the actual damping are listed in the Significant Mode Coefficients
(Including Damping) table.Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationSpectrum Analysis