Group Damping is used to assign different damping ratios resulting from
time history analysis to different element and boundary groups.
When the materials, vibration control devices or vibration isolators
in a structure have different damping ratios, elements with different
damping ratios can be assigned to different Element Groups.
It is common to assign structural members such as beams and columns
to a Structural Group and assign boundary conditions (including structures
assigned with vibration isolators or vibration control devices) to a Boundary
Group in order to define different damping ratios.
From the Main
Menu select Model
> Properties > Group Damping
Select Geometry >
Properties > Group Damping in the Menu tab of the Tree Menu.
Group
Damping dialog box
Default Values for Damping Ratio for Unspecified Elements
and Boundaries
Apply damping ratio to the elements and boundaries
unspecified by groups.
Strain
Energy Proportional Damping
Specify a Damping Ratio, which will be used
to calculate modal damping ratios in the case where Strain Energy Proportional
is selected as the Damping Method in Time
History Load Cases.
Element
Mass & Stiffness Proportional Damping
Enter a coefficient, which will be multiplied
with the element stiffness matrix to calculate modal damping ratios (Cele=Beta
x Kele) and will be used in the case where Element Mass & Stiffness
Proportional is selected as the Damping Method in Time
History Load Cases.
Damping matrix for each element is applied
when the direct integration method is used for analysis.
Alpha:
Mass matrix coefficient for calculating damping matrix for each element
Beta:
Stiffness matrix coefficient for calculating damping matrix for each element
Damping Ratio for Specified Elements and Boundaries
Assign damping ratios by element and boundary
groups.
Type of Group
Structure:
Select to assign damping ratios by element groups.
Boundary:
Select to assign damping ratio by boundary groups. Available boundary
conditions are Elastic Link, Nonlinear Link and Point Spring.
Name
of Group: Select a group name, which was defined in Model>Group>Structural
Group or Boundary Group.
Strain
Energy Proportional Damping
Specify a Damping Ratio, which will be used
to calculate modal damping ratios in the case where Strain Energy Proportional
is selected as the Damping Method in Time
History Load Cases.
Element
Mass & Stiffness Proportional Damping
Enter a coefficient, which will be multiplied
with the element stiffness matrix to calculate modal damping ratios (Cele=Beta
x Kele) and will be used in the case where Element Mass & Stiffness
Proportional is selected as the Damping Method in Time
History Load Cases.
Damping matrix for each element is applied
when the direct integration method is used for analysis.
Alpha:
Mass matrix coefficient for calculating damping matrix for each element
Beta:
Stiffness matrix coefficient for calculating damping matrix for each element
Note
When elements or boundary
conditions have not been assigned to Groups, damping ratios specified
under 'Default Values for Unspecified Elements and Boundaries' are used
for time history analysis. When elements or boundary conditions have been
assigned to Groups, damping ratios specified under 'Damping Ratio for
Specified Elements and Boundaries' are used for time history analysis.
Calculate Only When Used
If this option is checked on, modal damping
ratios are calculated only when 'Strain Energy Proportional' is selected
as the Damping Method in Time History Load Cases. If this option is not
checked on, modal damping ratios are calculated for each Damping Method.
(Result>Modal
Damping Ratio Based on Group Damping)
Default Values for Damping Ratio for Unspecified Elements
and Boundaries