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STAAD Commands and Input Instructions
Section 5
5-140
5.26.4 Modal Damping Information
Purpose
To define unique modal damping ratios for every mode. If all
modes have the same damping, then enter damping with the Define
Time History Load or with the Dynamic Loading commands.
Damping may be entered here
1.
EXPLICITly for some or all modes;
2.
by specifying that STAAD EVALUATE each mode’s
damping based on the frequency of the mode and the
minimum and maximum damping entered here. The
formula used to evaluate the damping is given below.
3.
by specifying that STAAD CALCULATE each mode’s
damping based on the frequency of the mode and the mass
factor, ALPHA, and the STIFFNESS factor, BETA. The
formula used to calculate the damping is given below.
The damping entered will be used in Dynamic Analyses such as
Time History load cases; Response Spectrum load cases that use
the CQC/ASCE4 methods and/or Spectra vs. Period curves versus
damping; and Steady State load cases.
General Syntax
DEFINE DAMPING INFORMATION
⎧ EVALUATE dmin (dmax) ⎫
⎨ CALC ALPHA c1 BETA c2 MAX c3 MIN c4 ⎬
⎩ EXPLICIT d1 (d2 d3 d4 d5 ….) ⎭
END
1.
Enter dmin and dmax as the minimum and maximum
damping ratios respectively to be used in the formula
below, or
2.
Alternatively enter c1 and/or c2 (optionally include the
limits of c3 and c4) to be used in the formula below, or