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STAAD Commands and Input Instructions
Section 5
5-294
ACC or DIS indicates whether Acceleration or Displacement
spectra will be entered.
SCALE f4 = Scale factor by which the spectra data will be
multiplied. Usually to factor g’s to length/sec
2
units.
DAMP , CDAMP , MDAMP. Select source of damping input.
f5 = Damping ratio for all modes. Default value is 0.05 (5%
damping if 0 or blank entered).
DAMP indicates to use the f5 value for all modes.
CDAMP indicates to use Composite modal damping if
entered, otherwise same as MDAMP.
MDAMP indicates to use the damping entered or computed
with the DEFINE DAMP command if entered, otherwise
default value of 0.05 will be used.
LIN or LOG. Select linear or logarithmic interpolation of the
input Spectra versus Period curves for determining the spectra
value for a mode given its period. LIN is default. Since
Spectra versus Period curves are often linear only on Log-Log
scales, the logarithmic interpolation is recommended in such
cases; especially if only a few points are entered in the spectra
curve.
For LIN or LOG , when FILE fn is entered, the
interpolation along the damping axis will be linear. The last
LIN-LOG parameter entered on all of the spectrum cases will
be used for all spectrum cases.
MIS f6 = Use Missing Mass method to include the static effect of
the masses not represented in the modes. The spectral
acceleration for this missing mass mode is the f6 value entered
in length/sec
2
(this value is not multiplied by SCALE). If f6
is zero, then the spectral acceleration at the ZPA f7 frequency
is used. If f7 is zero or not entered, then the spectral
acceleration at 33Hz is used. The results of this calculation
are SRSSed with the modal combination results. If the MIS
parameter is entered on any spectrum case it will be used for
all spectrum cases.