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TR.37.6.6 Harmonic Ground Motion Loading

This set of commands may be used to specify harmonic ground motion loading on the structure, the modal damping, and the phase relationship of ground motions in each of the global directions. 

Response at all of the frequencies defined in TR.37.6.2 Define Harmonic Output Frequencies will be calculated.

General Format

HARMONIC GROUND { DAMP f2CDAMP | MDAMP } { ABSOLUTE | RELATIVE }

Where:

ParameterDescription
DAMP f2 Damping ratio for all modes when DAMP is selected.  Default value is 0.05 (5% damping if 0 or blank entered).

This command specifies the damping.  The steady state response will be calculated for each specified output frequency entered or generated, see TR.37.6.2 Define Harmonic Output Frequencies

DAMP, MDAMP, and CDAMP select source of damping input:
  • DAMP indicates to use the f2 value for all modes
  • 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
  • CDAMP indicates to use the composite damping of the structure calculated for each mode. You must specify damping for different materials under the CONSTANT specification

ABSOLUTE or RELATIVE. Ground motion results in output file will be relative to the ground unless ABSOLUTE is specified. Graphical results are relative. This option has no effect for the force loading cases.

General Format

GROUND MOTION { X | Y | Z } { ACCEL | DISP } f3 PHASE f4

Enter the direction of the ground motion, the acceleration and the phase angle by which the motion in this direction lags (in degrees).  One Ground Motion command can be entered for each global direction.

Where:

ParameterDescription
f3 Ground acceleration in g’s or displacement in length units.
PHASE f4 Phase angle in degrees

Next is an optional amplitude versus frequency specification to be used when the ground motion acceleration is a function of frequency. For any forcing frequency an amplitude can be determined, from the data below, which will multiply the acceleration f3 entered above.  If no amplitude data is entered for a direction then the acceleration is f3 for that direction.

AMPLITUDE ( A a B b C c )
where
Amplitude
=
a×ω2 + b×ω + c
ω
=
forcing frequency in rad/sec.
a, b, c
=
Constants. a and b default to 0.0 and c defaults to 1.0.

Or

AMPLITUDE
(f1 a1  f2 a2  … fn an )
ParameterDescription
f1 a1  f2 a2  … fn an Frequency - Amplitude pairs are entered to describe the variation of acceleration with frequency. Continue this data onto as many lines as needed by ending each line except the last with a hyphen (-). These pairs must be in ascending order of frequency.  Use up to 199 pairs.  Linear interpolation is used.

One Ground Motion and Amplitude command set can be entered for each global direction.