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 f2 | CDAMP | MDAMP } { ABSOLUTE | RELATIVE }
Where:
| Parameter | Description |
|---|---|
| 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 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 } { ACCELERATION f3 | DISPLACEMENT f4 } ( PHASE f5 )
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:
| Parameter | Description |
|---|---|
| ACCELERATION f3 | Ground acceleration as multiple of ‘g’ (i.e. acceleration due to gravity 9.81 m/s2 or 32.17 ft/s2 ). For example, for an ground acceleration of 5 m/s2, use ACCELERATOIN 0.51 ( where 5 / 9.81 ≅ 0.51). |
| DISPLACEMENT f4 | Ground displacement in current units of length (e.g., DISPLACEMENT 5 for 5 cm when that is the current units of length). |
| PHASE f5 | Phase angle in degrees. If not specified, this is take as zero (i.e., no shifting). |
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
| = | ||
| = | ||
| = |
Or
AMPLITUDE
(f1 a1 f2 a2 … fn an )
| Parameter | Description |
|---|---|
| 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. |