TR.37.6.4 Steady Ground Motion Loading
This set of commands may be used to specify steady ground motion loading on the structure, the ground motion frequency, the modal damping, and the phase relationship of ground motions in each of the global directions.
General Format
This command specifies the ground motion frequency and damping.
STEADY GROUND FREQUENCY f1 { DAMP f2 | CDAMP | MDAMP } { ABSOLUTE | RELATIVE }
Where:
| Parameter | Description |
|---|---|
| FREQ f1 | the steady state frequency in cycles/sec at which the joint loads below will oscillate. |
| DAMP f2 | Damping ratio for all modes when DAMP is selected. Default value is 0.05 (5% damping if 0 or blank entered). |
- 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
Enter the direction of the ground motion, the acceleration magnitude, 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.
GROUND MOTION { X | Y | Z } { ACCELERATION f3 | DISPLACEMENT f4 } ( PHASE f5 )
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). |