TR.32.10.2 Time Varying Load for Response History Analysis
General Format
The following set of commands may be used to model time history loading on the structure for response time history analysis. Nodal time histories and ground motion time histories may both be provided within one load case.
TIME LOAD
joint-list *{ FX | FY | FZ | MX | MY | MZ } It Ia f2
GROUND MOTION { ABSOLUTE | (RELATIVE ) } { X | Y | Z } It Ia f2
Where:
| Parameter | Description |
|---|---|
| ABSOLUTE | nodal results are absolute (elastic response + motion of ground). If entered on any ground command, all results will be absolute. |
| RELATIVE | nodal results are relative (elastic response). (Default if neither ABS or REL are specified) |
| It | sequential position in the input data of type number of time varying load. To refer to first type number entered, use a 1 here regardless of actual type number entered. Ground Motion must have an Acceleration Type; Time Load forces must have a Force type; and Time Load moments must have a Moment Type (refer to TR.31.4 Definition of Time History Load). |
| Ia | Arrival time number (integer). This is the sequential number of the arrival time in the list explained in TR.31.4 Definition of Time History Load. Thus the arrival time number of a3 is 3 and of an is n. |
| f2 | The Force, Moment, or Acceleration Amplitude at this joint and direction will be multiplied by this factor (default = 1.0). For accelerations, if the amplitude-time curve was in g’s, please use the Scale Factor in the Define Time History command to convert g’s to the acceleration units used in that command. This is recommended due to possible unit changes between that command and this command. |
Either TIME LOAD, GROUND MOTION, or both may be specified under one load case. More than one load case for time history analysis is not permitted.
For TIME LOAD data, multiple direction specifiers can be in one entry as follows (the direction specifiers must be on one line and missing values are assumed to be 1):
TIME LOAD 2 3 FX 1 FZ 1 4 -2.1 MX 2 2 6 7 FX FY FZ
Notes
- A Time History analysis requires the mode shapes. These are calculated using the mass matrix determined from the loading specified in the first dynamic load case. (refer to TR.34.2 Modal Calculation Command)
- The Node Displacement table reports the maximum displacement that occurs at each node over the entire time range.
- The displacement of the model at a specific time instance can be displayed by using the time slider bar on the Results toolbar when displaying a load case with time history loading.
- If any Node Groups are defined, the time history graphs can be set to display the average results for the group. The name of the selected node or group being displayed is given in the graph title bar.
- Time history loading can only be defined in a single load case before the first analysis command.
Example
LOAD 1 SELFWEIGHT X 1.0 SELFWEIGHT Y 1.0 SELFWEIGHT Z 1.0 MEMBER LOADS 5 CON GX 7.5 10.0 5 CON GY 7.5 10.0 5 CON GZ 7.5 10.0 TIME LOAD 2 3 FX 1 3 5 7 FX 1 6 GROUND MOTION REL X 2 1
In this example, the permanent masses in the structure are provided in the form of selfweight and member loads (refer to TR.32 Loading Specifications and G.17.3 Dynamic Analysis) for obtaining the mode shapes and frequencies. The mass model can also be created using a Reference Load case with the Load type of Mass (refer to TR.31.6 Defining Reference Load Types). The rest of the data is the input for application of the time varying loads on the structure. Forcing function type 1 is applied at joints 2 and 3 starting at arrival time number 3. (Arrival time number 3 is 1.8 seconds in example shown in TR.31.4 Definition of Time History Load). Similarly, forcing function type 1 is applied at joints 5 and 7 starting at arrival time number 6 (4.4 seconds). A ground motion (type 2) acts on the structure in the x-direction starting at arrival time number 1 (0.0 seconds).