Analysis Case Data - Linear Modal History Form

Access the Analysis Case Data - Linear Modal History form as follows:

Click the Define menu > Analysis Cases command to display the Analysis Cases form.
Click the Add New Case button or highlight a previously defined analysis case and click the Add Copy of Case or the Modify/Show Case button to display the Analysis Case Data form.
Select Time History from the Analysis Case Type drop-down list and select the Linear option for Analysis Type and Modal for Time History Type.

The Analysis Case Data - Linear Modal History form is used to view and change the definition of a linear modal time-history analysis case. A modal time-history analysis uses the method of mode superposition, compared with a direct-integration time-history, which solves equations for the whole structure at each time step.

Note: If a modal analysis case has not been defined, first define a modal analysis case before defining the modal time-history case.

Time History Motion Type Choose the type of motion to consider:

Transient: This is the usual case, where the structure starts at rest and is subjected to the specified loads only during the time period specified for the analysis.
Periodic: This is a special-purpose analysis. The specified loads are assumed to be periodic; that is, they repeat indefinitely with a period given by the length of the analysis. The response is also periodic. The program automatically adjusts the displacement and velocity at the beginning of the analysis to be equal to the values at the end of the analysis. The result is the steady-state response to a periodically applied load of arbitrary time-variation.

Initial Conditions For linear transient analysis, the initial conditions are always zero. For linear periodic analysis, the initial conditions are set equal to the final conditions at the end of the analysis, as stated above (see Periodic bullet item).
Modal Analysis Case and Use Modes from Case drop-down list. Modal time-history analysis is based on modal superposition. Select the name of the modal analysis case whose modes are to be used as the basis for the time-history analysis.

Tip: Most commonly, one modal analysis case will be available, and its name will already be selected by default.

Loads Applied Apply the loads from one or more load cases or built-in acceleration loads, each scaled by the same or different time functions. All loads specified will be added and applied in combination.

Load Type drop-down list. Choose whether the load to be applied is a load case or a built-in acceleration load.
Load Name drop-down list. Choose the load case name, or the direction of ground acceleration, depending on the type of load. For acceleration loads, choose direction U1, U2, or U3. See Applying Acceleration Loads for more information.
Function drop-down list. Select the name of a previously defined time-history function that specifies the magnitude of the load as a function of time.
Scale Factor edit box. Enter a scale factor that multiplies the load before adding it to other loads applied. For acceleration loads, the scale factor has units of acceleration, and should be consistent with the length units currently in use. For load cases, the scale factor is unitless.
Show Advanced Load Parameters check box. Check this box to reveal the following additional columns in the applied loads table.

Time Factor edit box. Use this parameter to specify a positive factor that multiplies the time scale of the time-history function. The default value is one, which does not change the function. This should normally be used for ground-motion records. However, modifying the time scale can be useful, for example, to change the period of sinusoidal loads, or to increase or decrease the time to apply ramp or pulse loads.
Arrival Time edit box. The arrival time specifies when the time function starts during the analysis. A positive value delays the start of the function. If the arrival time is negative, the program ignores that portion of the function occurring before the analysis begins.
Coordinate System drop-down list. Specify a coordinate system in which the acceleration directions are measured. See Applying Acceleration Loads for more information.
Angle edit box. Specify an angle by which U1 and U2 are rotated from UX and UY in the specified coordinate system. See Applying Acceleration Loads for more information.

Add button. To add a load to the set of applied loads, enter the load type, load name, and other parameters at the top of the table, then click Add button.
Modify button. To modify a load in the set of applied loads, click on the load in the table to select it, make any changes to the load type, load name, and other parameters at the top of the table, then click Modify button.
Delete button. To remove a load from the set of applied loads, click on the load in the table to select it, then click Delete button.

Number of Output Time Steps edit box and Output Time Step Size edit box. Accept the default or enter a value for the number of output time steps and the size of the output time step. The total time of the analysis is the number of output time steps multiplied by the output time-step size. Results will be saved only at time zero and the subsequent output time steps, although the analysis will compute intermediate results at every time step of every applied-load time-history function.
Modal Damping. Click the Modify/Show button to access the Modal Damping form and specify the modal damping to use for the analysis.