Analysis Case Data - Linear Direct Integration History Form

Access the Analysis Case Data - Linear Direct Integration 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 Direct Integration for Time History Type.

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

Stiffness to Use Choose whether to solve for the response using the stiffness of the unstressed structure (i.e., Zero Initial Conditions - Unstressed State option), or at the end of a nonlinear static or nonlinear direct-integration time-history analysis case (i.e., Stiffness at End of Nonlinear Case option--select the case from the drop-down list). See Stiffness to Use for more information. If you are uncertain which option to use, choose the Zero Initial Conditions - Unstressed State option. Note that the Stiffness at End of Nonlinear Case option is available after a nonlinear analysis case has been defined for the model. Select the nonlinear case from the drop-down list.
Modal Analysis Case area and Use Modes from Case drop-down list. This area and drop-down list become available when Modal is selected for the Time History Type.
Loads Applied area. Loads from one or more load cases or built-in acceleration loads may be applied, each scaled by the same or different time functions. All user-specified loads 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 the 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 the 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 the 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.
Damping display box. This display box shows the type of damping being applied. Click the Modify/Show button to access the Mass and Stiffness Proportional Damping form to specify the damping to use for the analysis.
Time Integration display box. This display box shows the analysis method to be applied to time integration. Click the Modify/Show button to access the Time Integration Parameters form and specify the Time Integration Parameters to use for the analysis.