Load Case Data - Nonlinear Modal History (FNA) Form

When the Load Case Type/Subtype is set to Time History/ Nonlinear Modal use the form to view and change the definition of a nonlinear modal time-history load case. A modal time-history analysis uses the method of mode superposition, compared with a direct-integration time-history, which solves equations for the entire structure at each time step. Nonlinear modal time-history analysis is also called Fast Nonlinear Analysis (FNA). It is a highly efficient, special-purpose algorithm for analyzing structures with limited nonlinearities. Only nonlinear material behavior in Link objects, and hinges modeled with links, is considered; other hinges and geometric nonlinearity are excluded.

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

Note: Please refer to the Load Case Data Form topic for information related to the Load Case Name edit box, the Load Case Type and Load Case Subtype drop-down lists, and the Notes Modify/Show and Design buttons.

Initial Conditions
Load Applied options. Apply the loads from one or more load patterns 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 pattern or a built-in acceleration load.
- Load Name drop-down list. Choose the load pattern 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 patterns, 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, that portion of the function occurring before the analysis begins is ignored.
  - 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. Click this button to display the Load Type, Load Name, Function, and Coordinate drop-down lists, and the Time Factor, Arrival Time, and Angle,deg edit boxes. Use these items to specify the load to be applied in the load case.
- Delete button.
1. 1. Highlight the load pattern to be deleted in the set of applied loads.
  2. Click the Delete button.

Modal Load Case and Use Modes from Case drop-down list. Modal time-history analysis is based on modal superposition. Select the name of the modal load case whose modes you want to use as the basis for the time-history analysis.

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

Number of Output Time Steps and Output Time Step Size edit boxes. The total time of the analysis is the number of output time steps multiplied by the output time-step size. Results will only be saved 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 display box. This display box identifies the type of damping to be applied. Click the Modify/Show button to specify the Modal Damping to use for the analysis.
Nonlinear Parameters display box. This display box identifies if the nonlinear parameters are Default or User Defined. Click the Modify/Show button to define user-specified Nonlinear Parameters for the analysis.