Initial Conditions

For nonlinear analyses, specify the initial conditions at the start of the analysis:

• Zero Initial Conditions - Start from Unstressed State —  the structure has zero displacement and velocity, all elements are unstressed, and there is no history of nonlinear deformation.

• Continue from State at End of Nonlinear Case —  the displacements, velocities, stresses, loads, energies, and nonlinear deformation histories from the end of a previous analysis are carried forward. Note that this option is available after a nonlinear load case has been created in the model. Use the drop-down list to choose the "previous" analysis.

Important: An unlimited number of sequences or branches of nonlinear load cases can be created. However, be sure that the first case in every sequence or tree of nonlinear cases starts from Zero Initial Conditions.

When continuing from the state at the end of the nonlinear case, the following restrictions apply:

• • Nonlinear static and nonlinear direct-integration time-history cases may be chained together in any combination, i.e., both types of analysis are compatible with each other.

  • Nonlinear modal time-history (FNA) cases can continue only from other FNA cases that use modes from the same modal load case.

Note: When continuing from the state at the end of a nonlinear case, all applied loads specified for the present load case are incremental, i.e., they are added to the loads already acting at the end of the nonlinear case.

When considering a sequence of FNA cases, static loading may be applied by defining a case in which:

• • The loads are applied using a ramp function that ramps-up over a length of time that is long (for example ten times) compared to the first period of the structure, and then held constant for an equal length of time.

  • Modal damping is set to a high value; for example 0.99.

Note: Linear time-history load cases always start from Zero Initial Conditions.

Display the Initial Conditions options on the Load Case Data form as follows:

1. Click the  Define menu > Load Cases command to display the Load Cases form.

2. Click the Add New Case button, or highlight a previously defined case and click the Add Copy of Case, or Modify/Show Case button, to display the Load Case Data form.

3. Set the Load Case Type to Nonlinear Static or Nonlinear Staged Construction, or set the Load Case Type to Time History and the Subtype to a Nonlinear option.