Geometric Nonlinearity Parameters

The geometric nonlinearity options available for nonlinear static and nonlinear direct-integration time-history analysis consist of the following:

None option. All equilibrium equations are considered in the undeformed configuration of the structure.
P-Delta option. The equilibrium equations take into partial account the deformed configuration of the structure. Tensile forces tend to resist the rotation of elements and stiffen the structure, and compressive forces tend to enhance the rotation of elements and destabilize the structure. This may require a moderate amount of iteration
P-Delta plus Large Displacements option. All equilibrium equations are written in the deformed configuration of the structure. This may require a large amount of iteration. Although large displacement and large rotation effects are modeled, all strains are assumed to be small.

Tips: The large displacement option should be used for cable structures undergoing significant deformation, and for buckling analysis, particularly for snap-through buckling and post-buckling behavior. Cables (modeled using frame objects) and other elements that undergo significant relative rotations within the element should be divided into smaller elements to satisfy the requirement that the strains and rotations within an element are small.

For most other structures, the P-delta option is adequate, particularly when material nonlinearity dominates.

If reasonable, it is recommended that the analysis be performed first without geometric nonlinearity, adding P-delta, and possibly large-displacement effects later.

When continuing one analysis case from another, it is recommended that they both have the same geometric-nonlinearity settings.

Note: Geometric nonlinearity is not available for the FNA method.

The Geometric Nonlinearity Parameter options appear on the Load Case Data form when the analysis type is nonlinear static or nonlinear direct-integration time-history.

Click the Define menu > Load Cases command to display the Define Load Cases form.
Click the Add New Load Case button or highlight a previously defined Load case and click the Add Copy of Load Case or the Modify/Show Load Case button to display the Load Case Data form.
Make one of the following selections:
- Select Static from the Load Case Type drop-down list and select the Nonlinear or Nonlinear Staged Construction option for Analysis Type.
- Select Time History from the Load Case Type drop-down list and the Nonlinear option for Analysis Type and the Direct Integration option for Time History Type.