Stiffness to Use

For linear load cases, choose the structural stiffness matrix to use.

• Zero Initial Conditions - Unstressed State option.  In the most common case, choose the stiffness at Zero Initial Conditions - Unstressed State, which means to consider the entire structure without any effects resulting from prior loading.

• Stiffness at End of Nonlinear Case option. To consider pre-existing P-delta or large-displacement effects, the state of the structure after incremental construction, or the effects of material nonlinearity resulting from prior loading history, choose the stiffness at the end of a nonlinear load case option. The nonlinear case may be of type nonlinear static or nonlinear direct-integration time history. Note that this option is available after a nonlinear load case has been created in the model.

For example, perform several linear analyses that consider the P-delta effects caused by only gravity load as follows:

1. Define a nonlinear static load case that applies the gravity load, and specify that P-delta effects be considered (see Geometric Nonlinearity Parameters for more information). Call this case "GravityPdelta."

2. Define as many linear load cases as needed that all use the stiffness from case GravityPdelta. They can be of various types: static, modal, moving-load, and so forth. Because the cases are all based on the same stiffness matrix, they can be superposed in additive combinations.

When performing a linear analysis, no additional nonlinearity is considered. For example, additional P-delta effects caused by the load applied in the linear case are not included. Similarly, if the nonlinear case had caused certain tension-only braces to go into compression, thereby removing their stiffness, the linear loading cannot cause them to go back into tension. Use linear analysis only if you think that the nonlinear effects will be small compared to those in the nonlinear stiffness case. Otherwise, perform a full nonlinear analysis instead.

Note: Response-spectrum and modal time-history load cases are based on modal superposition. These cases implicitly use the stiffness matrix that was used to calculate the modes.

Note: Linear load cases based on a nonlinear stiffness do not include the load that was applied in the nonlinear case.