Mechanical APDL Structural Analysis Guide

If prestressing effects develop because of nonlinear behavior (as in the case of residual thermal

stresses in a casting), several load steps might be required to complete the static prestressing

phase of your analysis. In the case of geometric nonlinearities (large deformation effects), you

can capture the prestressing effect by issuing NLGEOM,ON.

2. For all subsequent load steps, turn time integration effects on (TIMINT,ON), and proceed using the
   full transient dynamic analysis procedures described previously. Once all load steps are written to
   files (LSWRITE), you can initiate the multiple load step solution (LSSOLVE).

Note

If you intend to define initial conditions (IC), perform the static prestress solution as a separate

solution. To activate the gyroscopic damping matrix in a prestressed transient analysis, perform

a separat e static solution with Coriolis effects activated (CORIOLIS,ON,,,ON) in a stationary

reference frame.

(Main Menu> Solution> Define Loads> Apply> Initial Condit'n> Define)

The IC command is valid only in the first load step.

5.5.2. Prestressed Mode-Superposition Transient Dynamic Analysis

In order to include prestress effects in a mode-superposition analysis, you must first do a prestressed
modal analysis. See Modal Analysis (p. 39) for details. Once prestressed modal analysis results are
available, proceed as for any other mode-superposition analysis.

5.6. Transient Dynamic Analysis Options.

The following sections provide additional details about defining integration time step, automatic time
stepping, and damping.

5.6.1. Guidelines for Integration Time Step

The accuracy of the transient dynamic solution depends on the integration time step: the smaller the
time step, the higher the accuracy. A time step that is too large introduces an error that affects the re-
sponse of the higher modes (and hence the overall response). A time step that is too small wastes
computer resources. To calculate an optimum time step, adhere to the following guidelines:

1. Resolve the response frequency. The time step should be small enough to resolve the motion (response)
   of the structure. Since the dynamic response of a structure can be thought of as a combination of modes,
   the time step should be able to resolve the highest mode that contributes to the response. For the
   Newmark time integration scheme, it has been found that using approximately twenty points per cycle
   of the highest frequency of interest results in a reasonably accurat e solution.That is, if f is the frequency
   (in cycles/time), the integration time step (ITS) is given by

ITS = 1/(20f)

Smaller ITS values may be required if acceleration results are needed.

The following figure shows the effect of ITS on the period elongation of a single-DOF spring-mass

system. Notice that 20 or more points per cycle result in a period elongation of less than 1 percent.

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Transient Dynamic Analysis