Control Time Step Sizes

To access advanced time step settings, go to: Setup Model Setup Parameters Advanced Time-Step tab.

The information on this page applies to the following analysis types, except if indicated otherwise:

Note: In a static analysis, time has two effects on the analysis. Firstly, the analysis uses pseudo time to solve static analyses. Secondly, when the model does not converge, a smaller time step results in a smaller load increment (assuming the load curve is not constant) and smaller nonlinear effects. Both factors can help with convergence problems (provided there is a static solution).

The Output interval field can be used to limit how often the results are written to the output files. A value of 1 will write the results after each time step, a value of 2 will write every other time step, and so on.

To start the analysis at a non-zero time, specify this time in the Starting time for event field. View the starting time as shifting the load curve and time step label shown in the results. However, the Starting time for event has no affect during a resume or restart analysis. In particular, the Starting time for event functions as follows:

During the analysis, the processor will reduce the time step if convergence cannot be achieved or if distorted elements are encountered. To avoid this, activate the Use a constant time-step size check box. However, you may need to allow more iterations to achieve convergence (change the Maximum number of iterations field in the Equilibrium tab) and/or to recalculate the stiffness matrix more often than once per time-step (change the Number of allowable stiffness reformations per time step field on the Equilibrium tab).

You can control the situations that cause the time step to be decreased using the Decrease trigger: rate of convergence drop-down box.

Finally, if the Automatic option is selected, the following will be chosen based on other model settings:

Two additional decrease trigger options are available:

The Time step change factor drop-down box controls the factor by which the time step is reduced whenever the analysis fails to converge. (Likewise, the time step increases by the Time step change factor when the model is converging and a larger time step can be used.) In particular, the current time step size DT is related to the time step level L as follows:

where n is the Time-step change factor. (Also see Setting Up and Performing the Analysis: Performing the Analysis: Performing A Nonlinear Analysis.)

You can also control the situations when the time step is increased after a solution is converged at the reduced time step:

The Time step reduction if there are distorted elements check box is activated by default. The time step size will be reduced if you obtain overly compressed or severely distorted elements during the simulation. When this option is disabled, an alternative method is used to help the solution to converge. Specifically, the integrity of the elements is maintained by the imposition of an artificial stiffness. While this alternative method does help the solution to converge, it also decreases the accuracy of the solution. Therefore, the default behavior is generally preferred.

If BOTH of the following statements are true, it may be appropriate to deactivate the Time step reduction if there are distorted elements checkbox: