What is a step?

A step corresponds to a set of loads for which you want to obtain a solution and review results. In this way every static or transient dynamic analysis has at least one step. However there are several scenarios where you may want to consider using multiple steps within a single analysis, that is, multiple solutions and result sets within a single analysis.

A static or transient analysis starts at time = 0 and proceeds until a step end time that you specify. This time span can be further subdivided into multiple steps where each step spans a different time range.

As mentioned in the Role of Time in Tracking section, each step spans a ‘time’ even in a static analysis.

When do you need Steps?

Steps are required if you want to change the analysis settings for a specific time period. For example in an impact analysis you may want to manually change the allowable minimum and maximum time step sizes during impact. In this case you can introduce a step that spans a time period shortly before and shortly after impact and change the analysis settings for that step.

Steps are also useful generally to delineate different portions of an analysis. For example, in a linear static structural analysis you can apply a wind load in the first step, a gravity load in the second step, both loads and a different support condition in the third step, and so on. As another example, a transient analysis of an engine might include load conditions corresponding to gravity, idle speed, maximum power, back to idle speed. The analysis may require repetition of these conditions over various time spans. It is convenient to track these conditions as separate steps within the time history.

In addition steps are also required for deleting loads or adding new loads such as specified displacements or to set up a pretension bolt load sequence. Steps are also useful in setting up initial conditions for a transient analysis.

How do you define steps?

See the procedure, "Specifying Analysis Settings for Multiple Steps" located in the Establish Analysis Settings section.

What are substeps and equilibrium iterations?

Solving an analysis with nonlinearities requires convergence of an iterative solution procedure. Convergence of this solution procedure requires the load to be applied gradually with solutions carried out at intermediate load values. These intermediate solution points within a step are referred to as substeps. Essentially a substep is an increment of load within a step at which a solution is carried out. The iterations carried out at each substep to arrive at a converged solution are referred to as equilibrium iterations.