A load step is a configuration of loads for which a solution is obtained. In a linear static or steady-state analysis, you can use different load steps to apply different sets of loads - wind load in the first load step, gravity load in the second load step, both loads and a different support condition in the third load step, and so on. In a transient analysis, multiple load steps apply different segments of the load history curve.
The program uses the set of elements which you select (ESEL) for the first load step for all subsequent load steps, no matter which element sets you specify for the later steps.
This figure shows a load history curve requiring three load steps:
The first load step is for the ramped load, the second load step is for the constant portion of the load, and the third load step is for load removal.
Substeps are points within a load step at which solutions are calculated. You use them for different reasons:
In a nonlinear static or steady-state analysis, use substeps to apply the loads gradually so that an accurate solution can be obtained.
In a linear or nonlinear transient analysis, use substeps to satisfy transient time integration rules (which usually dictate a minimum integration time step for an accurate solution).
In a harmonic analysis, use substeps to obtain solutions at several frequencies within the harmonic frequency range.
Equilibrium iterations are additional solutions calculated at a given substep for convergence purposes. They are iterative corrections used only in nonlinear analyses (static or transient), where convergence plays an important role.
Consider, for example, a 2D, nonlinear static magnetic analysis. To obtain an accurate solution, two load steps are commonly used:
The first load step applies the loads gradually over five to 10 substeps, each with just one equilibrium iteration.
The second load step obtains a final, converged solution with just one substep that uses 15 to 25 equilibrium iterations.
