The time history analysis is performed using numerical time step integration methods. The Newmark integration algorithm is used along with nonlinear Newton-Raphson iteration. Either single or spectral analysis can be performed.

The single analysis dialog allows the user to look at the time histories, plot computed quantities during the whole record in its Main Results Window, or follow an animation of the system. The spectral analysis dialog only holds the spectral (peak) values from each analysis, and allows the results to be shown in both two-dimensional (with different series) or  three-dimensional space.

The single analysis function steps through the time history at a time step less or equal to the time history digitization time step. For shorter period lengths, that is replaced by a lower limit which is T/25. That is meant to improve the convergence process as well as the significance of the results. Smaller values may provide better performance in case of severe yielding, but may significantly slow down the analysis because of the increase in the number of time steps to be performed. The time step size can be specified through the Analysis Options dialog.

Spectral Analysis is performed by varying the properties of the model or other dynamic properties and running the analysis repeatedly (by varying the strength for example for "Constant Strength" spectra), or by performing an iterative search to match a certain quantity, like ductility or damage index. In a ductility search, the strength required for elastic behavior is computed and gradually decreased by small increments until the required ductility is obtained. Local minima and nonlinear behavior require the use of a very small steps to avoid missing the solution corresponding to the strongest possible system.