# Analysis Wizard : Linear Time History
This page defines the analysis parameters for linear time history analysis. If no modal dynamic analysis results are present, this option is disabled and this page cannot be opened.
# Definition
Modal Analysis Task
If more than one set of modal dynamic analysis results exist it needs to know which set of results are to be used. If only one modal dynamic analysis task is available, it will be used by the analysis.
Excitation
Excitation can be a force excitation which requires a magnitude and locations of the dynamic loads, the syntax of the description of the loads is the same as that for static analysis, e.g. 1.4L1+1.6L2. The other excitation option is base excitation where the base acceleration is defined by the load curve and the direction of the excitation can be in global X, Y or Z direction or in any direction defined by global X, Y & Z vectors. This direction vector only defines the direction of the base acceleration and its length is not used in the analysis, e.g. direction vector (1, 2, 3) will be the same as direction vector (2, 4, 6) . the ground accelerations defined by the load curve can be scaled up and down by the scaling factor that is also defined at this page of the wizard.
Output Options
Two output options are offered;
- "Save results at intervals of specified seconds" is to define the time interval when the full set of analysis results are saved in the analysis cases;
- "Interval for time-history results" is to define the time interval for plotting analysis results (e.g. displacements and acceleration etc) versus time curves on chart view.
Time Step
This defines the time step in the analysis, if it is too large, the analysis may diverge. Normally the automatic time step should be used but this can be overridden if required.