The Modal Analysis with Automatic Definition of Seismic Cases allows you to define a modal analysis case and a seismic analysis case simultaneously. 
- Click Analysis 
 Analysis Types.  The Analysis Types dialog opens.  
- Click New.  The New Case Definition dialog opens. 
 
- Select Modal with automatic definition of seismic cases, and then click OK.  The Modal Analysis dialog opens. 
 
- Specify the necessary parameters, and then click OK.