Given dimensions and strength class of the concrete section and the steel grade of longitudinal bars and stirrups you can require to the program a first design of longitudinal bars and stirrups selecting the design option in Section Data dialog window (frame "Type of calculation"). For a T section of a beam such dialog appears as below:
The design is referred to all ULS combination forces assigned in the pertinent dialog window (Forces Tab). For SLS forces program execute only checks calculation after ULS design of longitudinal bars.
Selecting Checks Tab the program performs the design and a subsequent complete checks. After first check you can modify, as many time you want, the reinforcement data in order to refine the results to your need.
In order to do automatic design only for stirrups (if longitudinal bars already assigned) you shall input zero value in the Diameter (or spacing) text box in the stirrups frame. The stirrups design also relates to the control of the local ductility in critical section of columns in seismic zones.