In Staad the floor vibration simulation per Design guide -11 is only available if the composite floor system is modeled.
However, if you do not want to model the composite floor, then you can separately determine the fundamental natural frequency of different components like joist panel , Girder panel. and then combine them by Dunkerly relationship.
For non- composite floor system, you can model the slab with the specified material and define time varying forcing forcing function at certain location of the floor and perform the Time-History Analysis. But as the walking process is a location varying action (point of application of dynamic loading changes its location), it is a very complex way to keep this dynamic loading moving and then perform the analysis. In such case, it is suggested to follow the walking excitation simulation rules defined by several design codes like Steel Construction Institute - P354 and American Institute of Steel Construction – Steel Design Guide 11.
If you refer to AISC_DG-11, you can see that they have categorized this footfall analysis, in two parts.
(1) Irregular footfall( Walking) (2) Rhythmic
Although both the actions have unpredictable or unknown forcing frequency and its fixed point of application, but for the later case, you can somehow relatively have the data of its exact forcing frequency and the application zone.
For walking exciting, DG-11 specifies a Time Dependent repeated force can be represented as
. where f step is the step frequency of activity and P is the person's weight.
So, for the evaluation of the peak acc. due to walking can be estimated the equation 4.1
which needs to be compared with the recommended limited as specified in the table 4.1.
Now for calculating the "ap/g", you need to determine the fundamental natural frequency of the floor system.
You can determine the same by using the modal analysis in Staad. see the attached example(Please visit the site to view this file)
In addition to that, you may want to use RAM Concept, where DG-11 is already implemented for such simulation.
Please see the link below.