You can refer to-

https://communities.bentley.com/products/ram-staad/f/ram-staad-forum/94795/how-to-calculate-natural-frequency-in-staad-for-steel-structures#:~:text=b)%20The%20modal%20calculation%20method,method%20than%20the%20Rayleigh%20method.

You can use "Modal Calculation Requested" command. In a particular direction, the mode which have the maximum mass participation factor and the lowest frequency, that mode will be considered as the fundamental natural time period of that particular direction

Good Morning Shreyanka & DSANJU,

I am aware about the "modal calculation requested" command and my question was about which frequency or mode I should consider for natural time period calculation. However same has also been clarified but I have some further queries.

What if the lowest frequency is not corresponding the mode with maximum participation, we should take lowest frequency or the one corresponding to mode with maximum participation?

Can we use it to calculate static base shear value (Not the one which STAAD anyway provides) ? 

For the better explanation I will mention the steps which I would follow  

1. Get the natural frequency from STAAD (corresponding to mode with max participation)

2. Calculate corresponding time period.

3. Calculate design spectral acceleration for the time period using the formula from code or the acceleration plot

4. Calculate design horizontal seismic coefficient Ah

5. Finally Vb = Ah*W

6 Vb should be compared with the design base shear from STAAD and calculate the enhancement factor in case Vb is greater than design base shear value provided by STAAD.