A Structural Engineers Practical Guide To Seismic Design: Fundamental Period
You can determine the approximate fundamental period of a structure from ASCE 7 Eqs. 12.8-7, 12.8-8, or 12.8-9. The type of SFRS you have will determine what equation you use.
The approximate period helps determine initial loading.
Your computer model will provide your your actual period.
But what do I do now?
This is an iterative process. You should check loads with the actual period.
Computer programs can be powerful tools, but you need to learn how to use them. If your approximate period and actual period are largely different, check your work.