Imagin that you want to find the natural period or frequency of a building in PLAXIS. You should model the building and apply a constant horizontal load to the roof level. Run a plastic analysis. In the next phase, remove the load and run a dynamic analysis (e.g., 5 seconds) and draw the horizontal displacement plot for the roof. You will observe a decay in the response. Use the relevant equation for the decay (find in one of the Structural Dynamics reference books) and calculate the time interval between to successive maximum displacements. This interval will give you the natural period of the building.
Dear @ Mohsen Seyedi, three small queries:
1. What do you mean by dynamic analysis? Is that giving periodic load at roof level again or some ground motion?
2. Does the magnitude of dynamic load needs to be very small as compared to initial horizontal load?
3. Why do we need to apply a constant horizontal load to begin with?
Bala Harsha Teja Srusti
After constructing your building in PLAXIS, the next step is to apply a constant load (let say 100 kN) to the roof level and perform a plastic analysis. Subsequently, change the analysis type to dynamic, remove the load, and conduct a dynamic analysis for 5 seconds, allowing the building to vibrate freely. Plot the horizontal displacement of the roof and determine the building's natural period from this plot.
Magnitude of the constant load should be large enough to give a visible lateral deformation to your building.