Dear Houda Okay, you answered where the vibration is; at the base nodes and you said the duration of the vibration is 30 seconds.
What direction is the vibration motion? I assume it is in the plane of the ground, and not vertical (Y axis). But at what angle? Is it along the X axis or the Z axis or at some angle in between? The kind of vibration is called ground motion.
Vibration is a very general word. There are different types of vibration. The simplest type is sinusoidal. For example x(t) = A*sin(wt) where A is the amplitude, w is the circular frequency and t is time. Do you want to apply sinusoidal vibration?
The amplitude can be in terms of acceleration, velocity or displacement. Acceleration amplitude is sometimes in the units of G or g which is the acceleration due to gravity, commonly set to 9.8 m/s^2. What amplitude do you have specified for the ground motion? Make sure to include the units.
The frequency of vibration is another required part of a sinusoidal vibration specification. The frequency of vibration is usually specified in the units of Hz, which is cycles per second where a cycle starts at the center, goes left, goes right and back to the center. Frequency f in Hz is related to circular frequency w by the equation: w = 2*Pi*f because that is how many radians are in one circle.
Another type of vibration input of ground motion is an earthquake. This motion is not described by a sinusoidal function, but the acceleration of the ground motion is recorded by seismic instruments and that data can be used in a Transient Structural analysis to replay that ground motion as input to a building in simulation.