TRANSIENT RESPONSE
Direct method
Structural response (displacement) is solved at discrete times, with an integration step Δt constant or not.
Velocity and acceleration are computed using finite differences.
Solution is obtained for each time step.
Modal method
This method uses mode shapes to transform the problem to modal co-ordinates, reducing the size of the system of equations, as in frequency response.
If no damping or only modal damping is present, the system of equations can be decoupled.
If other types of damping are present, the coupled system is solved in modal co-ordinates.