Many industrial problems involving slender structures in transient dynamics require a 3D model for a better understanding of local or non linear effects that occur along a small period of time, whereas a simplified beam model can be sufficient for simulating the linear phenomena occurring for a long period of time. This paper proposes a method which enables to switch from a beam to a 3D model during a transient dynamic analysis solved with a time integration technique. Thus, this method allows to reduce the computational cost while preserving a good accuracy. Starting with the beam model, at switch moment ts, a 3D solution is constructed as the sum of a cross-section rigid body displacement corresponding to the classical Timoshenko kinematical assumption and a 3D correction which accounts for cross-section deformation. This correction is obtained from the solution of a static problem and may be computed on three consecutive time steps (the switch instant, the previous and the following steps) thus allowing to make a velocity and acceleration corrections based on the three static computations. The dynamic simulation is initialized at ts by the displacements, velocities and accelerations being computed. The method is validated through comparisons with a reference solution corresponding to the 3D model solution used during all the simulation.