The effects of seismic excitation on a reinforced concrete beam have been studied on the shaking table Vesuve in Saclay. Pure flexural behavior tests, up to rupture, have been performed. Experience obtained with different dynamic loadings (including fatigue effects) shows a decrease in stiffness and natural frequency as well as growth of damping. Analytical models describing the behavior of reinforced concrete were worked out, particularly for the region where the concrete is widely cracked whereas the reinforcement still remains elastic. After comparison of the experimental data with the results of a conventional elastic analysis, significantly higher safety factors have been derived, thus allowing a more realistic analysis of nuclear structures.