In the framework of developing a non-destructive vibration testing method for monitoring the structural integrity of constructions in civil engineering, it is important to be able to determine the dynamic stiffness in each section of the structure from measured modal characteristics. From the dynamic stiffnesses, one obtains directly an idea of the extension of the cracked zones in the structure. In an experimental program, a concrete beam of 6 meter length is subjected to an increasing static load to introduce cracks. After each static preload the beam is tested dynamically in a free-free set-up. The change in modal parameters is then translated into damage in the beam. The technique to predict the damage location and intensity that will be presented in the paper, is a direct stiffness derivation from measured modal displacement derivatives. Using the bending modes, the dynamic bending stiffness can be derived from modal curvatures. Using the torsional modes, the dynamic torsion stiffness can be derived from modal torsion rates.
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