Experimental System Identification and Dynamic Assessment of Damaged Concrete Beams in Bridges

One of the major factors contributing to structure collapse is the appearance and development of damage (visible or concealed) in structural components by time. In this paper, the effect of damage on dynamic characteristics of reinforced concrete beams usable in bridges, including natural frequency, damping ratio, bending stiffness and vibration amplitude are experimentally investigated. Damage is considered as a reduction in the flexural stiffness by extension of the flexural cracks. The force-deformation curve becomes a linear curve (no hysteretic loop) if the cyclic load is applied slowly enough, hence in this experiments slow dynamic force is created by the vibration motor. The results indicate that the damping ratio in the vicinity of the cracked region is not merely a viscous damping, but rather is a combination of viscous and frictional damping, and the contribution of the frictional damping will increase by increasing the crack or damage. The natural frequency of beam and its maximum amplitude of vibration increase with increasing eccentric mass and therefore with increasing dynamic load. The damping ratio is significantly influenced by the degree of cracking, increase of eccentric and number of cycles. In general, damping ratio calculated by averaging the values for different cycles, increases with increasing degree of cracking and with increasing dynamic load. The results indicate that maximum amplitude of vibration, bending stiffness and natural frequency of beam decrease and damping ratio increases with increasing degree of cracking. Also, in this paper, two new extended methods for determining the stiffness of concrete beams using dynamic force-displacement curve instead of static experimentation methodology are presented. Precision and accuracy of the both proposed methods are verified. It is observable that the results related to the 2nd proposed method for dynamic secant stiffness curves have best correlation with the static stiffness curves. Results indicate that by increasing the number of cycles N in logarithmic decrement method, the dynamic force-displacement curve closes to the static force-displacement curve.