Experimental investigation of amplitude dependency of dynamic characteristics in elastic and inelastic stages of reinforced concrete pier model

Structural health monitoring of existing bridges relies on vibration testing, but the free- and ambient-vibration testing techniques in current use for the determination of dynamic characteristics have a previously unknown dependency on vibration amplitude. The main purpose of this study is to investigate the dependency of the dynamic characteristics of reinforced concrete (RC) piers on the vibration amplitude in elastic and inelastic stages by applying free vibration tests to RC pier models. A series of free vibration measurements over a wide range of amplitudes were carried out with relatively small amplitude of excitation on intact models; these measurements were repeated after the models had been progressively tested to failure on a shaking table, and again after repair with carbon fiber sheets (CFS). The repair method used is one in common use for seismic repair of bridge piers: the damaged cross-section was recovered by wrapping carbon fiber sheets around the cross-section in the damaged zone, after injecting the relatively wide cracks with resin epoxy. Generally, our results indicate that overall natural frequency and damping ratio are very sensitive to the vibration amplitude level. Particularly, whenever the acceleration amplitude increases, the damping ratio significantly increases while the natural frequency slightly decreases; which means that, when considering the dynamic characteristics of a structure, we need to pay special attention to the expected vibration amplitudes before choosing design values. We also found that the CFS repair method was effective in restoring performance characteristics of the damaged piers.