Evaluation of Vibration Characteristics of an Existing Harbor Caisson Structure Using Tugboat Impact Tests and Modal Analysis

Ambient and forced vibration tests are proposed to evaluate dynamic characteristics of a caisson-type breakwater, including natural frequencies and modal damping ratios. The feasibility of a numerical analysis model with fluid-structure-soil interaction effects, which play an important role in evaluating structural performance and safety, was investigated by comparing the numerical results with experimental results. The Oryukdo breakwater in Busan, Republic of Korea, was employed as the target structure. This breakwater was once heightened by installing additional parapet structures of about 4 m tall to improve the harbor tranquility in 2005. Vibration tests were carried out in 2000 (before heightening) and in 2011 (after heightening). Most caissons were tested in the first test, while only one caisson was tested in 2011. It was found that natural frequencies were reduced by 1.7%–4.3% after heightening, and similar results were observed from the numerical analysis. It was also found that forced vibration tests can yield more reasonable results than ambient vibration tests. Even though there is some discrepancy between experimental and numerical results, numerical analysis can be carried out to analyze dynamic characteristics and evaluate structural performance and safety.