Application of local reference-free damage detection techniques to in situ bridges

There are increasing demands to apply structural health monitoring (SHM) techniques to bridge structures. To meet these demands, our group has developed a suite of local SHM techniques that can detect particular types of damage without using baseline data obtained from the pristine condition of a structure. They are coined as reference-free damage detection techniques. The main advantage of the reference-free techniques is that they can provide improved damage diagnosis even when the system being monitored is exposed to ambient variations such as external load and temperature changes. Since the reference-free techniques do not rely on a direct comparison with previously obtained baseline data, they are less vulnerable to false alarms due to undesirable variations which in-site structures are frequently subjected to. In this study, the applicability of two specific reference-free techniques to real bridge structures and associated implementation issues are examined through field tests of a decommissioned curved steel box girder bridge and an in-service single-span steel girder bridge in Korea. Our monitoring efforts are focused on detection of cracking in critical steel members with additional structural features such as stiffeners and diaphragms. External loading effects on the reference-free techniques are investigated through static and dynamic loading tests using a 35-t dump truck. Furthermore, daily/seasonal temperature variations are observed over a 1-year period. The test results confirm that the reference-free techniques successfully identify cracks on the test bridges even under varying external loading and temperature conditions. In addition, the long-term durability issues of piezoelectric transducers (PZTs) are addressed, and an enhanced PZT design has been proposed to improve the PZT durability.