Long-term monitoring of two highway bridges using microwave interferometer-case studies

The microwave interferometer used in the present study is capable of multiple-point displacement measurements for bridge health monitoring. In this paper, two highway bridges were monitored once a year for a period of five to six years. The first case is an extradosed bridge with prestressed concrete (PC) multicell box girders. The second case contains prestressed concrete box girder with tapered cross-section. For the two PC bridges, the vertical displacements of the tetrahedron metal reflectors fixed at the railing corresponding to ambient vibration due to traffic loads are remotely monitored. The modal frequencies as well as the modal shapes were analyzed by Frequency Domain Decomposition method. The results show the first bridge is perfectly healthy but constant decreasing of the modal frequencies and the Modal Assurance Criterion were found for the second bridge. The reasoning behind the measured results is given.