Investigation on the damage identification of bridges using distributed long-gauge dynamic macrostrain response under ambient excitation

Abstract

In this study, an output-only method for extraction of modal macrostrain and damage identification of bridges under ambient excitation is presented. It is theoretically proved that the modal macrostrain within the gauge length of a long-gauge macrostrain sensor is uniquely determined by the peak value of the power spectral density (PSD) of dynamic macrostrain response. Then damage occurred within the gauge length of a sensor can be identified by the ratio change between the peak value of PSD of this sensor response and that of the reference sensor response. The damage extent has also been verified to have corresponding relationship with the ratio change. Numerical simulation was carried out to confirm the feasibility of the proposed method. Analysis results of the numerical simulation reveal that the maximum error of the identified modal macrostrain relative to that of the modal analysis is 3%. Results of simulation show that the proposed PSD-based method can not only accurately localize the damage but also assess the damage extent. The influence of several typical excitations on real bridges was also investigated that further proves the robustness of the method. It is worth mentioning that only the first-order mode is necessary for the method to identify damage. Finally, the proposed method is employed for condition assessment of a real bridge located in New Jersey, wherein 17 long-gauge macrostrain sensors with the gauge length of 1 m were distributedly arranged along the critical region of the girder. Analysis results of the field measurements further verify that the PSD-based method can be utilized to assess the damage state of structures under ambient excitation.