Modal analysis for damage identification of partially continuous multi-span bridges

Among the compendium of highway civil infrastructures built in the last decades, many repetitive or quasi-periodic configurations can be found, such as multi-span simply supported bridges. Structural Health Monitoring (SHM) strategies for this kind of structures should properly consider such structural periodicity. Literature studies already demonstrated that multi-span bridges are particularly challenging target structures for Operational Modal Analysis (OMA) since the modal properties typically appear as dense clusters of poles with closely spaced frequencies and mode shapes with similar wavelengths. Hence, this circumstance considerably hinders the identification of physical poles using standard stabilization diagrams. The imperfect independence between spans is typically due to the presence of a continuous deck over the simply supported girders, weak deck/asphalt connections and/or imperfect expansion joints. In this context, the coupling degree of spans manifests through global mode shapes and frequencies spanning between the limit cases of simply supported and continuous multi-span conditions. In this light, this work proposes a novel method to assist the clustering of modal poles extracted using OMA. The proposed method is based on the analytical modal solution to the free vibration problem of multi-span beams with weak rotational coupling between adjacent spans. The developed method is validated through the modal identification of a real-world in-operation multi-span reinforced-concrete girder bridge.