Vibration-based structural health monitoring from operational long-gauge fiber optic strain data

Vibration monitoring from strain data is a promising alternative to acceleration-based monitoringbecause a dense measurement grid can be achieved at a relatively low cost and because strain modeshapes are more sensitive to local stiffness changes than displacement mode shapes. However, thefeasibility of monitoring strain mode shapes of full-scale civil structures, where the operationaldynamic strain levels are of very low amplitude and temperature changes can influence the modalcharacteristics, has remained an open question. The present work provides a proof of concept inwhich the deck of a steel bowstring railway bridge is instrumented with 80 Fiber-optic Bragg Gratingstrain sensors, multiplexed in four fibers, that are interrogated with a technique that achieves highaccuracy and precision. For more than a year, the natural frequencies and strain mode shapes of 10modes have been automatically identified from operational strain time histories, with typical rootmean-square values of 0.01 microstrain, on an hourly basis. Furthermore, using these modal data,the influence of temperature fluctuations and that of a retrofitting of the hangers connecting thebridge deck and the bow, which took place during the monitoring period, are extensivelyinvestigated. Both have an influence on the overall stiffness of the bridge and therefore they resultin clear changes in the natural frequencies. They do not have an influence on the local stiffness andtherefore they do not influence the strain mode shapes, except when the retrofitting induces aninteraction between previously well-separated modes.

Publication year:2021