Monitoring Bridge Displacement Using Pre-Existing Telecommunication Fiber

Abstract

Monitoring the condition and performance of bridges is critical for enhancing the resilience and safety of urban infrastructure. Sensor-based approaches (both mobile and in-situ) have been developed for monitoring bridges; however, such systems are limited in temporal information, or costly to install and maintain, and thus not scalable. Our previous works have demonstrated that ubiquitous pre-existing telecommunication fiber optic cables can be turned into a dense array of vibration sensors using distributed acoustic sensing (DAS). This enables scalable, high spatial resolution (meter-level) bridge health monitoring, as a single DAS interrogator can turn up to 100 km of telecommunication fiber into distributed strain sensors. While only the dynamic (1-10 Hz for short to medium-span bridges) response of the bridge using telecommunication fiber has been studied before, quasi-static (0.01 - 1 Hz) response of the bridge is critical for BHM tasks such as load rating and serviceability assessment. To this end, we aim to estimate quasistatic displacement response of bridge from telecommunication fiber response. The key challenge is the coupling between the fiber and the bridge that distorts the bridge response measurements. The telecommunication fiber optic cables are often laid inside conduits, which are attached to bridges. As the bridge vibration passes through these attachments, it gets distorted. To address this, we characterize the vehicle-induced quasistatic strain measured by telecommunication fiber, examining its response under different load positions and magnitudes. Based on this characterization, we model the transfer function as a linear relationship between DAS measurements and actual bridge response and infer the parameters using a reference sensor installed for calibration. Our evaluation results from two real-world in-service highway bridges demonstrate that DAS can serve as a displacement sensor with sub-millimeter accuracy.