Optimal Sensor Placement for Condition Assessment of a Cantilever Truss Bridge

Structural Health Monitoring and Structural Identification (St-Id) applications are often criticized for deploying too many sensors without regard for their benefit. This paper discusses a project where St-Id was used to provide a sensor-light, yet informative field testing application. An initial numerical assessment of a long-span cantilever truss bridge indicated unsatisfactory load ratings and an unfavorable estimate of fatigue life. As a result, the overarching project objectives were to identify the (1) live load distribution throughout the primary and secondary load carrying systems and (2) nominal stress levels at fatigue-prone details. Given the nature of the issues, and the potential benefit of field testing, the St-Id framework was applied to the bridge. Financial resources for the project were limited, constraining the type and number of sensors. Therefore, a comprehensive sensor placement study was conducted to optimize information learned from the testing. An automated sensitivity study was conducted utilizing an FE model. Several load positions were simulated and the corresponding results were extracted for different structure section cuts. Those sections with the greatest sensitivity (not the largest response levels) to the parameters with the most uncertainty (e.g. boundary stiffness, pin stiffness) were selected for field instrumentation. The testing results were then used to calibrate the model, which showed the presence of multiple load paths depending on load position and levels of participation of “dummy” members. The calibrated model was then used to update the load ratings for all members of interest.

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