Railroads carry 40% of the U.S.’ freight tonnage. Railroad bridges are the most critical component of this network. Measuring transverse displacement of railroad bridges under train-crossing load is essential for the safe and cost-effective operation of railroad network. However, bridge displacement is difficult to collect in the field with traditional sensors due to the lack of fixed reference frame. Although reference-free sensors provide flexibility overcoming the aforementioned challenge, they often fail to capture pseudo-static components observed in timber bridges. This study proposes a novel reference-free sensing system to measure the total displacements of railroad bridges under train-crossing loads. A novel passive-servo electro-magnetic-induction (PSEMI) sensing technology provides accurate direct reference-free dynamic displacement measurement. Furthermore, researchers utilize two reference-free accelerometers to record inclination measurement and transform to pseudo-static displacement. Total bridge displacement is obtained by adding dynamic and pseudo-static responses together. Shake table experiments employing a bridge pier model excited by bridge displacements measured in the field has validated the effectiveness and accuracy of the novel sensing system.

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