Direct Reference-Free Dynamic Deflection Measurement of Railroad Bridge under Service Load
Today, railroads carry 40% of the US freight tonnage and this demand will double in 20 years. North American railroad infrastructure includes approximately 100,000 bridges spanning over 140,000 miles of tracks. Half of those bridges are over 100 years old. Measuring deflection time history of railroad bridges under train load can assist in quantifying the reliability and increasing the safety of railroad operations throughout the network. However, obtaining bridge deflection is often difficult to collect in the field due to the lack of fixed reference points from where to measure. Although reference-free acceleration can be used to estimate the dynamic deflection through double integration, the algorithms are difficult to develop and apply because of the complicated integration constants selected for the data post-processing. This research studies the reference-free dynamic deflection (vertical displacement) acquisition approaches, a sensing system composed of one passive-servo electro-magnetic-induction (PSEMI) velocity sensor and one built-in hardware integrator unit. This research has presented two promising reference-free dynamic deflection acquisition approaches, direct reference-free displacement measurement from a sensing system composed of one passive-servo electro-magnetic-induction (PSEMI) velocity sensor and one built-in hardware integrator unit, and a reference-free displacement estimation from accelerometer by Lee-Method, that can be used for evaluating the performance and safety of railroad bridges under service load. Using the passive-servo feedback electrical control technology, the PSEMI velocity sensor provides a low-frequency direct reference-free measurement performance with its small size and light weight. Using a finite impulse response (FIR) filtering instead of double integrating, the displacement can be estimated from acceleration without the integration errors from unknown integration constants and boundary conditions. Researchers used an ASCE steel truss bridge model and an MTS actuator to quantify the accuracy of the PSEMI sensing system. The actuator replicated various harmonic motions and real bridge vertical displacements under train-crossing events measured in the field. The direct dynamic reference-free displacements measured by PSEMI sensing system and the indirect dynamic reference-free displacements estimated by acceleration using Lee-Method were compared to reference displacements measured by LVDT. The experimental results show that the direct reference-free dynamic displacement sensing system and indirect reference-free displacement estimation method from acceleration are two promising alternatives to railroad bridge deflection under train loading, without the need to a fixed reference frame.
KeywordsDirect reference-free displacement Indirect displacement estimation Railroad bridges Train-crossing load
The financial support for this research came from the following sources that are gratefully acknowledged: The Center for Teaching and Learning at the University of New Mexico; the National Natural Science Foundation of China (No. 51208107); and the China Scholarship Council Foundation (No. 201604190028). The authors of this paper thank the Canadian National Railway (CN) for the bridge displacement data collected in the field, and Mr. Rahulreddy Chennareddy from the Department of Civil Engineering, University of New Mexico for his support in the configuration of MTS actuator system. The conclusions of this research are solely those of the authors.
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