Lateral Loads and Displacements of Railroad Bridges from Field Investigations

Abstract

The progressive increase in loads and frequency of trains traversing over aging railway infrastructure can lead to large lateral forces and displacements that could potentially cause derailment and problems related to safety. Reliance only on design thresholds for safety assessment of existing bridges can lead to conservative evaluations, unnecessary replacements, and increased maintenance costs. However, there is a lack of data on lateral behavior of existing railroad infrastructure. This article examines the recommended lateral load and displacement specified in codes. It specifically analyzes the responses of railroad bridges under revenue service traffic, using field-collected data. The goal is to establish a contextual understanding of how modeling, field data, and the code relate to each other. First, this paper presents an investigation of measured lateral forces of existing railroad bridges using different instrumentation techniques such as truck performance detector, track loading vehicle, and linear variable differential transducers (LVDTs). Second, a series of lateral displacement field monitoring data of three railroad steel bridges using the LVDT and the time history analysis of two railroad timber bridges under train load events with different speed and axle load using laser are presented. Time history responses of railroad bridges under train load provide an objective qualification to evaluate their current condition. Finally, the data help to identify maximum lateral displacement, train speed, symmetry of bridge response associated with the construction, the number of cars, and freight versus passenger different responses. These measurements are recommended to be used for field evaluations of bridge lateral responses under traffic. This study focuses on lateral load and displacement in railroad bridges. The authors suggest using field monitoring data to ensure the safe operation of existing railroad bridges, because the design code’s conservative values may not be sufficient. To assess lateral load, the authors recommend using a pair of forces rather than a single concentrated lateral force. For lateral displacement assessment, the authors suggest the use of a field monitoring technique with LVDT or sensors. This approach enables the creation of a displacement time history that includes train speed and symmetrical movement data. The practical application of this method may be limited by budget, bridge access under revenue service traffic, and safety restrictions for sensor location. Additionally, the amount of data that can be collected depends on the number and location of sensors. Sensor placement must be guided by prior engineering knowledge of the bridge. Finally, data collection may be time-consuming, which may be cost-ineffective and render the whole exercise difficult. In summary, this study proposes a field monitoring approach for lateral load and displacement assessment that requires a careful consideration of practical limitations.