Strain-based evaluation of a riveted steel railroad bridge

Recent structural failures such as the I-35W Mississippi River Bridge in Minnesota have underscored the urgent need for improved methods and procedures for evaluating aging bridges. The aim of this research was to develop methods for Structural Health Monitoring (SHM) of railroad bridges utilizing a fatigue model to estimate the remaining life of the structure. Additionally, a method is developed for extracting experimental influence lines from measured strains. In the literature, it was found that fatigue models based on the Miner's sums or the effective stress ranges are the most common for bridge evaluation. SHM for bridges typically involves tracking changes in the dynamic properties from measured accelerations, and equating a loss of stiffness to damage. Strain based methods included tracking changes in structural properties and/or estimating the remaining fatigue life. Acceleration-based SHM systems were generally unable to reliably detect damage outside of the laboratory, so this research focused on applying a strain-based SHM approach. Four finite-element models were created and validated based on diagnostic load tests of a railroad bridge in northern New Mexico that carries daily passenger rail. It was found that the end-fixity of the floor system members generally had little effect on the behavior of the structure. Modeling the tracks, rail ties, and ballast had little effect on the girders and floorbeams, but a significant effect on the stringers. Ultimately, the simulated strains significantly overestimated the measured strains. The most accurate model was determined for each member type, and subsequently used for a fatigue evaluation as per the AREMA Specifications where representative historic traffic was estimated and used to determine the fatigue damage to date. The remaining life was then determined based on current traffic. A method was also developed to extract experimental influence lines from measured strains by multiplying a row vector containing the measured strains by the inverse of a matrix that represented the moving train load as it traversed the structure. The strain histories obtained using these experimental influence lines yielded results very close to the measured strains. The methods presented in this dissertation can be used to more accurately evaluate bridge structures.