Fatigue assessment of centenarian railway bridges with structural health monitoring data

Transit agencies own and operate thousands of bridge structures subjected to repetitive train loading. The majority of these bridges have been built at the turn of the century and many have exceeded their theoretical fatigue life spans. In New Jersey, recent increase of railcar weight limits from 1170 kN (263,000 lbs) to 1272 kN (286,000 lbs) raised additional concerns for railway bridges which were not designed based on the increased railcar weight and were in service for more than a century. There is a need to establish procedures to estimate the impact of the increased railcar weight on the remaining life and damage accumulation of bridges to prioritize and schedule repairs and rehabilitation events. In this study, a typical railway bridge on various New Jersey passenger rail lines that is used for freight was selected to investigate the impact of the heavy freight railcar on fatigue life of the bridges. Field testing was performed for the selected bridge to understand the behavior (strain, deflection and velocity) of the structure under various train loads. The collected field testing data were used to validate the 3-D FE model of the bridge. A probabilistic model is proposed for fatigue evaluation of a railway bridge located in NJ Transit line. On the loading side, the dynamic impact and the annual train frequencies f_ti were considered as random variables to derive the probabilistic train load spectra with Monte Carlo simulation. The calibrated 3-D finite element model of the selected bridge was used to calculate the stress histories from train loading. Fatigue load spectra for this bridge was then develop by converting the stress histories into stress ranges through rainflow counting method. On the resistance side, the relevant S-N curve is randomized with constant variance in fatigue strength. The performance function for fatigue evaluation is given by a modified form of the Palmgren-Miner damage law. The reduction in the remaining fatigue life of the selected bridges under increased loading is estimated as 24 years and 27 years, if the target reliability indices are assigned as 3.5 and 2.5, respectively.