Estimating the remaining fatigue life of steel bridges using field measurements

As bridges continue to age and budgets reduce, transportation officials often need quantitative data to distinguish between bridges that can be kept safely in service and those that need to be replaced or retrofitted. One of the critical types of structural deterioration for steel bridges is fatigue-induced fracture, and evaluating the daily fatigue damage through field measurements is one means of providing quantitative data to transportation officials.

When analyzing data obtained through field measurements, methods are needed to properly evaluate fatigue damage. Five techniques for evaluating strain data were formalized in this dissertation. Simplified rainflow counting, which converts a stress history into a histogram of stress cycles, is an algorithm standardized by ASTM and the first step of a fatigue analysis. Two methods, effective stress range and index stress range, for determining the total amount of fatigue damage during a monitoring period are presented. The effective stress range is the traditional approach for determining the amount of damage, whereas the index stress range is a new method that was developed to facilitate comparisons of fatigue damage between sensors and/or bridges. Two additional techniques, contribution to damage and cumulative damage, for visualizing the data were conceived to allow an engineer to characterize the spectrum of stress ranges. Using those two techniques, an engineer can evaluate whether lower stress cycles (concern due to electromechanical noise from data acquisition system) and higher stress ranges (concern due to possible spike from data acquisition system) contribute significantly to the accumulation of damage in the bridge.

Data from field measurements can be used to improve the estimate of the remaining fatigue life. Deterministic and probabilistic approaches for calculating the remaining fatigue life were considered, and three methods are presented in this dissertation. For deterministic approaches, the output of the equations is the year when the fatigue life has been exceeded for a specific probability of failure, whereas for probabilistic approaches, the probability of failure for a given year is calculated.

Four different steel bridges were instrumented and analyzed according to the techniques outlined in this dissertation.