Use of Strain Data to Estimate Remaining Fatigue Life of a Fracture-Critical Bridge

A fracture-critical steel I-girder bridge was instrumented with strain gauges to estimate the remaining design fatigue life. The two girders on the bridge had extensive fatigue cracking. Continuous, dynamic strain data were collected for nearly 2 months to determine an effective stress range and cycle count according to Palmgren–Miner's rule. A simplified rainflow counting algorithm was developed and used to calculate the amplitude of each fatigue cycle. The effective stress range and cycle count were combined with AASHTO's S (stress range)-N (number of cycles to failure) curves to estimate the remaining design fatigue life of certain bridge details. The data revealed that the estimated design fatigue life was exceeded in the east girder (right lane), whereas some life remained in the west girder (left lane). The distribution of observed cracks in the girders was closely correlated with the calculated fatigue life. A method is presented in this paper to index the effective stress range so that strain measurements can be compared over extended periods.