Time-dependent considerations of I-35W St. Anthony falls Bridge including long-term monitoring applications

Structural health monitoring of in situ concrete structures is complicated by variations in the behavior of the structure due to ambient environmental conditions, transient loads, and time-dependent effects (e.g., creep and shrinkage of concrete, prestress losses). As a case study, the I-35W St. Anthony Falls Bridge, a post-tensioned concrete box girder bridge instrumented with over 500 sensors to monitor structural behavior, was studied over the first five years of operation. A data normalization scheme was developed to extract the time-dependent behavior of the bridge from long-term data, removing the instantaneous response due to thermal variations and adjusting the timescale with the Arrhenius equation to account for the temperature-dependent rate of time-dependent behavior. The resulting time-dependent behavior followed a line in log time, and served as the baseline for anomaly detection. Short-term and long-term checks were developed to identify anomalous, and thus potentially damage-related, changes in the time-dependent behavior. The efficacy of these checks was investigated by introducing perturbations into the monitoring data from the St. Anthony Falls Bridge, assumed to contain no damage, to simulate instantaneous bearing lockup and a slow degradation process through introduction of a two-year drift in the data. The proposed method succeeded in successfully identifying the perturbations while minimizing false positives.