Behavior of an integral abutment bridge in Minnesota, US

The behavior of an integral abutment (IA) bridge near Rochester, Minnesota, was investigated from the beginning of construction through approximately 7 years of service using data collected from more than 150 instruments installed in the bridge during construction. Long-term bridge shortening was observed to increase with time, based on the readings of horizontal extensometers installed behind the abutments and strains measured using concrete embedment gauges in the girders. Abutment rotations and measured pile curvatures also steadily increased with time. The time-dependent behavior of the bridge can be explained from a combination of factors including the concrete creep and shrinkage, the increase of backfill soil pressure, the yielding of the reinforcement across the joint between diaphragm and pile cap, and the alternation of the relative humidity and temperature. In comparing the measured pile axial force and moment data to axial force-moment interaction capacity curves, it was estimated that approximately 20% of the pile flange cross section yielded in compression under the abutment. The likelihood of low-cycle fatigue was determined to be small. On the basis of the findings of this research, it is recommended that the time-dependent effects of concrete IA bridges should be considered in structural design and analysis for both long-term bridge shortening and abutment rotation if these long-term effects are anticipated to be significant. More research is needed to determine the parameters for the cases when a time-dependent analysis of an entire integral bridge system is warranted, that is, bridge length limit and soil condition.