2019 CSCE Annual Conference - Laval (Greater Montreal)

2019 CSCE Annual Conference - Laval (Greater Montreal) Conference

Fatigue Life Evaluation of the Diefenbaker Bridge Using Structural Health Monitoring

Mr. Christopher Morgan, WSP Canada Inc. (Presenter)
Dr. Leon Wegner, University of Saskatchewan
Dr. Bruce Sparling, University of Saskatchewan

As bridge infrastructure continues to age, public agencies must reliably determine which structures can remain in service, and which structures require rehabilitation or replacement. Structural fatigue is a common problem for many aging steel structures and its evaluation is one that carries a high level of uncertainty. Structural health monitoring is one technique that infrastructure owners can employ to reduce this uncertainty, thereby allowing them to make the necessary investment with confidence.

Structural fatigue occurs when steel components of a bridge are subjected to stress cycles, with every detail able to withstand only a limited number of cycles. The challenge in determining the remaining fatigue life of a bridge is the uncertainty in historical stress cycles and in-situ structural behaviour. The Canadian Highway Bridge Design Code (CSA S6-14) does not address fatigue life evaluation directly, and therefore, creates an even larger challenge for engineers. Structural health monitoring is a technique that engineers and owners can use to reduce this uncertainty because it helps to reveal actual stress levels and cycle counts.

Structural health monitoring was used to inform the determination of the remaining fatigue life of the Diefenbaker Bridge. The Diefenbaker Bridge is located in Prince Albert, Saskatchewan, Canada. The 304 meter long, seven span bridge consists of two separate fracture critical superstructures, each comprising a cast-in-place concrete deck supported by two welded steel I beams. The separate superstructures share a cast-in-place concrete substructure. Given the age of this bridge, and its history of frequent rehabilitation, an understanding of the remaining fatigue life was of critical importance to its owner since asset management plans depended on the outcome.

To perform the evaluation, the structure was instrumented with strain gauges, accelerometers, and a weather station. Data was collected for one year, and was used to characterize in-situ bridge behaviour (i.e. lateral load distribution, degree of composite action, dynamic load influence, bearing restraint) and to evaluate the bridge’s remaining fatigue life. Lastly, various methods of fatigue life evaluation were compared including deterministic methods, probabilistic methods, and AASHTO’s method.

This research proved that costly improvements to the fatigue prone details were not required, and that under current conditions, fatigue would not govern the service life of the structure. In addition to this, unexpected composite action and dynamic load influence was found to exist on the bridge.