In August 2022, a transporter carrying a highly flammable liquid crashed at the Sikanni Chief River Bridge. The resulting fire generated intense heat causing extensive structural damage and a full closure of Hwy 97, cutting off the primary link to Northern Canada and Alaska. With this route servicing 3 hospitals and facilitating more than 2,500 vehicles per day, including more than 860 commercial trucks, there was severe social and economic loss associated with the closure and extreme pressure for rapid re-opening of the highway. An emergency response was initiated immediately along with a site review and rapid damage assessment that enabled reinstatement of passenger vehicles within 24 hours of the incident. While the quick re-opening to light vehicles was an important success, a significant challenge followed in determining how to evaluate the residual bridge strength so that full truck loads could be restored: no existing codes or standards exist to provide direction on how to carry out post-fire structural evaluations. Significant and irreversible loss of capacity can occur in concrete and steel when they are exposed to the intensity of heat generated a fire of this magnitude, with particular vulnerability for the high strength steel strands in the prestressed concrete girders that support the Sikanni bridge. Heat exposure during the incident was severe enough to cause significant concrete spalling which exposed the girder’s prestressing strands. Extreme levels of heat exposure present a serious risk for irreversible strength loss, particularly within the strands which could lead to a sudden and catastrophic collapse if either the ultimate strength or pre-tension has been compromised. Determining heat profiles within the girders and applying the results of the heat isotherms is critical in a post-fire structural evaluation as material test results need to be paired with heat exposures to accurately predict residual capacity.

After re-opening the Sikanni Chief River Bridge to light vehicle traffic, the primary focus was forensic review of the structure and evaluation of the residual capacity. Given that heat exposure of this magnitude can significantly reduce the capacity of a bridge, WSP needed to determine what the heat profiles were within the key structural components of the bridge during the fire. Through quick engagement of Subject Matter Experts, we determined that petrographic examination of the concrete was a reliable method for determining what the heat exposures were during the fire, including temperatures at critical depths within the girders such as the location of the prestressing strands. In addition to performing petrographic examination, we extracted prestressing strands, mild reinforcing steel, and concrete cores from the fire damaged zones. Material strengths obtained by laboratory testing were adjusted for heat profiles determined from the petrographic examination and were then used in the structural evaluation for the bridge. Load testing was also completed with the results applied to validate a Finite Element Model of the bridge to accurately determine structural demands. In parallel with the structural evaluation, a robust contingency plan was initiated immediately following the incident by mobilizing concurrent Teams to prepare two separate sealed strengthening designs as a risk management strategy.

The structural evaluation determined that the bridge had sufficient capacity and full truck traffic was restored in just 24 days. The timely re-opening of this highway kept communities supplied with daily goods and provided access to health care while maintaining supply chains critical for northern development and economic opportunities including oil and gas, mining, and other large-scale projects. A subsequent structural rehabilitation of the bridge to restore durability was completed in 2024.