Reducing Longitudinal Demands on Tall Bridge Piers with an Anchored Abutment

摘要

The Highway 53 relocation project in Virginia, Minnesota, includes a tall bridge across a currently inactive iron ore mine pit. The 1,132-ft long bridge is a three-span, plate girder structure. Two intermediate piers are approximately 165.5 ft and 182.5 ft tall. Horizontal demands acting on the piers in the longitudinal direction of the bridge are significantly reduced and buckling capacity of the piers significantly increased by resisting longitudinal load through an anchored abutment and bracing the top of the piers through the superstructure. The reinforced concrete East Abutment is constructed against strong chert, iron ore bedrock. Eight 218-kip tieback anchors are included in the abutment. The combination of tieback anchor tension and passive resistance against the bedrock behind the abutment allow it to serve as an anchor point against longitudinal bridge loads. The bridge girders are attached to the East Abutment with fixed disc bearings, which allow rotation but not translation. Because the piers are tall, a small lateral force applied to the head results in a large overturning moment at the base and ultimately the foundation. By anchoring the East Abutment, significant cost savings were realized in the substructure and foundations. The combination of a tall bridge and rock bearing abutment made this structure particularly well suited for including an anchored abutment in the design. This paper will present the key design considerations from both a geotechnical and structural engineering standpoint.