ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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footing or any formwork to construct the columns. The piles within an open pile-bent pier function as both the foundation and as a permanent form to place the concrete infill while still being structurally utilized in the design. This type of pier lends itself to this application and was selected as the preferred concept for the Trunk Highway 101 Bridge. Typically, the Minnesota Department of Transportation (MnDOT) limits the use of open pile-bents to sites where the height from top of pier to streambed is a maximum of 20 ft. For this project, the piers were significantly beyond this limit, with the tallest pier reaching a height of approximately 32 ft. Although these heights reached beyond the typical limit for open pile-bent piers, use of this type of pier was still the preferred concept. Therefore, further investigation was carried out to determine whether open pile-bent piers could meet structural demands and provide a safe route across the floodplain. Based on the hydraulic report for the site, water flow rates during a flood event are minimal, if not static, and ice floes are not expected. Due to the low hydraulic demand on the site, an encasement wall around piles was deemed unnecessary and ice impact did not have to be included in the design. Unique design criteria were developed to address the long-term durability and serviceability of the structure and its components while minimizing future maintenance needs. Serviceability— specifically, lateral deflection—was the governing factor in the design of the piers. There is a natural tendency of long bridges with relatively tall, flexible piers to "walk" or "migrate" in the direction of the longitudinal axis of the bridge under cyclic lateral loads such as braking and thermal forces. Criteria for limiting the lateral deflection of the piers were set to address this phenomena by reducing these longitudinal movements through the stiffening of the open pile-bent piers. Also, this stiffening of the piers aids in protecting the expansion joint glands from tearout and deterioration, and limits undesired expansion bearing movements. All of these measures should reduce future maintenance needs and costs. Initially, multiple rows of smaller- diameter battered piles were considered to increase the stiffness of the piers. W ith 40 piers on the project, this approach would have resulted in a significant increase in the amount of piling required and presented challenges in maintaining reasonable construction tolerances at the heads of the piles, considering their heights above the ground. Instead, a single line of larger-diameter piles was selected for the design. Although the individual piles were more expensive on a per-foot basis, the overall project cost was reduced because fewer piles were used and the simplicity of driving piles plumb also saved money. During final design, two concrete-filled composite steel pipe pile sections were chosen. To ensure and maintain a clean inside wall on the steel pipe pile, the pile bases were driven closed-ended and filled with concrete later. For the shorter piers at both ends of the bridge, a 16-in.- diameter steel pipe pile was used, and, for the taller piers, a 30-in.-diameter steel pipe pile was used. The 30-in.- diameter composite steel pipe pile section was the first of its size and the largest composite pipe pile section used on any open pile-bent pier in Minnesota. The final design of the pile-bent piers included an analysis that considered both combined axial and lateral load effects along with nonlinear soil- structure interaction. This analysis was used to determine the fixity point of the pile below the surface of the soil and the associated effective length for buckling capacity computations of the piles. Because the design was governed by serviceability criteria (specifically, lateral deflection), it was paramount that the design and properties of the piles maximize the available composite action between the steel pipe pile CARVER COUNTY, MINNESOTA, OWNER BRIDGE DESCRIPTION: A 4225-ft-long, 41-span, precast, prestressed concrete I-girder superstructure with a deck area of 333,456 ft 2 on open pile- bent piers STRUCTURAL COMPONENTS: Three-hundred sixty-nine 45-in.-deep precast, prestressed concrete I-girders; open pile-bent piers composed of 408 concrete-filled steel pipe piles (30-in. and 16-in. diameters) with cast-in-place concrete pier caps; abutments supported by fifty-one 12-in.-diameter concrete-filled steel pipe piles; and a 9-in.-thick monolithic concrete deck BRIDGE CONSTRUCTION COST: $24.4 million bid cost (approximately $73/ft 2 ) The bridge was constructed in multiple segments. The spiral welds are visible on the steel pipe piles that were filled with concrete after driving. ASPIRE Winter 2018 | 19

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