The new Peter Courtney Minto Island Bridge is the last link connecting 1,300 acres of riverfront parks and trails across Salem, Oregon. The steel arch pedestrian crossing, which was completed in August, 2017, extends the renewal of the downtown by linking it to the expansive natural park on an island in the center of the river.
Building a bridge to a nearby island may seem like a simple enough project, but the extensive geometric and environmental constraints and administrative hurdles made designing and building this bridge incredibly challenging, says Jim Bollman, senior project engineer for OBEC Consulting Engineers in Salem.
To meet the requirements of the owner and regulations, the bridge had to clear the 100-year flood elevation, meet existing path grades with a limited approach, accommodate grade limits for the Americans with Disabilities Act (ADA), and over-cross an existing park path while providing adequate head room for pedestrians -- all while accommodating a "no-rise" criterion for the 100-year flood elevation upstream of the bridge because the project is located within a FEMA floodway.
And that was just the beginning. Once on site, the project teams had to avoid multiple hazards, including buried large woody debris and industrial waste, while minimizing disruption to the nearby wetlands, and to the possibility that the project was encroaching on Native American burial sites.
“Meeting the combined site constraints with any bridge would be a challenge, but to meet them all simultaneously and with stunning style and aesthetics, and at reasonable cost is the great achievement of this project,” Bollman says.
The combination of long span and very shallow superstructure required a creative solution for the main span and approaches to achieve a coherent and seamless structural arrangement. “Choosing precast concrete was instrumental to simultaneously meeting all of these obstacles while facilitating the extensive permitting process,” he says. Precasting the deck in a panel system offered numerous and crucial advantages in quality, construction practicality, minimizing temporary construction disturbance, and long-term durability. “The precast panels also reduced the cost and duration of main span construction by precluding the need for falsework and formwork for the main span floor system over the slough,” he says.
The overall configuration is a five- span bridge, with a main span tied-arch of 304.5 feet at the springline chord, and thin cast-in-place (CIP) post-tensioned haunched slab approach spans. The main span features a pair of tied arches to support a precast panel stress-ribbon deck. “The tied arch allows the main supporting structure to be situated above deck level and does not require the arch to extend to ground level,” he says. The presence of the arch ties was leveraged into a constructability asset by using them to position the precast deck panels. The main span features a five percent parabolic vertical curve profile centered at the arch mid-span, which aligns with ADA requirements, and shallow approach spans to create minimal backwater during flooding events.
The precast panels also function as stay-in-place forms for a CIP topping slab. Full-length deck post-tensioning and the topping slab of the arch span provide capacity to the approaches and precludes live load tension at the arch span precast panel joints. The resulting structure met all of the owner’s requirements while adding a beautiful piece on architecture to the waterfront community, he says. “While the finished structural solution has a simple, uncluttered and graceful appearance, that outward appearance belies the complexity of the site constraints and the design attributes for meeting those constraints.”