Task Force chooses suspension bridge type for bridge over Willamette River
During a meeting to decide which type of pedestrian/bike bridge would run across the Willamette River near Wilsonville, French Prairie Bridge Task Force members had differing views on one overarching question: Should the bridge stand out or blend into the surrounding environment?
Most viewed the cable-stayed bridge type — with its tall, diagonal cables that form a distinctive triangle— as a potentially remarkable sight while some said the suspension bridge features a more understated, utilitarian look.
Ultimately, the suspension bridge ruled the day less for its aesthetics and more because of its lower cost and better construction feasibility.
"The engineer sold me," task force member and Wilsonville resident Jeremy Appt said. "The suspension bridge is more efficient and better."
Though about a quarter of the members initially advocated for the cable-stayed bridge type, the task force unanimously voted to recommend the suspension bridge during the meeting Tuesday, June 4. Wilsonville City Council and the Clackamas County Board of Commissioners will ultimately decide which bridge will be chosen at a later date but City Councilors Charlotte Lehan and Susie Stevens are on the task force and the council and board of commissioners typically approve the task force's recommendations.
The French Prairie Bridge would be a bike, pedestrian and emergency vehicle bridge across the Willamette River that would land along Boones Ferry Road on the Wilsonville side of the river and Butteville Road on the south side of the river. It has yet to be funded but City staff and consultants recently completed conceptual designs for the bridge, which was funded through a Federal Highway Administration (FHA) grant and City parks system development charges, which are one-time charges to developers to fund infrastructure impacts.
Months prior to the meeting, the task force and the council narrowed down the bridge types from five to two — using input from a public survey — and chose the suspension and cable-stayed bridge types as finalists. The final two types were chosen largely because the task force liked the bridges' architecture and because they wouldn't require piers to be placed in the Willamette River or — so they initially thought — the Boones Ferry Marina parking lot. However, they were both projected to cost 70-90% more than the cheapest option: steel girder.
"They (task force members) felt those two bridges would be the best for tourism interest," Wilsonville Capital Projects Engineering Manager Zach Weigel said last December. "They are more aesthetically pleasing bridges than say the steel girder or steel truss."
The meeting started with a presentation from Bob Goodrich, a consultant from OBEC Consulting Engineers.
He said the cable-stayed bridge would be much taller than the suspension bridge (180-to-200 feet compared to about 100) and that the configuration would create a structural imbalance.
"Because the cables themselves on each side of the pylon are not at the same angle, it pulls the pylon in one direction, which makes you have to reinforce, add concrete and rebar, to the pylon to resist that force," Weigel said.
"So you end up with design inefficiencies, extra materials, extra costs for those forces that are generated because of the suboptimal configuration," Goodrich said. "What we really want to make the cable stayed more optimal is a longer back span."
Goodrich said the two bridges are similar in many ways but that the suspension bridge is more efficient from an engineering point of view because it would be more structurally balanced.
And many of the task force members who preferred the suspension bridge cited its more suitable engineering as the primary reason for their preference. Some also appreciated its design.
"It (the suspension bridge) is a classic looking design and it's timeless," Lehan said.
Goodrich also said that, unlike the suspension bridge option, the cable-stayed bridge would require a pier to be placed on one of the few parking lot truck trailer spaces in the Clackamas County-owned Boones Ferry Marina, which is on the other side of the south side of the river.
"Every parking lot space is important to the functioning of the marina," said Karen Buerig, a task force member and the long range planning manager for Clackamas County.
The City and Goodrich also projected the cable-stayed bridge to cost between $39.1 and $51.6 million while the suspension bridge would cost between $37.1 million and $49.3 million. Goodrich said the cost variance is largely due to uncertainty about whether the area where the bridge would be built has liquefiable soil. If so, they will need to make ground improvements, which could cost about $5 million, to ensure that the bridge is seismically resilient
"In this case the seismic resiliency is a big piece of the project so having that accounted for and the bridge serviceable after the Cascadia (subduction zone earthquake) event is critical," Goodrich said.
Task force member Steve Van Wechel, who created the Salmon Cycling Classic bicycling event in part as a way to promote and accentuate the French Prairie Bridge project, felt that the cable-stayed option could be more of a tourist attraction due to its more pronounced look.
"I think the cable-stayed bridge, because it's bigger, would be a better attraction," he said. "It could be a tool for economic development rather than a (bridge for) 'Let's take a walk on a Sunday afternoon.'"
However, some did not like the idea of a bridge that would stand out from the rest of the scenery.
"I like the idea of a lower profile bridge that doesn't take away from the natural environment," Wilsonville resident Leann Scotch said.
If the suspension bridge type is upheld by council, the City will then undergo an analysis to determine how it will fund the project, which Weigel said will likely take six months.
If it eventually is funded, Weigel said construction would take about 2-3 years.
"I imagine it will be a combination of federal and state funding. The analysis will help identify where those opportunities are available," Weigel said.
Weigel also said the City is waiting for approval from the FHA to extend the grant so that they can perform a more in-depth analysis for the project.
"We want to be able to do the geotechnical field work earlier rather than later so we can narrow that cost range down," he said.