The new and improved Manning Crevice Bridge is only the seventh of its kind in the world
By Brian Alvarado
Since 1934, the Manning Crevice Bridge over the Salmon River in Riggins, ID has served as a key point of access to resorts, residences and commercial rafting routes in the Nez Perce National Forest. The timbered-tower suspension bridge was originally part of President Franklin D. Roosevelt’s plan to create jobs amid the Great Depression.
As decades passed and the service life of the bridge ended, the owner, the Federal Highway Administration, decided in 2010 it was time for a replacement to address the deteriorating structure. The FHA called on engineering firm Atkins North America to design the bridge, while Record Steel Construction acted as the builder of the project. The geotechnical engineer was Shannon & Wilson Inc., while the civil engineer was Horrocks Engineers. Lastly, Inland Foundation Specialities handled crane and erection.
“The original Manning Crevice Bridge was built in 1934 as a timbered-tower suspension bridge with width, height and turning limitations and was load restricted,” said Atkins Technical Manager David Konz. “Extremely sharp curves on the bridge from both ends required wheel rub rails to direct the wheels away from support posts to keep vehicles on the structure.”
Additionally, concrete anchor blocks showed extreme deterioration and cracking, while timber support towers were showing evidence of extreme decay. After exploring six structural configurations, the development team and the local public decided that the replacement would be an asymmetrical single-tower suspension bridge with weathering steel. The Manning Crevice Bridge is only the seventh structure in the world built as a single-tower asymmetrical bridge.
After two years of construction, the project was completed in June 2018, with the cost coming in at just under $10 million.
“Several years of successful planning, design, and construction of this bridge was a result of effective collaboration between the owner, designer, and contractor,” Konz said.
The major stakeholders included the project’s sponsor and primary advocate Western Federal Lands Highway Division; Idaho County, who maintains the roadway and bridge; the U.S. Forest Service; Bureau of Land Management; National Park Service and the Nez Perce Tribe.
Several years of successful planning, design, and construction of this bridge was a result of effective collaboration between the owner, designer, and contractor.” -David Konz, Technical Manager, Atkins
Deciding on Design
One aspect that helped steer the design of the bridge was the location of the construction site, which was remote and in a deep canyon. This limited access for trucks as well as space for construction. The site features a narrow shelf road with steep drop-offs in hard rock terrain.
“The hard rock and pristine canyon location also made traditional construction techniques both cost prohibitive and inappropriate,” Konz said.
A single-tower asymmetrical configuration was chosen, as competent bedrock at the site allowed for anchoring large horizontal forces. Rock adjacent to the north tower of the existing bridge required a minimum tower height of 60 feet to place anchorages on favorable rock geometry.
The limited space also meant a cable suspension would be more constructible than an arch because of the light weight and flexibility of steel cables. A temporary crane platform was located on the north side of the river for erection of the tower and anchorages. A stacked superstructure framing configuration was conceived to permit easy assembly from the bottom up, allowing for smoother tower construction. The high-strength concrete deck has low permeability and an epoxy-based traffic topping for supplemental durability and protection from freeze/thaw cycles.
Lessening the Environmental Impact
In regards to materials used, Grade 50 weathering steel was chosen for the main tower and steel superstructure due to the remote location and rustic aesthetic appeal.
“The material requires very little maintenance as the densely rusted patina creates a protective coating on the steel,” Konz said.
To avoid future painting that can be harmful to the sensitive ecosystem, remaining steel components were galvanized. The design decisions kept environmental impacts to a minimum by significantly reducing maintenance activities over the Salmon River.
“Additionally, the salmon fishery in the river below is prized as one of the best in the northwest United States. The visualizations prepared by Atkins clearly showed that no matter which structure type was selected, the construction equipment would not enter the pristine river,” Konz said.
The reception from the local community has been overwhelmingly positive, Konz said. The ribbon-cutting ceremony included Riggins, Mayor Glenna McClure and local historians that showcased five story boards memorializing the construction and life of the original bridge from 1934, which was built by the Civilian Conservation Corps.
The timber towers of the original suspension bridge were moved to Riggins City Park as a landmark that will last for generations. The original rock anchors also remain visible in the same rock face as the new anchorages, memorializing the efforts of past generations.
After opening of the new Manning Crevice Bridge, the project has received multiple awards from a number of organizations, including honors as the 2018 ASCE Region 8 – Outstanding Project (under $10 million); the 2019 ACEC Idaho Engineering Excellence Award (1st place); the 2019 ENR Mountain States Best Small Project (Under $10 million) Award; the 2020 American Institute of Steel Construction Prize Bridge Award; and most recently, the 2020 ACEC National Engineering Excellence Award – Grand Award – American Council of Engineering Companies.
As the new and improved Manning Crevice Bridge continues to serve Riggins, ID, it’s the careful planning and execution of the project has resulted in a major victory for the development team and the bridge’s owner.
“Ultimately, the CM/GC team identified, planned for, and mitigated the majority of the foreseeable challenges leading to an extremely successful project,” Konz said. “Maintaining clear communication among the many project stakeholders regarding the value of the natural resources and balancing the requirements and concerns of all parties were a high priority from the outset of the project.
Brian Alvarado is the editor for American Infrastructure Magazine. He can be reached at firstname.lastname@example.org.