Angkula Seo Suspender Bridge

• Bridge Specs: Span = 148′: Travel Width = 3′: Deck = Pultruded Fiberglass Decking: Mainlines = 1″ dia.: Anchors = Grouted DWYIDAG 150 KSI Anchors.
• Services: Design/Engineering, Fabrication, Installation.
• Location: 39.260334, -121.033388

The Sierra Fund has spent years driving construction of a memorial trail along Deer Creek in Nevada City, California. The Immigrant Tribute Trail is an homage to the native Nisenan people of the region and the waves of immigrants who have made the area home since the 1800s; Seattle Bridge was pleased to be approached about installing a bridge to close a loop across Deer Creek.

The Angkula Seo Suspender Bridge project presented a perfect marriage of people, site, and bridge design. The Sierra Fund’s Izzy Martin and her team were dogged in her pursuit of this ‘missing link’, and excellent collaborators. Keith Monohan, a longtime bridge builder and former principal with Sahale LLC, spearheaded the project locally, helping Martin Walz and Austin Thompson to locate and survey the bridge site. Together, this project team found a location spanning a spectacular section of Deer Creek, and, as it happened, the distance was ideal for a novel bridge design that had been exciting Senior Engineer Albert Highberger for years. Mr. Highberger had been honing plans and refining analyses for a novel bridge design, a tensioned “suspender” bridge reliant on an array of tensioned mainline cables, since at least 2009. It was a bridge design searching for a location; at the Angkula Seo site, we found our location.

The Angkula Seo Suspender Bridge employs six 1″ A586 mainline cables, three on each side, with those cables tensioned beyond 18,000 lbs such that the vertical deflection under dead load is 1’ 10”. The resulting structure is a slender ribbon of deck and railing pulled taut between the two opposing banks; this tension is absorbed by an array of 150KSI grouted anchors. The structure bears similarities to an “Indiana Jones” bridge, but without the deep sag and side-to-side deflection; it is a pleasantly dynamic structure.

This intensely collaborative effort began in February 2014, bringing together Mr. Highberger’s vision and structural analysis, the know-how of Seattle Bridge and Keith Monohan, foundation design by Holdrege & Kull in Nevada City, bridge design and engineering support from Sturgis Engineering in Loomis, California, and the specialized skills of Neil’s Controlled Blasting for installation of sets of anchors on each shore. In addition, we developed a great working relationship with a group of trailbuilders and artists who were tying the Tribute Trail into this ‘missing link,’ the Forest Trails Alliance.

Keith Monohan led foundation work on the site–including, excavation, placing and testing anchors, and concrete forming and pouring–through the Spring. The principal issue with the Angkula Seo project emerged during this period: it was the shared expectation of geotechs, engineers, and bridgebuilders that this bridge was located on a thin layer of soil atop competent bedrock perfect for rock anchors; this was, unfortunately, not the case. Adapting on the fly, Neil’s Controlled Blasting, H&K, and Seattle Bridge switched to grouted earth anchors of greater depth. Setting and testing of these anchors was complete in August.

Meanwhile, in parallel, Ken Gilman led fabrication of the bridge structure in Seattle. Fabrication began in the spring, encompassing eight tons of A588 weathering steel, more than a thousand A325-type 3 fasteners, and hundreds of other hardware and decking elements. Houston Structures supplied mainline cables cut and socketed to spec. This effort culminated in shipment of the bridge kit in early August.

The Seattle Bridge field crew, led by Martin Walz III, arrived in late August 2014 to begin bridge installation. Working in tandem with friend, collaborator, and temporary landlord Mr. Monohan–who had already set up rigging in the trees for casting material and supplies across the Creek–the field crew cast the 1,600 lb ‘tall’ towers to the far shore and set them in place within two days. With that, we were off and running. End posts were secured by August 26, superstructure was in place by August 30, and decking and fencing cables were installed by September 6. Everything was complete with well over a month to spare before the grand opening of the bridge, on October 21, 2014.

We are grateful to everyone in Nevada City who helped to make this project so successful, in particular the Monohans. In addition, we cannot thank Mr. Highberger enough for his creativity, care, and diligence in continuing to expand the boundaries of cable footbridges.

View Angkula Seo Suspender Bridge Images.