Opportunity
In 2007, leaders of the Boy Scouts of America began searching for a permanent home for the National Scout Jamboree, which brings together tens of thousands of Scouts and visitors every four years.
After visiting more than 80 sites in 28 states, the “Project Arrow” committee chose a site in West Virginia near the 70,000 acres of the New River Gorge National River Area. Nearby is the iconic New River Gorge Bridge — for many years the longest single-arch steel span bridge in the world.
In 2009, the Boy Scouts announced the acquisition of a 10,600-acre site for the creation of the Summit Bechtel Family National Scout Reserve. Flat bench areas left by long-abandoned surface coal mines simplified construction. However, a 100-foot-deep ravine that crossed the site made a pedestrian bridge a necessity.
The Boy Scouts wanted a bridge that would be exciting yet safe, would last a long time with minimum maintenance, would become an iconic structure for the facility, and would offer scenic views while making the least possible impact on the natural environment. And they wanted it in time for the National Jamboree scheduled for July 2013.
Solution
To choose a design, the Boy Scouts sponsored a competition in 2010. The competition was won by the team of Mott MacDonald, which managed the overall project and designed the bridge foundations, and the German firm Schlaich Bergermann und Partner. (See our videos Shaping the Summit and National Jamboree Construction Update.)
The ultimate design, featured in Civil Engineering magazine, was a back-anchored cable suspension bridge with a 15-foot-wide main deck and cable walkways. The bridge is 786 feet long and runs on an east-west axis.
Each end of the bridge passes through a V-shaped structure made of six steel masts, raked outward at the top. Between the masts and abutments, the cables are splayed to provide extra stability against lateral motion. The resemblance of the slanting masts to the wing tip feathers of a flying eagle inspired the name Wing Tip Bridge.
The base of each V rests on a concrete pier that is supported by micropiles, and the four steel cables of the bridge pass over the tops of the raked masts. Above the masts at each pier location is an observation platform. From the masts, the cables dip down below the main deck as they support the 520-foot main span of the bridge.
This “underslung” span maintains an efficient cable profile while creating a dynamic visual form. Where the cables cross from above the deck to below the deck, a special structural member is specified, comprising a strong box section to connect the cables on both edges of the deck. Through the underslung portion of the bridge, the vertical members must transmit load through compression. At the belly of the span, a third observation deck allows users to rest and enjoy the views out over the valley.
A unique feature of the bridge is that pedestrians can cross in one of two ways: on a traditional level deck or on secondary walkways that follow the cables and pass above and then below the deck, sometimes at an angle of up to 10 degrees.
Outcome
The CONSOL Energy Wing Tip Bridge was designed not only to be a worthy companion to the historic New River Gorge Bridge, but to complement its natural setting. To help ensure this, only enough trees were thinned from the ravine to allow enough clearance for the central span.
Safety is ensured in part by the foundations designed by Mott MacDonald. Cables at each end of the bridge are anchored to the ground using concrete abutments, each one about 80 feet wide, 16 feet long, and 15 feet deep. The abutments are fastened to the underlying sandstone using post-tensioned rock anchors up to 135 feet long.
The final planks of the Wing Tip Bridge were placed at an unveiling ceremony on June 4, 2013, six weeks before the opening of the Jamboree. The CEO of CONSOL Energy, which donated $15 million to build the bridge, called it an icon that symbolizes the Boy Scouts’ next 100 years.
To ensure that it lasts far into the future, fully locked cables were used to increase durability and minimize corrosion. The deck and handrails were made from locally sourced black locust wood, which requires no paint or stain and withstands harsh weather. The unique cable walkways allow all parts of the bridge to be inspected without expensive equipment.