The Linn Cove Viaduct - The World's Most Complicated Bridge
"Ecology was a main concern," said Bob Hope, the landscape architect who represented the Park Service during the completion of the final section of the Blue Ridge Parkway from Linville to Blowing Rock. "Particularly in the Black Rock area of Grandfather Mountain. We'd always been concerned about building a Parkway that fit the terrain, but we realized that because of he elevation the road would take across Grandfather Mountain that it would be highly visible from very far off."
Studies and engineering schemes were done through the early 1970s in search of a plan for routing the Parkway through that area. Engineers from the Federal Highway Administration would present a scheme on paper, Hope and the engineers would look at it on the ground, and it would be rejected.
"The whole Black Rock area is one large mass of boulders," explained Hope. "It is not one solid piece of granite, but all cracked and loose boulders, so conventional road-building practices would not have worked in that area."
The proposal/rejection process went on for five or six years as the Federal Highway Administration fine-tuned their understanding of the geographic problems and possible solutions.
Connecting the dots
After all other possibilities had been exhausted, a group of Federal Highway Administration engineers and Blue Ridge Parkway representative spread out a map of the Black Rock Section of Grandfather Mountain on a table at the Banner Elk Holiday Inn. Weary from hiking the untamed rhododendron hell earlier in the day, the group finally agrees on the possible route through the fragile area.
"Rex Cocraft (Federal Highway Administration Engineer) sat down with a red pencil circling suitable pier sites and connected the red dots," said Hope with a laugh. "It just kinda fell into place."
But, in fact, connecting the dots was no laughing matter. The challenge of designing a road that could be successfully supported by piers set in the relationship proposed by those red dots would include every kind of alignment geometry ever used in highway construction.
At that point, the Federal Highway Administration presented the question to the engineering community: could a bridge be built along that configuration?
Jean Muller of Paris, France said YES. Using a computer program developed by his Paris firm, Europe Etudes Gecti, and refined by application on several Figg & Muller jobs, Muller designed the S-shaped configuration that would connect the dots on Rex Cocraft's map.
The design, prepared by Figg & Muller of Tallahassee, Florida, called for 153 fifty-ton segments to connect seven permanent piers set 150 feet apart. The complicated bridge was 1,243 feet long and 39.5 feet wide. The tallest pier is 65 feet from the top of the footing to the top of the pier. The specification book on the bridge was 1,500 pages thick.
Calculations of stress involved in the alignment of the piers, contortions of the curves, post tension, super elevation and the cantilever all came into play.
"Horizontal alignment included spiral curves going into circular curves with radii as small as 250 feet curving in two directions," said designer Jean Muller with an inflection in his voice that implied that his statement was as mind-boggling to those who understood it as it is to those of us who don't. "Only a small portion is on a horizontal tangent. No two of the 153 segments have the same dimensions, and only one segment in the entire bridge is straight."
Jasper Construction Company of Minneapolis, Minn. Was awarded the $7.9 million contract by the US Department of the Interior National Park Service, in September 1978 and work began in earnest in June 1979.
Built from the top down
The Linn Cove Viaduct was only the second bridge in history to be built from the end of a span, called a cantilever, which is anchored only at one end. In this case, the cantilever was the road itself. To protect the fragile terrain, all construction was done from the top down and no machinery was allowed more than 50 feet from the base of the piers.
Segments were trucked from a nearby storage area over the completed portion of the bridge to the end of the cantilever. There a stiff-leg crane lifted the segment, swung it out and lowered it to within six inches of the cantilever end. Epoxy was then applied to the joint face and the segment was moved to the cantilever end where the temporary thread bars were installed and stressed.
The entire operation, from the time the segment reached the end of the cantilever on the truck to attachment, took between an hour and an hour and a half.
Specifications required a temperature of at least 40 degrees for the epoxy to cure properly, so the contractor had to develop a special heating system to heat joints for the work to continue through the winter. Twenty segments were erected between November 1981 and March 1982.
The concrete used in making the Viaduct was tinted with an iron oxide pigment developed specifically for this project so that the color of the finished bridge would match the color of the one-billion-year-old boulders and cliffs that surround it, completing the Park Service's commitment to making the man-made structure a natural compliment to nature's magnificence.
The bridge was completed on schedule in November 1982 at a final cost of $9.9 million. $6.6 million of that covered the cost of the superstructure. The other $3.3 million went to the foundations, drilled microshaft piles that were reinforced and grouted. Unknown rock conditions caused the need to the additional microshaft pilings that accounted for the project being over budget.
Dedicated on September 11, 1987, the Blue Ridge Parkway Viaduct has won over a dozen national and international design awards.
