Frame and Form

Linn Cove Viaduct is a bridge that was built over 25 years ago (and 1983), but it is a stark example of a structure in which design we find modern values such as technological innovation, landscape integration, and a proper respect for the environment.

This structure was designed by Jean Muller and Eugene Figg, and built by Jasper Construction Company. The viaduct, following an S-shaped alignment, skirts the slopes of the Grandfather mountain in North Carolina.

Overview of the Linn Cove Viaduct. Author: Hugh Morton en www.grandfather.com

The main problem was to able the road (Blue Ridge Parkway) through the Grandfather Mountain. The mountain side called”Black rock area”, has a steep slope, and therefore, the route of the road becomes very visible in this area. In addition to the steep, the fracturing of the rock in this part of the mountain made inadvisable a conventional mid-slope road.

Surveying hicks were made to locate the areas where foundations could be build , and from them an impossible road axis with variable radius, curvatures, and slopes was draft. The solution proposed by Muller and Figg consisted in a factory prefabricated beam divided into short segments (2 m approx.) the work was assembled with adhesive resins, and prestressed cables to form a helical beam that fits the needs of the natural layout.

Photo of Kenny Carter and www.pbase.com

To fit all segments a technique called 'paired keystones' was used. The technique consists in casting each segment (keystone) using as mold for its rear face , the front of the predecessor segment or keystone previously cast. Thus, when facing two consecutive pieces, their faces fit perfectly,and they perform a shape with a small geometrical error.

Photo of the Scott Morgan www.panoramio.com

The construction of prefabricated segment bridges it has been applied since 1958 (Russian bridge Kranoholonski), but in this case the technique was taken to its limits due to the geometric complexity of the bridge. All segments were different from another, what in 1983 was a significant challenge because the technology was much more limited, not only in office but on site.

Foto de Houck Medford de www.brpfoundation.org

Moreover, to preserve as much vegetation on the mountain,piles of the bridge and its foundations were built without any auxiliary access road. The materials and machinery were supplied through the road and the bridge itself. The deck was built in cantilever up to the next pile. Then,got off the machinery to build the foundation and the elevation of the pile. The machines could not work further than 8 m from the pile.

Photo of Laura Poss the www.panoramio.com

In this case the construction technique allowed to materialize a winding road that fits the contour of the mountain, and did not just alter the landscape and natural environment. Note that the concrete was stained with iron oxide pigment to mimic the color of the bridge with the rock of Grandfather Mountain.

Photo of the Scott Morgan www.panoramio.com

Unlike some modern bridges which seek beauty through the formal design of the structure, the key to the success of the Linn Cove Viaduct is the design of the layout of the road in search of a perfect match to the environment. The designers understood that the structure and its construction should be subordinated to the magnificent local geography.

In an economic environment like the present, looking back to the past may be instructive. A seemingly simple structure as the Linn Cove Viaduct, a heavy prestressed concrete beam,offers us many lessons that today can be applied, Why not?,in new structural designs.

Credits:

Length of bridge = 379 m
Platform width = 11.4 m
Maximum span = 55 m
Depth of the deck = 2.7 m
Minimum radius curve = 75 m

Meta-Information:

Linn Cove Viaduct en Figg Engineering Group.
Linn Cove en Structurae.
Linn Cove Viaduct and Wikipedia.
Brief history of the viaduct.

Probably the deck of the viaduct was constructed with temporary braces to reduce the effort when the spans were temporarily cantilevered. We could not find photos that might confirm this hypothesis. The only photos of the viaduct under construction are these. Photo 1, Photo 2.

A couple of videos on the experience of driving over the bridge. Video 1, Video 2.

For those interested in technical details: Construction of bridges by segments on wikipedia (English). Technical Articles (in Spanish) on concrete bridge construction by segments. Article 1 of ROP, Article 2 of ROP.