VALLEY CITY, NORTH Dakota, takes its bridges seriously. And well it might, for the municipality of 7,000, covering a bit more than three square miles, has no fewer than 14 of them, dating back as far as 1901. Crisscrossing the Sheyenne River as it wiggles through town, they amount to a working museum of bridge technology. And for nearly eight decades, one of the most cherished was the Rainbow Bridge, which served as a gateway at the city’s eastern entrance.

The Rainbow Bridge opened in 1926 and served the city well for decades, holding up under sometimes heavy traffic on the main road through town. As time went by, however, vehicles got bigger, heavier, and more numerous, and the bridge became increasingly inadequate. By the late 1990s, under the pounding of nearly 5,000 vehicles per day, including almost 200 trucks, it was in need of rehabilitation, and some in the community thought it should be replaced. The bridge was quite narrow, with two 12-foot lanes and no shoulders. It also stood a mere 5 feet above the river’s normal level, raising concerns about flooding.

Many residents and preservationists wanted to keep the beloved bridge, which had been placed on the National Register of Historic Places in 1997, and renovate it. But emergency personnel and city authorities insisted that modern life had bypassed the bridge’s design limits. After much discussion, numerous hearings, and the slow process of consensus building that attends many civil engineering projects these days, Valley City and the state department of transportation made a decision: A new bridge would be built, higher and wider than the old one —but it would use the same design, even at the cost of an fxtra million dollars.

This decision made life interesting for Kadrmas, Lee & Jackson (KL&J), the firm that won the design contract, because the Rainbow Bridge was a Marsh arch, a type that had not been built in more than 70 years. The design consists of a steel arch embedded in reinforced concrete, which both strengthens and protects the steel. Its inventor, James B. Marsh, wrote in his 1911 patent that his bridge would “expand and contract under varying conditions of temperature and moisture”—an important consideration in the Midwest, where most examples of the design were built. Eric DeLony, the former head of the Historic American Engineering Record (HAER), sums up the advantages of Marsh’s design: “Strong, durable, aesthetically pleasing, and maintenance free.”

Aesthetics may have been the Marsh arch’s biggest selling point. After the Civil War, with the introduction of the Bessemer process, steel became the material of choice for many bridges, but the spidery frames that resulted looked unappealing to some critics. In 1911 Engineering Record wrote: “There is one feature of city bridge building that still remains in the dark ages, and engineers ought to give it more attention. The usual criticism of our public works is that they are needlessly utilitarian and consequently ugly. Now it must be admitted that an ordinary highway truss bridge is not so charming as a well proportioned masonry arch.”

You could build a steel truss and put an ornamental concrete or masonry shell around it, but that often ended up looking tacky. Marsh’s arch, however, used concrete as an integral structural part of the design. The exterior could be molded or embellished as much as the client wished—for example, by being incised with decorative patterns. Scores of Marsh arch bridges were built in the 1910s and 1920s, especially in places where appearance was important. But when the Depression arrived, the cost advantage of a plain steel bridge became too great to ignore, and changing sensibilities made bridge designers realize that simplicity could be elegant in itself.

This meant that when it came time to design the new bridge, no one could tell KL&J’s engineers how it was done. So, like programmers studying ancient computer languages to prepare for Y2K, they had to immerse themselves in an outdated technology. “We were fortunate to have a set of the original plans for the existing bridge to use as our primary resource,” says KL&J’s Wade Frank, P.E., the project manager. “We also reviewed the original patent, and we spent a considerable amount of time reviewing HAER documents for other Marsh bridges that have been removed.” They were pleased to find that “some of the documents provided pictures and narratives of how the Marsh arches were constructed. That’s important, because how the bridge will be constructed controls the design.”

Removal of the old bridge and construction of the new one began in April. If all goes according to plan, it will open in November 2005. Some advances that make construction easier nowadays include larger, more sophisticated cranes; prefabricated forms for casting the concrete, instead of wooden ones built from scratch; and modern surveying technology. On the other hand, says Frank, “this design is a very laborintensive bridge to construct, and labor is a much higher percentage of overall project cost today than it was in Marsh’s time.”

So don’t expect the Marsh arch to make a comeback anywhere else. Besides the increased labor, the extra material needed for a steeland-concrete bridge also boosts the cost. In addition, Frank points out, “Bridges like this, with the main supporting members located above the roadway, are very difficult and sometimes impossible to widen in the future if it becomes necessary.” In fact, many Marsh arches have been demolished in recent years; according to the National Trust for Historic Preservation, only 14 remain in the entire country. In Valley City, though, where bridge lovers, local, state, and federal funding agencies, and users of the span got together and worked out a compromise, a technology that flourished early in the twentieth century is proving to be exactly the right choice for the twenty-first.