Double Arches

Double Arches, a photo by kevin33040 on Flickr.

Via Flickr:
"The last segment of the Natchez Trace Parkway's Double Arch Bridge [was] put into place on October 6, 1993. The $11 million, 1,572-foot–long bridge carries the parkway over Route 96 near Franklin, Tennessee. It was the first precast segmental concrete arch bridge to be built in the United States. (These bridges are more cost-efficient than traditional ones, because workers at the bridge site simply need to assemble concrete pieces that have already been cast.)


The Natchez Trace Parkway is not just famous for its sleek, modern bridge; on the contrary, the road has been around and in use for thousands of years. It was a well-worn trail through Choctaw and Chickasaw territory (along with a handful of prehistoric Native American mounds) that connected the southern Mississippi River with the salt licks of central Tennessee. Starting at the end of the 18th century, the Trace became the primary route north for the famous "Kaintucks," farmers and boatmen who floated produce, coal and livestock from the Ohio Valley down the Mississippi to markets in Natchez and New Orleans. Once they got there, they sold their boats for lumber and trudged back home. (The 500-mile trip from Natchez to Nashville took about 35 days on foot; on a horse, it only took about 20 to 25 days.) In 1800, the Natchez Trace became a national mail road. It became obsolete, however, once steamboats made it possible for people and goods to travel up the Mississippi River as well as down.


Since 1938, the National Park Service has been paving the Natchez Trace a little bit at a time, turning it into a scenic modern highway. At the Franklin crossing, engineers had to figure out how to elevate the bridge over Route 96 and the densely wooded valley below while preserving the natural beauty of the site. Engineer Eugene Figg settled on an open, double-arched bridge that supports its deck without spandrel columns, preserving most of the view across the valley. The first arch is symmetrical and has a span of 582 feet and a rise of 145 feet. (That is, the bridge is 145 feet above the highway beneath it.) The second arch, thanks to the elevated topography at the bridge's southern end, is not symmetrical: It has a span of 462 feet and a rise of 102 feet. The arches are made of 122 hollow boxes, each precast nearby in Franklin. They were assembled in what engineers call "progressive cantilever," with steel cables holding them together until each one was finished. The foundation and vertical piers were put together more traditionally: They were cast on-site as they were built.


The bridge officially opened on March 22, 1994 and the Natchez Trace Parkway was finally complete."

The above was taken from www.history.com/this-day-in-history/natchez-trace-parkway... as it appeared on November 3, 2013. Edits are so noted.