New River Gorge Bridge

Completed in 1977, the New River Gorge Bridge in West Virginia is the third longest single-span arch bridge in the world, spanning more than 3,000 ft. The truss arch has a span of 1,700 ft and varies in depth from 53 ft to 34 ft at its center. The bridge deck itself is 876 ft above the river and is popular with BASE jumping enthusiasts. 

Modern Steel Construction covered the construction of the bridge in its Second Quarter 1975 issue:

"...this bridge is 3,030 ft in length and consists of two 34-ft roadways supported by 18-ft deep deck trusses. The deck trusses are divided into three continuous units. Two of the units serve as the approach structures and are supported by welded box column bents anchored into the mountainside. The box columns are tapered, with the tallest nearly 400 ft in height. The remaining deck unit is supported by similar column bents mounted on a truss arch.

The truss arch has a span of 1,700 ft and varies in depth from 53 ft near the skewbacks to 34 It at its center. The center of the arch is 800 ft above the bottom of the gorge. The truss chords are welded boxes up to 6 ft in depth, with web plates up to 4 in. in thickness. All of the material in the bridge is unpainted corrosion-resistant steel (ASTM A588). During the design, the engineer used the computer to analyze stress patterns and determine the final geometric shape.

All of the main field connections in the deck truss spans and most of the main arch field connections were N/C drilled. This resulted in the elimination or reduction of the amount of shop assembly with a sizable cost savings for the owner.

The field material handling system consists of twin 3,500-ft cableways with two main cables suspended over the gorge from 330-ft towers. Each cableway has a 50-ton capacity. The cables are continuous over sheaves that are mounted at the top of the towers and run down to anchorages located 700 ft behind the towers. Each tower can be luffed 36 ft to each side by adjustable luffing guys. The first line for the cableway was flown across the gorge by a helicopter.

The approach spans were the first items erected. Due to the large capacity of the cableway, it was possible to assemble the deck trusses in the yard and then lift an entire truss span utilizing the cableways. A bent and an entire span of trusses, floorbeams, and stringers could normally be erected in less than one week.

Once the approach spans were completed, the tieback system for the cantilever arch halves was built. The forward cable ties of this system are connected to jacking rods at the arch tieback points. The jacks are used both to attach and stress the wire ropes as well as to ensure an equal stress distribution among the various parts. These forward ties extend back and up from the arch to connect to a rocking device that is mounted at the front end of the approach span. The horizontal component of these ties is carried back a distance of 600 ft to a concrete anchorage by four lines of high-strength oil drilling casing.

As succeeding sections of arch truss are erected and secured with new forward tieback cables, the preceding cables are released. At the time of closure, with the arch halves cantilevered 850 ft, each half will be supported by tieback cables from two different panel points.

The arch is being erected higher than its final position so that the arch halves can be precisely lowered until bearing is obtained. This lowering is done with jacks located in a jacking frame embedded in the casing tieback anchorage. After closure, all tieback cables will be removed and erection of the center deck truss unit with its bents will start from the center and move toward the approach spans."

We also covered the bridge for Bridge Day in the October 2016 issue of Modern Steel Construction. 

Best wishes for the holidays from AISC and NSBA!