Dent Bridge

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The Dent Bridge is a suspension bridge located near Orofino, Idaho, United States. It is located 17 miles (27 km) up the North Fork of the former Clearwater River, now the Dworshak Reservior. Completed in 1971, the bridge has a main span of 1,550 feet (470 m).

[edit] History

Built in conjunction with the Dworshak Dam by the Army Corps of Engineers during the early 1970s, this bridge provides local access over the flooded river, now Dworshak Reservoir. Without the bridge, vehicle access from Ahsahka to the small community of Elk River would be blocked off. The US Army Corps of Engineers carefully weighed the possibilities for getting traffic to Elk River from the Orofino side of the Clearwater River. Building a new road all the way around the 53-mile (85 km) long reservoir would not only be expensive, cutting into miles and miles of extremely rocky terrain, but would also be a painful inconvenience for travelers. The only reasonable answer was a bridge somewhere in the middle of the future reservoir. The exact location was based on the topography most suitable for road access as well as for the bridge construction itself.

The 1,550-foot (470 m) long suspension bridge is an attractive wonder, both in size and in appearance. The Dent Bridge was named for Charles and Katherine Dent, who homesteaded the land on the west end of the bridge in 1895.

[edit] Trivia

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The only noteworthy accident on the entire project involved a back hoe, which had not been properly parked by the operator and consequently rolled into the river down the edge of the canyon. No fatal accidents ever occurred.

In 1972 the Dent Bridge earned the "American Institute of Steel Construction Award of Merit 1972 Long Span," having been judged as one of the 18 most beautiful structures in America.

In December of 1969, a $7,848,950 contract between the Corps of Engineers and the Hensel Phelps Construction Company of Greeley, Colorado was signed. The new structure would stand 17 miles (27 km) above the Dworshak Dam at an elevation of 1,640 feet (500 m). The design called for an overall length of 1,550 feet (470 m), a clearance of 40 feet (12 m) over the highest water level possible after the water blockage at the dam, and a height of 610 feet (190 m) above the floor of the North Fork of the Clearwater River. The Dent Bridge design called for an enormous amount of steel and concrete. The April 30, 1970 issue of the Clearwater Tribune noted that about 1000 tons of reinforcing steel was used for the footing for the pier on the Dent side of the bridge. By July 1, 1971, 20,000 cubic yards (15,000 m³) of concrete and 4150 tons of steel and cable had been hauled in to the site. In April of 1970, 60 men were active in the Dent Bridge construction project. It was the largest bridge built by the Hensel Phelps Construction Company, and at the time the largest in the Pacific Northwest. Supervisors to the building project included Ivan Anderson, Civil Engineering inspector, Don Wilder and Dave Pedersen, assistant superintendents for the Hensel Phelps Construction Company, Captain Pat Hunt, bridge inspector/Civil Engineer, and Jim Morgan, project manager.

The Corps decided that the width and height requirements called for a suspension bridge. Engineers and geologists explored either side of the river, in search of the slope and rock type suitable for the bridge cradle.

The Dent Bridge project would prove to be a tremendous challenge to the engineers and contractors involved, due to its remote site, its isolation from established towns, load limit requirements (i.e., 110 ton logging truck traffic), and low-standard access roads for transporting building supplies to the site.

While working on the structure ironworkers would be operating 130 feet (40 m) above the bed of the structure to get proper tension in the cables. The main span (between the two 140-foot (43 m) tall towers) is 1,050 feet (320 m). One major task in supporting this span was binding together 19 cables three-inches (76 mm) in diameter into one 12-inch (300 mm) diameter supporting strand. All the cable was built on either side of the bridge roadway. At the tops of the towers special caps were installed to transmit the tension from the cables directly to the towers. The builders were generally working 600 feet (180 m) above the river.

Several aspects of the deck construction of the bridge are unusual and interesting. All the deck forms were installed as permanent components of the structure. The deck was further stiffened by plate girders. The cracks between the concrete sections on the deck surface were so wide that engineers worried about motorcycles and bicycles falling into the cracks, so steel plates were installed to cover the cracks. The initial deck surface was made of lightweight concrete, but later the county poured a blacktop coating over this surface as a protective coat to increase the lifespan of the surface.