The Hoover Dam, widely regarded as one of America?s most impressive engineering feats, now has a soaring companion piece, with the opening in November of the looming Mike O?Callaghan-Pat Tillman Memorial Bridge.
The new bridge, which connects the US States of Arizona and Nevada, is 579 metres long and is supported by the longest arch (323m high) in the Western Hemisphere. The arch holds up a roadway that leans on 91m long concrete pillars. It contains 7.25 million kilograms of steel, nearly 23,000m3 of concrete and 600,000m of cable.
The new bridge, the seventh highest in the world, cost $US240 million to construct over five years by 1200 labourers and 300 engineers.
The bridge was opened a fortnight after the Hoover Dam celebrated the 75th anniversary of its own grand opening. The dam, constructed from 1931 to 1935 by over 5000 men (112 lost their lives), was declared by then US President Franklin D Roosevelt as ?an engineering victory of the highest order?.
Much like the dam, the by-pass bridge had its dramas. It was in the pipeline for over 40 years, but was delayed time and again. Even when the US Federal Highway Administration (FHA) approved the plan for the by-pass bridge in March 2001, it was beset by Federal and State funding issues. There were also fears the bridge would not meet the standards to withstand the type of powerful earthquake that only occurs every 500 years. Ultimately the September 11 terrorist attacks hastened the bridge construction, with fears the Hoover Dam might be the next terrorist target.
The impressive twin-ribbed arch design was decided in 2002 and an engineering team appointed to determine how the arch could be erected. Work on the bridge started on Valentine?s Day in 2005. To reach the job site, workers spent two years blasting their way through rock to build two short stretches of freeway that would connect the US93 to the bridge. Seven pairs of pre-fabricated concrete columns (from 440 concrete blocks) had to be erected first from the slopes of the canyon?s rock walls to hold up the roadway. These columns enabled crews to build short stretches of roadway jutting partly into the canyon and brought the roads at each side of the river 275m closer. Work then began on the arch foundations.
A cable was strung across the Black Canyon to carry workers and equipment over to the emerging arch, 1000m above the Colorado River. To support the arch temporarily, they erected on each side of the canyon huge steel towers, which were anchored to the ground by thick steel cables. These cables held up the partially constructed arch for three years.
There were only two major setbacks on the project. In 2006, the towers and rigging crashed to the ground after 96km per hour winds swept through the canyon. No workers were harmed and the arch work was not damaged. In 2008, the project suffered its only casualty when a worker loosening cables with a hydraulic jack punctured his chest.
The gap in the arch was closed in August 2009 and the two segments that began 323m apart were out by just 0.95cm. ?The completion of the arch was an incredible sense of accomplishment for the team,? FHA?s by-pass bridge manager Dave Zanatelli said.
Just as the Hoover Dam was viewed in the Great Depression as an example of America?s ?can do? spirit, it is hoped the new bridge will similarly inspire a nation again in the throes of a deep recession.
Colleen Dwyer, of the Bureau of Reclamation that manages the dam, said the dam and bridge were ?two engineering wonders constructed at times when America was looking at itself and wondering what the future held.
?We?re looking at what can be done even at these worst of times to make these wonderful structures come to be, to create something new and different which enables America to keep moving ahead.?
Sources: The Age, Aggregate Research, LA Times, AZ Central (Arizona)