Colorado river bridge relieves congestion

Built in the shadow of the Hoover Dam, a new bridge is set to takes its place as another major tourist attraction. Patrick Smith reports
Road Structures / February 7, 2012
Highline hoisting girder
The highline hoists and transports a steel girder out onto the arch for placement between Pier 10 and 11 spandrel column pier caps during construction

Built in the shadow of the Hoover Dam, a new bridge is set to takes its place as another major tourist attraction. Patrick Smith reports

On 30 September, 1935, US President Franklin Roosevelt dedicated the Hoover Dam, a construction effort that produced one of the nation’s iconic structures.

Involving thousands of workers, the dam in the Black Canyon of the Colorado River, on the border between the states of Arizona and Nevada, was built between 1931 and 1936 and cost over 100 lives.

It impounds Lake Mead, 40km south-east of Las Vegas, Nevada, and its generators provide power for utilities in Nevada, Arizona, and California, while it has also become a major tourist attraction.

The Hoover Dam has now been joined by another iconic landmark, following completion of the US$240 million Hoover Dam Bypass, which was thrown open to the public for six hours at a special Bridging America event on 16 October. Those who missed out on the oversubscribed event will now be able to access a pedestrian walkway on the bypass bridge during daytime hours following its opening to traffic.

The heavily trafficked, two-lane US 93 (14,000 vehicles and trucks crossing per day) that runs across the crest of the dam has been diverted to the new bypass and its new Mike O’Callaghan-Pat Tillman Memorial Bridge, which crosses the Colorado River some 450m downstream of the dam. The dam roadway will be restricted to use by visitors.

The dramatic new 323m long concrete arch bridge, the fourth longest such span in the world, and the longest in the United States, was officially named by the United States Congress after two prominent local citizens who “dedicated themselves to public service and the greater good.” Mike O’Callaghan, former Governor of Nevada, community leader and businessman, died in March 2004 aged 74. Pat Tillman graduated with honours from Arizona State University (ASU) and played professional football for the Arizona Cardinals before joining the Army. He was killed in Afghanistan in 2004 at the age of 27.

David Goodyear, chief bridge engineer at 2404 T. Y. Lin International, and chief engineer responsible for design of the new bridge, was recently the keynote speaker at the 2400 New Zealand Concrete Industry Conference 2010.

First suggested in the 1960s, the need for a new Colorado River crossing became a matter of urgency as traffic congestion increased.

“The existing highway Route 93 over the dam mixed the throng of tourists for whom the dam is a scenic destination with heavy highway traffic and commercial trucking, crawling at 25km per hour. The blend of these two created hazard and hardship for both, and served as a bottleneck for commerce along the major north-south route of US93,” said David Goodyear in his paper.

Although the old highway is narrow, winding and steep, there is no other route in the western states that can efficiently accommodate this traffic.

There have been six distinct, yet overlapping, phases of bypass construction including the bridge, which at some $114 million is the most expensive element. The others included relocation of portions of the 2407 Western Area Power Administration (WAPA) transmission system and switchyard; Arizona and Nevada approaches (3.2km and 4.8km of bypass roadway respectively); interim surfacing of the bypass, and the final surfacing and roadway tie-ins.

In total the bridge crossing, for which a joint venture of 2401 Obayashi Corporation and 2402 PSM Construction USA is the prime contractor, is some 579m long and its deck and pathway is located approximately 274m above the Colorado River.

Following the initial 1960s discussions, plans for a highway crossing of the Colorado River were advanced by the 2406 US Bureau of Reclamation to address the increasing highway traffic across the top of Hoover Dam. A series of studies ensued, sponsored by several of the project stakeholders throughout the next two decades.

The 2408 Central Federal Lands Highway Division (CFLHD) of the 2410 Federal Highway Administration serves as the project manager and leader of the multi-agency and consultant teams. The multi-agency team includes representatives from the Arizona and Nevada Departments of Transportation, 2412 National Park Service, 2413 Bureau of Reclamation, and the Western Area Power Administration. The CFLHD is responsible for the management of all the design, consultant, contracting, and construction activities.

“The project then advanced through the Draft Environmental Impact Statement, Final Environmental Impact Statement and Record of Decision leading to commissioning the project,” said David Goodyear in his paper.” A consortium of companies working under the name of HST (2418 HDR, 5750 Sverdrup, and TY Lin International) teamed with specialty sub-consultants to deliver the final design for approach roadways in Arizona and Nevada, and the Colorado River crossing.

A bridge design group of TY Lin International and HDR was directed by the Olympia, Washington office of TY Lin International for development of the bridge type study and final bridge design.

“The new bridge is a prominent feature within the Hoover Dam Historic District, sharing the view-shed with one of the most famous engineering landmarks in the US.

“The environmental document set a design goal to minimise the height of the new bridge crossing on the horizon, both from the dam and from a boater’s view on Lake Mead. The State Historic Preservation officers for both Nevada and Arizona (both members of the Design Advisory Panel) emphasised the need to complement and not compete with the architecture of the dam.” Following a thorough type screening process, the type study proceeded with only deck arch options for the crossing, whose construction was not without its problems. The final decision to proceed with the concrete composite alternative was made by the Executive Committee, comprised of the operations chiefs from the five Agencies on the Project Management Team (PMT).

“The final design went through an evolution of form dictated by the engineering demands on the structure to arrive at the twin rib framed structure.” In his paper, David Goodyear said the composite superstructure was selected for speed of erection and to lower weight on the arch. The spacing of spandrels was an extension of the erection concept to erect the bridge using a highline (tramway) crane system.

“The decision was made to target a 50ton (45.3tonnes) capacity for major superstructure elements. The span was set in the range that a highline crane could deliver the steel box sections, which resulted in a nominal 37m span. This same span also allows steel girders to be set within the range of most conventional cranes, should an alternative erection system be selected.” Integral concrete pier caps were selected over steel box cap sections.

A limited notice to proceed with construction was issued for November, 2004, with full field work starting in 2005.

The first challenge for the construction team was creating a foothold for foundation construction. Climbing on the side of the cliff 244m over the river below was difficult enough, but excavating (and doing so within the loss limits in the specification) was an incredible challenge.

“The subcontractor who met this challenge was 2420 Ladd Construction from Redding, California. It not only met the tight schedule for this work, but completed the excavation allowing about half of the rockfall into the river that was permitted,” said David Goodyear.

Initial bridge construction began on site with footing and abutment work, and in the precast yard outside [nearby] Boulder City where the contractor set up its own precasting facility, and self-performed the precasting. Column sections were trucked to the site as needed for erection, and set into place using both the highline crane and conventional cranes.

However, in September, 2006, the highline crane collapsed in a strong wind, and the contractor had to mobilise additional land cranes and a derrick to complete approach columns and set approach steel box girders until a new highline could be design, fabricated and erected.

Arch erection was being started at the time of the collapse, and the temporary cranes were able to service the first few arch segments from the springing, allowing the piece work for the starters and the initial set-up of the form travellers to proceed.

“Four form traveller headings were operated in concert for erection of the arch. After erection of the new highline and restarting the main arch erection, the contractor reached a reliable cycle of two weeks, and often bested that cycle on segments that did not have a stay.

“The arch was closed in August of 2009 within an impressive 19mm tolerance at closure.

“Demobilisation of the highline and related temporary works was a project in itself, and had to be completed before final paving and finishing could be completed for the roadway.”RSS

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