It stands like a sentinel, watching in the wind over one of America's most treasured landmarks, the Hoover Dam.

When the Mike O'Callaghan-Pat Tillman Memorial Bridge opens to traffic later this week, it will become an instant tourist attraction and provide a quicker, safer way to drive from Phoenix to Las Vegas.

Bridge photos, videos and more

The Hoover Dam bypass bridge is a study in superlatives. The highest and longest arched concrete bridge in the Western Hemisphere. The second-highest bridge of any kind in the United States and 14th in the world. The world's tallest concrete columns of their kind.

But what sets the bridge apart most of all is the setting. It is perched 890 feet above the turquoise Colorado River, wedged between rock cliffs that form Black Canyon, with commanding views of Hoover Dam and Lake Mead.

The bridge is also a testament to human perseverance.

For decades it was anything but a safe bet that a bypass bridge would ever stand alongside Hoover Dam. Once construction began, extreme heat and wind threatened completion.

The people who built the bridge faced the same challenges as those who built Hoover Dam 1,600 feet upstream. That industrial wonder of the world served as daily inspiration to workers on the bridge.

The bridge was dedicated on Thursday, just two weeks after the 75th anniversary of the dam's opening ceremony. Like the dam, it opens amid times of economic despair. From the country's top transportation officials to the journeyman carpenter, those who worked on the bypass bridge know they have created something special, a symbol for a hurting nation of what can be.

In 1902, Edison Electric Co. first studied a hydroelectric dam across the Colorado River but abandoned the idea. By 1922, the U.S. Reclamation Service recommended a large dam upriver from the eventual site. That year, six river states signed a legal compact to build a dam and manage the water. But it wasn't until 1928 that President Calvin Coolidge signed the law authorizing the dam.

In 1999, Dave Zanetell had finished repairing all the roads into Yosemite National Park, which had been washed out in a strong winter storm.

He was looking for the next challenge in his career at the Federal Highway Administration when he heard about plans to divert traffic off the badly congested Hoover Dam onto a bypass bridge. He put in to be the project manager.

All of Zanetell's mentors advised him to stay away from bridging the Colorado River.

Many had tried to start the project before. The first study recommending a bypass was written in 1968, and over the years, 27 more studies followed. But none had yet led to a bridge.

If Zanetell started the project, he would be staking his reputation on a successful completion.

"That job will never get funded. It'll never get built," his mentors told him. "It's career suicide."

Zanetell ignored the warnings.

"That's why I wanted the job - I wanted to make that project a reality," Zanetell said. "All the great works started with 99 percent of people saying it won't get done."

The Federal Highway Administration approved the plan for a bypass bridge in March 2001 but didn't identify a source for full funding. Normally, the feds put up the balance of the full cost before work starts.

Full funding would materialize only after Arizona, Nevada and various federal agencies agreed to split the cost. The deal hinged on the states lending part of the money, knowing complete funding was still years off.

Lacking any guarantee when all the money would be found, work would start on early phases. Cash would be meted out as early work was done. Success depended on an intricate flow of work and cash. If something went wrong, the money could dry up.

Victor Mendez took over as director of the Arizona Department of Transportation in 2001 and faced the tricky task of drumming up support for the state's contribution.

"Clearly in my mind I felt we were going to build this project," said Mendez, who now runs the Federal Highway Administration.

Then came Sept. 11, 2001. The terrorist attacks quickly made it clear that Hoover Dam wasn't just a traffic bottleneck and safety hazard but also a target.

Within minutes, trucks were permanently detoured. The urgency to build the bridge accelerated.

The government released bid documents for the dam in early 1931. Plans entailed 100 pages of description and 76 diagrams, an unheard-of level of detail for the day. Companies had to put up a $2 million bond - equivalent to nearly $30 million today - for the right to bid.

By mid-2001, David Goodyear had been brought in as chief bridge engineer at San Francisco-based TY Lin International. His job was to design the span across Black Canyon.

"The bridge isn't much without the site," Goodyear said. "It's just such a fabulous setting."

Right away Goodyear knew the choices were limited. The stretch of canyon had only two appropriate crossing points, and either one would be so long that it would require a concrete arch or a steel suspension bridge.

Goodyear knew that a concrete arch would be cheaper and more stable in the canyon's high winds. Plus, it wouldn't upstage views of Hoover Dam.

But several questions had to be answered before Goodyear knew an arch would work.

One question was about earthquakes. At the time, standards required the bridge to withstand a quake so powerful that it strikes on average only once every 500 years.

But Goodyear knew standards would tighten. He wanted the bridge to withstand a 1,000-year quake. Early research suggested a 1,000-year quake in that area could be catastrophic, but nobody was quite sure until geologists laser-mapped the gorge and modeled the Earth movements.

The other question was about the stability of the canyon rock. An arch structure would exert powerful outward forces at the base. If the rock wall wasn't solid enough, it would crumble under the pressure.

In early 2002, geologic reports gave Goodyear good news on both counts. Even a giant quake wouldn't be as violent as feared, and the canyon walls were strong enough to support foundations.

The findings cinched the design. The Colorado River would be spanned by a concrete arch, with the roadway on top.

Goodyear settled on a twin-ribbed arch, two parallel arches joined by 10 horizontal struts to strengthen the bridge against a quake's side-to-side motion. Seven pairs of concrete columns would rise from the slopes of the canyon's rock walls, and a series of eight more pairs would be built atop the finished semicircular arch, once it spanned the river. The columns would be made of prefabricated concrete blocks stacked like enormous Legos. The segments of the arch would have to be poured in place in midair.

At first, architects considered giving the bridge an art deco look. Historic preservation officials from Arizona objected, telling Goodyear they didn't want to emulate Hoover Dam but complement it.

Goodyear's team set about engineering the details and figuring out how to erect the arch. The cliffs were perfect to support the arches, but they would be a nightmare to work on.

Workers in 1933 finished blasting the cliffs and bedrock to make way for Hoover Dam's foundations. Workers called "high-scalers" were suspended by ropes as they jackhammered and dynamited the rock. Early on, falling rock and objects killed more workers than anything else, so they began dipping their cloth hats in tar and then leaving them in the sun to dry. The headgear became known as "hard-boiled hats" and worked so well the construction firms ordered thousands.

Work on the $114 million bridge started on Valentine's Day 2005.

Just to reach the job site, workers had hacked their way for almost two years through solid rock with dynamite and heavy machinery to build two short stretches of highway that would ultimately connect U.S. 93 to the bridge.

In the canyon, bridge work began tediously. Midway up the cliffs, workers carved out ledges to anchor the foundations.

A crane lowered workers, who chiseled away with handheld jackhammers. After a tiny notch had been created, bigger drills were brought in.

Eventually, workers set charges. Even that work was surgical. Rock couldn't be allowed to fall because buildings housing overflow pipes for the dam were below. Instead, all of the rock was carried out by clamshell buckets.

The first concrete of Hoover Dam was poured on June 6, 1933. If the dam had been poured all at once, it would have taken 125 years to cool and cure, and the concrete would have cracked. Workers built the dam as a series of concrete boxes, dumping the mix from huge buckets. To keep the mix cool, workers ran 582 miles of cooling pipes through the forms. At first they carried river water and later ice water from a nearby plant. Sixteen people at the site died of heat exposure in just one month that summer of 1931, in which the average high was 120 degrees. The last concrete in the dam was poured on May 29, 1935.

As cliff preparations were being finished, crews near Las Vegas were casting the first of 440 concrete blocks that would form the columns that hold up the roadway. The first columns allowed crews to build short stretches of roadway jutting partly into the canyon.

Those columns brought the roads at each side of the river nearly 900 feet closer. Then work on the 1,060-foot arch could begin.

To build the arch foundations, concrete was pumped through hoses from above. Work took place at night because of triple-digit heat, made worse as it radiated off the rust-colored rock.

Even at night, high temperatures threatened to dry out concrete too fast, which can cause fatal cracks. Workers pumped liquid nitrogen through tubes to cool the slurry.

The arch could rise from the rock.

A cable was strung from one side of the canyon to the other. It carried workers and equipment out one-quarter mile above the river, then lowered them to the emerging arch.

To support the arch temporarily, workers erected a huge steel tower on each side of the canyon. They anchored thick steel cables in the ground and looped them over the towers. These cables held up the partially constructed arch for almost three years.

As the arch's first segments formed, catastrophe struck.

In September 2006, wind gusts topping 60 mph roared through the canyon. The towers and rigging crashed to the ground.

Zanetell, the project manager, had just returned to his Colorado home when he heard the news. He returned immediately to Boulder City, Nev., just west of the bridge.

"That was a tough day," he said. "Often when something unforeseen of that magnitude happens, you immediately get finger-pointing, and your team fractures."

Within hours he had created three teams: one to clean up the mess and assure safety, another to assess damage, and a third to create a plan to keep the project moving.

Miraculously, nobody was hurt, and none of the arch work was damaged.

New rigging equipment was brought in from Scotland. Zanetell used the time to finish other parts of the job.

"I had to create an atmosphere that we're not giving up," he said.

The accident set work back two years.

The windstorm wasn't the only accident on the arch work.

In November 2008, a worker named Sherman Jones was loosening the cables using a hydraulic jack when something slipped. Part of the jack punctured his chest, killing him. He would be the project's only fatality.

Throughout construction of Hoover Dam, the remote site drew tourists from far and wide. Visitors came to be fixated by the daredevil work of the high-scalers, like a real-life circus act without the tent. One worker was dubbed "The Human Pendulum" because he swung co-workers and cases of dynamite along the cliff faces.

Over the months, the two fingers of the arch grew steadily together.

Carpenters built a wooden form in the shape of an arch segment. Supported by suspension cables, the form slowly traveled the distance of the arch. For each segment, concrete was poured into the form, then allowed to harden in place.

From turnouts on U.S. 93, Hoover Dam tourists stopped and gaped at the work high overhead. People worldwide flocked to a federal website to marvel at the construction photos and live webcam.

In August 2009, the gap in the arch was closed. The two segments that began 1,060 feet apart met just three-eighths of an inch off perfect. The most treacherous part of the job was done. The bridge could stand on its own weight for the first time.

"The bringing of the arch together was an incredible sense of accomplishment for the team and an incredible source of pride for the trade workers," Zanetell said.

While construction crews celebrated, Zanetell forced himself to stay away. It was the construction crews' night, and his mind was already on making sure nobody let down his or her guard for the final phases of work, he said.

It was a typical reaction from the man they called David Z, who held status meetings every Monday for six years. Goodyear, the designer, said it was the "most intensely managed project" he could remember.

Keeping everybody focused became one of Zanetell's biggest challenges.

On many big projects, work crews often compete for bragging rights and sometimes bonuses over which team can perform a task fastest. On the bridge, plaudits were earned by doing the job to perfection.

It mattered. Workers were erecting concrete columns atop the arch. A millimeter mistake at the bottom could result in an irreconcilable flaw at the top of a football-field-length column. Months could be lost.

With every new block they stacked, the entire structure settled a little. After each, engineers took precise measurements. Slowly, methodically, the bridge was finished.

President Franklin D. Roosevelt commemorated Boulder Dam on Sept. 30, 1935, with 10,000 people in attendance. The work was finished 18 months early. The dam had once borne the name of Herbert Hoover, who had been instrumental as U.S. Commerce secretary in getting the river-states agreement. But Hoover, no longer in office in 1935, was not invited, and his name was dropped. Congress restored his name to the dam in 1947.

The bridge and the new stretches of U.S. 93 leading to it will open to traffic this week. Combined, the projects came in within the $240 million budget.

Thursday commemorated the opening. Victor Mendez and his boss, U.S. Transportation Secretary Ray LaHood, were there, along with members of the Tillman and O'Callaghan families.

Congress had designated the name of the bridge in a 2004 transportation bill. It honors people from both of the states that the bridge connects.

Pat Tillman is the Arizona Cardinals linebacker who joined the U.S. Army after the Sept. 11, 2001, attacks and was killed by friendly fire during combat in Afghanistan. Mike O'Callaghan was a two-term Nevada governor and newspaper editor.

A public celebration was held Saturday, allowing visitors - some who came from as far away as Boston - the chance to walk on the monumental span for the first time. Police estimated a crowd in excess of the 20,000 predicted, as a steady stream of 60 double-decker buses continuously ferried visitors, who lingered to take photos and admire the view.

"The view is spectacular," said Mike Hoover, a 62-year-old from Phoenix whose father's name is Herbert Hoover. "The view is spectacular. People are really going to enjoy this."

Denver resident Mike Capps, 64, has relatives in nearby Kingman and visits them often.

"We've been watching this bridge being built for the last eight years," he said. "We made the trip just for this. Just to be a part of it."

Views from cars crossing the dam will be partly obscured by a barrier. But pedestrians are allowed to cross on the upstream side.

From there, they can look straight down through the railing at the rocks and river far below. They also get a new perspective on the dam. From the bridge, Hoover Dam seems more imposing, more impressive than from any other vantage point.

ADOT will finish widening the last 15-mile stretch of U.S. 93 before the bypass by year's end.

Studies from 2001 showed an average traveler will save 17 minutes when crossing from one side of the canyon to the other. On busy holiday weekends, the time savings could be more because the bridge eliminates tourist travel congestion and security checkpoints that have caused two-hour delays.

It remains unclear how significant the time savings will be on the entire Phoenix-to-Las Vegas route.

With the bridge opening, trucks will no longer be detoured to cross the river downstream at Bullhead City, as they have been since 9/11. About 2,000 trucks are expected to return to the more direct route on U.S. 93.

That is expected to add to the congestion through Boulder City, Nev., already a bottleneck 11 miles north of the new bridge. Nevada transportation officials still don't have the money or full political support for a bypass around Boulder City.

When work on Hoover Dam began, it was the depths of the Depression. About 20,000 desperate job seekers swarmed Las Vegas, then a town of 5,000 people. By the end of 1937, workers would finish the Golden Gate Bridge, the San Francisco-Oakland Bay Bridge and the first bore of New York's Lincoln Tunnel.

As the bridge was being built, Arizona and Nevada construction workers were hit harder than in any other state during the recession, the worst since the Depression. Since the boom-time peak in mid-2006, about 131,000 Arizona construction jobs, more than half, have evaporated. Nevada shed 86,000 jobs, or almost 60 percent.

So while the Mike O'Callaghan-Pat Tillman Memorial Bridge is an engineering feat, it symbolizes much more to its builders in these troubled economic times.

For Mendez, the project is a model and an advertisement to lawmakers reluctant to spend more money on public works.

"This bridge is a unique accomplishment for the nation," Mendez said. "This is the kind of smarts and attitude we need to bring more of these projects in and put more people back to work."

As the bridge rose, the 1,200 skilled workers and 300 engineers on the project found inspiration a quarter-mile upstream.

"If any one of us had a day when we felt tired or run-down or in doubt, working in the shadow of that dam really helped," Zanetell said. "Hoover Dam was the greatest engineering accomplishment in our nation's history. We had an opportunity to be as great for our generation."

Said Joseph Maietta, a carpenter who finished the arch: "We're building a national monument."