It looked good on paper: Use huge, golden-colored limestone rocks to make an overhang 130 feet above the entrance of the National Museum of the American Indian on the Mall. It would look like the overhang in a Southwest canyon. No ugly, concrete columns underneath: It would seem as though suspended in air.

Architects and engineers had designed it, but no one was exactly sure how well it would work. Making it work was up to Bethesda's Clark Construction Group LLC, the area's largest contractor with $2.7 billion in revenue last year.

Therein lies a tale of how one of the Mall's most unusual and ambitious buildings overcame several big structural obstacles behind the scenes to make its grand opening Sept. 21.

To make the limestone overhang work, for instance, Clark had to dig a hole 30 feet below street level; drive a tall steel beam into it; and reinforce the base of the beam with dirt and concrete. Then it had to make an arm of heavy-duty steel, the kind typically used for constructing bridges, that would extend 100 feet from the beam. The arm, made of V-shaped steel trusses and weighing 750 tons, would have large pieces of limestone hung on it to create the look of the canyon wall.

But Clark found the overhanging beam was too heavy. A dozen engineers, architects and designers spent four months going once a week to a Norfolk steel-maker and puzzling how to make the contraption hold the weight.

"It was like making a diving board that only King Kong could flex," said V. George Conard, a Clark vice president.

The overhang was one of the many challenges Clark said it faced in the almost four years it has worked on the 450,000-square-foot museum, which will house one of the most extensive collections of Native American artifacts in the world.

Building the roughly $200 million museum required 500 engineers, architects, carpenters, electricians, stone masons and other laborers. Some of the challenges: laying huge steel beams and slabs of concrete around underground supports and electric and water lines; carefully installing a huge steel dome in the ceiling; and building a structure that has few straight lines, requiring making walls of concrete curve. Speaking of concrete, it has enough to fill 250 swimming pools; enough limestone and granite to cover five football fields; and the steel in its basement -- if laid side-by-side -- would extend eight miles.

"This was like building a space shuttle versus building a go-cart," said William I. Magruder, senior vice president at Clark.

The 98-year-old company had built convention centers in Los Angeles, Chicago and the District; airport terminals in Baltimore and Orlando; sporting venues including Oriole Park at Camden Yards in Baltimore, MCI Center in downtown Washington and FedEx Field in Landover; plus hundreds of office buildings, including Discovery Communications Inc.'s new headquarters in Silver Spring.

But this was something new. And as soon as it got on the site after winning the contract in June 2001, it was hit with a problem.

The hole where the basement would eventually contain the loading docks, heating and air conditioning, storage units and mechanical systems wasn't ready for Clark to start pouring concrete. It had been dug a few years earlier to jump-start construction, but now it sat empty because there was no more funding.

Exposed to the wind and rain, dirt had seeped through gaps in the wood boards holding back the sides. And because Tiber Creek had once run through the Mall when parts of it were a swamp, the dirt had become soggy and like "black muck," engineers said. It wasn't strong enough to hold the concrete floor they needed. "Our equipment was sliding around," Conard said. Magruder, the senior vice president, said: "People were cursing. It was unsuitable for bearing any weight."

To fix it, workers dumped truckloads of a mix of sand, gravel and dirt. But the problems of the hole still weren't finished.

There were the "rakers," steel braces that a previous contractor had put in to help hold back the dirt wall. But they had been placed exactly where steel and concrete footings to build the museum also needed to go. The whole building would have to be moved a few feet from where the architects had planned to put it. "We couldn't move the building, so we had to work around the rakers," Conard said.

The problems cost time and money. Just the payroll while concrete was poured and laid was $10,000 an hour. But Clark, its subcontractors and the Smithsonian had agreed upfront to work together.

"Usually if you're unable to figure it out, you put your hands in your pocket and stop work and wait for the owner to solve it," Magruder said. "Here we were all working for the same goal: to finish it on time and within the budget."

Then the Smithsonian decided it wanted to cut costs by connecting the water lines for the heating and air conditioning system to a central plant for buildings on the Mall instead of putting in an expensive system on its roof as a typical office building would. Clark built the underground lines, but it caused the builder to juggle working on several parts at once.

"You usually like to get a flow to a project," Magruder said. "Where once the drywall guy is done for example, you get the painter in right behind him and then the rug guy."

Conard added: "Here we had to work in different areas like it was a checkerboard."

By the end of 2002, Clark had laid the concrete foundation and five floors of walls and floors for the museum and started to work on other parts. While workers were waiting to solve the problem of how to hang those steel trusses in the overhang, Clark switched to installing the curved dome over the atrium. "If we had sat waiting to figure out all the geometry on the trusses and not done anything else, we'd still be hanging steel," Conard said.

The dome is a huge ring of steel supported by 12 arcs. To attach them a worker had to straddle each arc 150 feet in the air while three large cranes helped keep it in place.

While the dome was going in, Clark's workers did scores of geometry problems to make sure the curved walls would meet. The entire museum was built for artistic reasons based on what the architects called the "Potomac Point" in the exact center of the museum. But building everything based on that point was tricky. It left Clark with no room for error.

"In a typical office building, it's built like a box," said James C. MacMichael, one of the lead project managers. "You only have to deal with straight lines, and keeping it lined up is easy. But here, you've got to deal with the radius of a circle from that point [because the walls are curved.] We had to have 21 engineers just in the field to check the geometry to make sure it was lining up."

Even a fifth-floor boardroom is built in a circle -- not a common feature in most office buildings. And there were almost some other unusual touches. The designers wanted a brushed-bronze-colored case for the fire extinguishers. They didn't get it. "That we couldn't do because it wouldn't meet fire code," MacMichael said.

MacMichael's next job, he said, won't be nearly as exciting: an office building in Rosslyn.

Dana Hedgpeth writes about commercial real estate and economic development. She can be reached at hedgpethd@washpost.com.

Executing the National Museum of the American Indian's unique design tested a team led by Clark Construction. Employee Fred Graham, left, installs an exterior fixture. Making the dramatic limestone overhang work above the National Museum of the American Indian was one of many challenges for Clark Construction Group.