The scaffolding at 15th and K streets NW is, in a word, big.

It's really big. It's 750 tons of metal standing 11 stories tall, extending for 148 feet along 15th Street and 193 feet along K Street.

It's so big that for most of the summer, as it grew around the 75-year-old Investment Building, passersby have stopped to wonder what all that metal is for.

The answer is at once obvious and complicated. Simply: It's holding the building up. Less obvious: It's how the architects and engineers are balancing historic preservation with the needs of modern office workers.

The Kaempfer Co., a D.C. development firm, last week formally began the demolition of the Investment Building. The site's redevelopment has been planned for more than a decade, but the project was repeatedly stymied by the ups and downs of business cycles.

It was resurrected last year when Kaempfer found a new money partner, the Blackstone Group, and a big anchor tenant, the law firm of Sidley & Austin. With the law firm's agreement to lease half of the planned 370,000-square-foot building, it was financially feasible to start this summer.

In 1924, the Investment Building was state of the art, one of the largest private buildings ever to be constructed in the District and one of the first on the East Coast to provide parking inside.

But the building is old now, its systems outmoded and its layout inefficient. To update it for lawyers and others willing to pay top dollar for prime office space, Kaempfer and Blackstone are knocking the whole thing down, except for the two Beaux Arts facades along 15th and K.

Saving the facades "was always a given," said Joey Kaempfer, chairman of the development company. "I don't think we would have been allowed to do it without that, just like the Warner."

Kaempfer's redevelopment of the Warner Theatre downtown, which restored the old theater while adding a big office building over and beside it, was one of the firm's major successes of the 1980s. At the Investment Building, the company worked closely with local preservationists to come up with a plan that would avoid a possibly costly conflict.

Company executives won't say how much saving the facades adds to the cost of the $120 million project. Said Kaempfer, "Frankly, though it's expensive, I don't think there is any way you could duplicate that today."

World-famous architect Cesar Pelli has designed a new building that will sit behind the Investment Building's facades. It will be 12 stories high, with a big atrium. Above-ground, it will have about 100,000 more square feet of office space than the old building, with its pre-air-conditioning, U-shaped layout, enclosed. Below ground, an additional four levels of parking will be excavated.

Although the facades will hide his work from passersby, Pelli praises the old building's elaborate cut-stone decorations. "What was relatively easy to do in the 1920s would be the utmost luxury today," he said.

Inside will be "an office building that will be as up to date as any office building in the world," said Pelli. "I believe this is the best way for cities to renew themselves."

The old facades must be kept intact during the year-plus it will take before the new building can support them. That's where the scaffolding comes in. In most cases, the technique is used to preserve facades of a few three- or four-story town house fronts while an office tower goes up behind them.

At the Investment Building, though, each facade is 11 stories high, a half-block long and just a foot-and-a-half thick.

"Envision a 100-foot wall standing by itself," said Paul Sowter, Kaempfer's senior vice president of construction. If a wind comes along, it won't be standing very long.

"People don't think of wind as a huge force," Sowter said, before last week's hurricane blew through the area to remind them. "By having a very high structure, and very large, then having the wind hit it, it creates a very large force" -- one that can easily knock down such a surface.

Washington architect Shalom Baranes, who is handling the preservation work at the Investment Building and has made a local reputation with similar projects, said that while other projects here have saved entire facades, he could not think of any project where the walls were as tall as these.

Because of their height, just propping them up wouldn't work -- diagonal supports would have to extend dozens of yards, which isn't practical on busy K Street. Any support system was restricted to closing no more than one lane of traffic.

The solution as designed by engineers Tadjer Cohen Edelson Associates involves a steel truss system resting on concrete caissons, Sowter explained.

The caissons are 30 inches in diameter, made of steel-reinforced concrete. They extend 60 to 80 feet into the ground. Four anchor bolts hold the truss to the caissons.

The truss is what most people would call scaffolding. It's made up of about 3,000 eight-inch-wide steel I-beam members that sit beside the old walls.

Unlike the more famous aluminum scaffolding on the Washington Monument a few blocks south, this truss is meant to support the building, not just provide access. So it's securely attached to each floor, with a total of about 600 6-by-6-inch by 1/4-inch-thick steel tubes that go through the windows and connect to support beams on either side to "sandwich the wall," as Sowter described it.

Pelli said the scaffolding and the old walls will create a beautiful effect during construction. "When they start taking the middle out and you can actually see through the window, it will be quite wonderful."