It took about three years for construction crews to erect Washington's old convention center at 11th Street and New York Avenue NW in the early 1980s.

It'll take about 20 seconds to bring it crumbling down.

At 7:30 a.m. Dec. 18, a Saturday, demolition experts plan to implode the 20-year-old building, reducing an 800,000-square-foot structure to roughly 50,000 cubic yards of concrete and 10,000 tons of steel.

Buildings are demolished all the time in Washington using conventional, nonexplosive methods such as cranes. But a longtime demolition expert in the region said the old convention center's destruction is the first implosion of a building in downtown Washington since a parking garage was razed in 1972.

About 500 concrete and steel columns will be rigged with explosives. Various charges will go off at different times to bring the steel roof down 50 feet to street level in a sequential manner.

"We're basically kicking the legs out of it, and it can't do anything but fall straight down," said Terry Anderson, 53, executive vice president of Wrecking Corp. of America, an Alexandria company. "The best description of this is, it's a very little bit of explosives and a lot of gravity."

Goel Construction Services of the District won the $6.4 million demolition contract, and Wrecking Corp. of America is a subcontractor.

Preparations began in September. The building's A, B and C halls have been gutted, and crews have removed drywall, roofing materials and architectural features to limit dust.

Convention center officials said they do not anticipate major disruptions from noise or dust. They said street closings -- including parts of Ninth and 11th streets and New York Avenue -- will cause some of the biggest inconveniences.

D.C. police will monitor a safety perimeter surrounding the convention center.

Many of the traffic restrictions will be in effect from about 5:30 to 9:30 a.m., but traffic will be allowed on Massachusetts Avenue and Seventh Street until 15 minutes before the implosion. Two hotels near the site -- the Grand Hyatt on H Street and the Renaissance Hotel on Ninth Street -- will have to relocate occupants of rooms facing the convention center during the implosion, officials said.

To prevent a premature detonation, the final wiring does not occur until the morning of the implosion. Detonation will last about 12 seconds, Anderson said, during which time a series of small and loud booms will sound as the blasting caps and main charges go off and the roof comes down.

Anyone expecting a thunderous, Las Vegas-style demolition with fireworks will be greatly disappointed, he said. "There's going to be no pyrotechnics involved in this project," he said. "This is more the workingman's demolition project."

A small portion of the building will remain standing, a 60-foot-high, two-block-long section on H Street that crews will demolish later by conventional means.

The two-level, dark-glass-and-concrete convention hall was shuttered in April after its bigger, sleeker offspring opened a couple of blocks away at Mount Vernon Square.

Officials said they expect the 10-acre site to be cleared by June, after which it will be turned into a parking lot. Tony Robinson, spokesman for the Washington Convention Center Authority, said the city also wants to use part of the site for outdoor weekend events and flea markets. "The expectation is that the lot will be in place for the next three years," Robinson said.

Permanent redevelopment of the site is in the planning stages.