Downtown may be rising, but first a part of it must collapse.

Early Saturday, as most Utahns enjoy their weekends sleeping late in quiet bedrooms, some 36,000 tons of concrete, steel and glass will come crashing down at the heart of the Downtown Rising revitalization project.

If all goes according to plan — and lightning is about the only thing that could stop it — the old Key Bank tower, 50 S. Main, will become the first Salt Lake City building to be imploded in nearly 25 years. It's coming down to make way for The Church of Jesus Christ of Latter-day Saints' City Creek Center development, which is on schedule for a mid-2011 opening.

And while much of the city is abuzz about the pending dramatics, certain to be a spectacle for anyone who manages to find a vantage point for the 6:45 a.m. show, construction officials want to drive this point home: It's an implosion, not an explosion.

"By all stretches of the imagination we're not blowing up the building," said Doug Loizeaux, vice president of Controlled Demolition Inc., which is managing the implosion.

In fact, it's not even a true implosion, which occurs when interior pressure is lower than external pressure and a building is crushed from the outside. Instead, the 20-story building is simply going to fall in on itself — with the help of some explosives.

It's not a whole lot of explosives considering the size of the building: only 180 pounds of it, mostly RDX inside extruded copper bars, which means each pound of explosive will take down about 200 tons of material.

"That's a lot of bang for your buck," Loizeaux said.

Crews have notched internal support columns on the building's fourth, seventh, 10th, 15th and 18th floors to weaken them. On Saturday, explosives will be placed at key points in the tower's basement, second floor and 10th floor, at nine spots on each floor.

When those explosives are detonated — in a southwest-to-northeast sequence — they will take out support beams, leaving the compromised columns with too much weight to bear, and the building will come tumbling down.

Loizeaux estimates about 90 percent of the building will end up in a 35- to 40-foot heap on the building's footprint, although crews hope to direct the collapse slightly to the southwest.

"The building wants to come down," Loizeaux said, noting that gravity will be the driving force behind the collapse. "It wants to come straight down."