Builders of Seattle's Kingdome said it was strong enough to last a millennium. Just a day shy of its 24th birthday that prediction came crashing to the ground. In a scant 16.8 seconds the world's largest thin-shell concrete dome dissolved into a cloud of dust and debris. Although explosives got credit for erasing the obsolete sports arena, it was the relentless tug of gravity that actually did most of the work, explains Mark Loizeaux. The president of Controlled Demolition Inc. (CDI) of Phoenix, Md., he is a leading authority on reducing massive structures, and the brains behind the big bang in Seattle.
The challenge in demolishing the Kingdome wasn't size, but geometry. A quarter of a century ago, structural engineer Jack Christiansen designed the Kingdome to take full advantage of the inherent stability and strength of the arch. The Romans used it to build their aqueducts, parts of which are still standing. Running arches around a circle produces a dome, which is stronger still. It's simple, strong technology, and a big headache when it's time to take things apart.
You knock down a building like you tackle a person: You go for the legs. In buildings, the legs are the columns that hold the floors and roof in place. This simple formula couldn't be applied to the Kingdome because it had no legs to kick out. The roof was a clear arching span suspended atop exterior columns. A massive steel tensioning ring, 2262 ft. long, 2 ft. thick and 24 ft. wide, encircled the base of the dome roof at the top of the 40 exterior columns. It held the arches in place against each other, under nearly 9 million pounds. Cutting the ring—easy work for explosives—would obviously drop the roof in one fell swoop. It was also out of the question. The resulting vibrations from the force of the concrete hitting the ground could shake nearby buildings to the point of inflicting serious damage. A more elegant solution was needed.
The Seattle Two-Step
Working with Rich Riggs and his concrete demolition experts from Aman Environmental Construction Inc., of Oakland, Calif., the Kingdome doom-makers decided to bring down the roof in two stages. Using concrete fragments from the demolished concrete ramps and bleacher stands, they created vibration-dampening berms that would act as an impact cushion. "We didn't know how much it would reduce the vibration because there was really nothing to compare this to," says Riggs. In the end they decided 12,000 cubic yards of concrete should cushion the fall of the 70 million-cu.-ft. dome.
By mid-January the overall plan of attack was in place. Crews began punching in for two 10-hour shifts, seven days a week. Working around, above and below each other, they tore out 10,000 seats, 5000 bleachers, miles of roof insulation and wall panels, and anything else that moved. Explosives experts followed, drilling nearly 5500 holes, half of them in the roof's 40 ribs.
One more critical decision remained. And it could be made only after a live explosives test. In late February, CDI did a small blast on a column to determine the best match between explosives and structure. Some 4450 pounds of explosives were carefully portioned among the 5500 holes, which were then two-way wired to blasting caps for redundancy.
Time To Pray
Success would be a matter of timing. "We wanted to force one portion of the structure to move while still holding the other," says Loizeaux. To do this, the Kingdome would be almost instantly cut into six pie-shaped sections. Choreographing this catastrophic ballet required the creation of imperceptible pauses within what would appear to be one explosion. The $9 million demolition job would require 400 primary detonation delays and 600 secondary delays.
By 8:30 am March 26, the Kingdome was wired to blow. It is traditional to demolish buildings on Sunday mornings, as fewer people are bustling about. At CDI it is also a tradition to pause for a moment to pray. Then, at 8:32 am, licensed blaster Thom Doud pushed the button.
The first sequence of explosions took out three of the six roof pie slices, pulling the structure inward. The second phase of explosions followed, as key points on the roof ribs and the columns detonated. After 20 weeks of work, the rest was up to Mother Nature. As the roof fell, the tensioning ring that had for so long held up the arches was being used to pull down the exterior columns.
Before the blast, CDI predicted it would take 17.6 seconds to drop the dome, leaving rubble as high as 70 ft. standing afterward. The dome cooperated, falling slightly faster, in 16.8 seconds, and leaving nothing taller than 25 ft. Even better, 99 percent of the debris landed within the blast circle. There were only five windows damaged in nearby buildings. Loizeaux was exhilarated but not entirely surprised. "We don't blow things up. We let things fall."
Charges were wired to delays to force the dome to fall in stages ...
... This caused the steel tensioning ring to pull the columns in ...
... And almost all of the rubble fell inside the site.