If 60,001 people show up …

May 24, 2007 

Is it possible that so many people could pile onto the new Tacoma Narrows bridge on opening day that they would make the bridge fall down?

Yes, it is theoretically possible, the state Department of Transportation says, but it’s not going to happen.

“We don’t think that many people are going to come,” said Claudia Cornish, a department spokeswoman, “but just as an added safety measure we’ll have some kind of counting technique going on that day.”

The state took the possibility seriously enough to ask Tim Moore, the top bridge expert on the Narrows project team, to run calculations to see when and if officials should worry.

The bridge is designed to comfortably carry a load of at least 9 million pounds, either traffic or people. Moore ran calculations for different weights of people and figured the bridge could hold 60,000 people weighing an average of 150 pounds.

That assumes the crowd is evenly spread out and not bunched up, which would increase the stress on the structure.

How crowded would the deck be with 60,000 people on it?

The deck is 5,400 feet long and 78 feet wide, for a total surface area of 421,200 square feet. People who study crowds say they typically range in density from about 2.5 square feet per person (in a subway rush hour, for example) to a more free-flowing 3 to 5 feet per person.

Using those densities, in subway conditions, 168,480 people (25 million pounds) could pack themselves onto the bridge. In a free-flowing crowd, 84,240 to 140,400 people (12.6 million pounds to 21 million pounds) could fit.

That’s well beyond the bridge’s loading capacity, but Cornish says things aren’t going to get to that point.

“We’re expecting 20,000 to 40,000 people to come and they’ll be arriving at various times throughout the day. So they won’t all be on the bridge at the same time,” she said. “If we need to remove people from the bridge, we will do that.”

Rob Carson, The News Tribune

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