Videos simulate how disasters could unfold on 520 bridge

Imagine winds so strong they would snap the Highway 520 bridge like a toothpick, sending pieces floating into Lake Washington.

Or an earthquake so strong, the western high-rise would collapse, pitching the cars into the water.

Computerized simulations of these two scenarios are now as close as a computer mouse click. It's been posted on the state Department of Transportation's Web site, at www.wsdot.wa.gov/projects/SR520Bridge.com, and on YouTube.com. Calling them "catastrophic failure videos," project manager John Milton said they were made so the public could see what could happen in a disaster.

In the windstorm video, where winds would be sustained at 75 mph, considered a 20-year event, the chains anchoring the pontoons break and pieces of the bridge float away. In this video, there are no cars on the bridge because it would have been closed because of the high winds.

In the earthquake video, the western high-rise pillars shake and that end of the bridge collapses, sending cars and buses to a watery grave. Milton said the earthquake video is based on a 155-year event; had the 2001 Nisqually quake lasted 15 seconds longer, there would have been major damage to the 520 bridge.

He said there are 30,000 linear feet of cracks along the 520 bridge, and the drawspan is the weakest portion.

He said the 1993 Inauguration Day storm almost sank the bridge.

"We want people to understand what could occur," Milton said. "When you show this in three dimensions, they are more able to understand."

Milton said the state hasn't calculated what the economic impact could be from such storms or earthquakes that could destroy, or at least shut down, Highway 520.

An estimated $4.4 billion to $5.3 billion is needed to build a six-lane bridge to replace the four-lane span constructed in 1963. Engineers consider it as vulnerable as the earthquake-damaged Alaskan Way Viaduct. The state has identified $1.6 billion in funding to replace the bridge, and that includes $1.1 billion from a tax package on the ballot in November.

The video, which cost about $21,000 to make, was produced by VanDevanter Associates of Vashon Island.

Susan Gilmore: 206-464-2054 or sgilmore@seattletimes.com