A deep dive on the final stages of repair to the West Seattle Bridge

Load testing is one of the final steps to perform before the West Seattle Bridge can reopen to traffic. We’ve talked about it a little bit before, but it’s time for a deep dive.

It’s one thing to say engineers are going to put dump trucks full of rocks at key points on the bridge to test its strength. I wanted to hear from an actual engineer on the subject.

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Greg Banks is the top engineer for this repair project. I met with him on the West Seattle Bridge during my tour inside the box girders Thursday. Banks said there will be static and dynamic testing, with those trucks being stationary and moving.

“We have six different load cases, and we’re positioning them at various locations,” he said. “Up to twelve trucks will be on the bridge at a time, but each of those trucks will weigh 80,000 pounds.  They’re strategically placed to create worst-case stress conditions.”

So what will Banks be looking for in these tests? He will be watching all the sensors which have been embedded in the bridge to make sure the numbers he sees are what he expects. Banks will be looking for an expansion of the concrete cracks under the load, and he’s looking for movement in the fractions of millimeters. That’s how small and precise this testing is.

What if the numbers don’t line up?

“If it doesn’t, then we go and look at the analytical prediction to make sure we didn’t mess something up with data input,” Banks said. “We’re going to make sure a sensor hasn’t popped or anything like that and is giving us faulty readings. We are going to ground-truth it first before we think it’s not behaving as expected.”

Heather Marx is the project director for the Seattle Department of Transportation. She expects this load testing to go as planned. Her confidence comes from the last two and a half years of the repair work, and the bridge has been responding as expected.

“When it was cold and we had a huge weight of snow on the bridge, it behaved as we expected it to,” she said.  “During the heat dome, the bridge behaved as we expected it to.  We have no expectation that any of those tests are going to come back anything other than exactly as anticipated.”

But if they do, Marx will not hesitate in delaying the reopening, which is now scheduled for September 18.

“We need to make sure,” she said.  “We are not going to open a bridge that isn’t safe for ordinary travel.”

You might find it interesting that live traffic doesn’t add that much weight to the bridge. Most of the weight stresses come from the weight of the bridge itself. Only about 20% of the weight comes from the vehicles using it.

Check out more of Chris’ Chokepoints.

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