'Potentially headed toward collapse:' Engineer reflects on West Seattle Bridge closure

SEATTLE — Deep inside the West Seattle Bridge, Matt Donahue points to cracks that changed his life and the city.

“So, these are single crack gauges, that are not just looking at one crack," Donahue said, as he pointed to a wall of sensors that are monitoring any potential spread of cracks of concrete.

He was the one that went inside the bridge the morning of March 23, 2020 for a routine inspection.

The bridge is essentially two caverness steel tubes reinforced by concrete siding side by side. One tube carrying the weight of eastbound traffic, the other westbound traffic.

As the city’s chief bridge engineer, he and SDOT other engineers had been tracking concrete cracks inside the bridge for years, but what he saw that morning startled him.

“I came up here saw all that rapid crack propagation and that confirmed for me the bridge was potentially headed toward collapse." Donahue said. “I told my deputy director, who told the department’s director, who told the mayor and she made the decision to close the bridge immediately."

He said the bridge steel tension system hidden in the concrete floor has started to stretch faster than expected, drastically shortening the life expectancy of the bridge. It was expected to last 75 years. It was shut down after 35 years.

“That post tensioning steel relaxed over time, stretched out and changed the way loading moved through the floor of the bridge and started cracking it,"Donahue said.

Nearly everywhere you look, there is some marking on the side walls highlighting a crack. It’s especially bad in two places on the south wall underneath the eastbound lanes.

“There’s lots of sun, rain and wind hitting the south wall," Donahue said.

The decision to call for an immediate closure was how the pattern of the cracks were starting surround the circumference of the bridge.

“All those cracks would link together into a hinge and basically cause a structure failure, where the bridge could no longer support its primary load even its own dead weight," Donahue said.

To stabilize the bridge, 10 miles worth of steel strands were bolted to the inner floor of the bridge and tightened. He says there four-million pounds of lateral tension preventing the bridge from separating. The stabilization work was completed in November.

“All that tension has slowed down the cracking," Donahue said.

Several key areas are now monitored electronically 24/7 with sensors and camera. Think of it as an early warning system that alerts engineers for crack expansion. The monitoring system will remain in place after final repairs have been made and the bridge reopens, which is expected to be in mid-2022.