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Sunday’s closure for nearly four hours of the westbound I-90 bridge in a windstorm caught many drivers off guard.

Although the old SR520 bridge would close in strong winds of 50 miles per hour for 15 minutes, drivers are not used to seeing wind closures of the I-90 bridge.

‘Extremely rare’ wind event could stall light rail over I-90

The Washington State Department of Transportation is using a new protocol to close the westbound bridge when winds from the north reach 26 mph for two minutes and generate waves at least two feet high.

Strong winter winds from the north do not occur often and WSDOT says it recently instituted the new protocol after a new analysis on retrofitting the I-90 bridge for light rail.

Officials say traffic on the Homer Hadley Bridge adds stress and that stopping traffic during high wind and waves could help preserve the bridge for the future.

“Our structural folks take a look at that and try to apply it to the bridge itself, and what can we do to try to extend the life of the bridge,” said agency spokesperson Bart Treece.

Preservation matters because in 1990, a year after the Homer Hadley Bridge opened, its companion bridge on I-90, the Lacey V. Murrow Bridge, sank when pontoons filled with rainwater during reconstruction.

The rebuilt Murrow bridge sits just to the south of the Hadley bridge and shields it from south winds.

John Sleavin, Sound Transit’s deputy executive director for design engineering and construction management, said north winds have a long distance on the lake to build up before hitting the bridge.

“It’s really the waves more than the wind itself that impacts the bridge,” Sleavin said.

A 2008 state analysis suggests with two fully loaded trains and three lanes of heavy traffic all on the bridge during unusually high north wind, the bridge could reach 97 percent of its capacity to twist.

Photos: I-90 bridge closure

Sleavin explains that is a conservative, theoretical analysis, and says post-tension cables Sound Transit is now adding to the bridge will make a big difference.

“It forces all the concrete together and makes it a lot stronger,” Sleavin said.

That work will be done later in 2018, as part of a $225 million budget increase for Sound Transit’s EastLink project.

After the post-tension work is done, WSDOT will decide how much it will change the threshold for closing the bridge to vehicle traffic.

The state first alerted the public on Saturday about the new protocol, a day before the first closure.

“This wind event from the north is a fairly rare occurrence on Lake Washington, so we did have a plan in place with the hopes of not having to use it,” Treece said.

Sound Transit has its own standards for when it runs trains on the bridge.

If there’s a north wind between 30 and 40 miles per hour, the agency will allow only one train on the bridge at a time.

That will probably happen about once a year.

If a north wind exceeds 40 miles per hour, it will stop light rail traffic.

Sound Transit expects that about once a decade.