The Seattle Times
Washington state possesses four of the world’s five longest floating bridges and 78 years of operating history. Despite that experience, the state couldn’t prevent a loud ka-thunk noise thousands of times every day, at the new Highway 520 bridge.
Cars produce a mighty din while crossing the broad expansion joints, over the hinges where road decks on fixed columns meet the mobile transition spans. These spans swing up and down to match seasonal changes in the levels of Lake Washington.
The bridge opened in April of 2016 and this fall, University of Washington scientists set up microphones in the bike lane and under the joints, in a quest for ways to quiet the bridge. “It’s a big deal for the neighborhood so I very much hope that we can make a difference for them,” said Per Reinhall, UW professor of mechanical engineering.
The study was prompted by complaints from Medina, but is meant to provide noise-reduction ideas for more than 50 bridges.
WSDOT has battled expansion-joint noise before, when the second Tacoma Narrows Bridge opened in 2007 above homes in Gig Harbor. The state added concrete walls lined with foam to absorb low-frequency sounds. However, foam panels fell into the roadway, causing a traffic hazard, said spokeswoman Claudia Bingham Baker.
When they designed the new Highway 520, engineers applied lessons from the Narrows bridge, by striving to confine the sound. A steel-and-concrete chamber under the new joints has contained noise so it doesn’t afflict lakefront homes about 70 feet below the bridge’s east high rise.
But a different problem surfaced. Sound waves travel up and out.
The UW team’s early hypothesis compared the effect to plucking a guitar string over the instrument’s sound hole, while Medina Councilmember Alex Morcos said the deep chamber amplifies sound “like a subwoofer.”
But the preliminary findings Nov. 7 and 8 showed otherwise, said lead scientist Alex Soloway. The noise “does not originate in the cavity below the expansion joint but instead comes from the top of the joint,” he said Thursday by email.
Because the new bridge is taller than the former 1963 bridge, this noise travels a full mile before it dissipates into the trees on Clyde Hill.
A medina council member who is an acoustic scientist, says she recorded sounds at Evergreen Point Way beyond 90 decibels, the same noise level as a lawn mower in your yard.
Neighbors have also complained on the Seattle side at Laurelhurst and Madison Park.
