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Scientists Attempt to Hush Noisy WA Bridge

Tuesday, December 11, 2018

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University of Washington scientists are investigating noise complaints that stem from cars passing over expansion joints belonging to the Highway 520 bridge, one of the longest floating bridges in the world, which carries Washington State Route 520 across Lake Washington from Seattle.

Researchers set up microphones in the bike lane and under the joints to help determine ways to quiet the bridge. More than 200 neighbors have complained since the structure opened in 2016.

Highway 520 Bridge

The Highway 520 Bridge, officially known as the Governor Albert D. Rosellini Bridge, also known as Evergreen Point Floating Bridge for the span it replaced, is a 7,710-foot-long bridge measuring 130 feet wide at its midpoint. The original structure was opened in 1963, a 7,578-foot-long floating span that cost $24.7 million to build, which translates to $198 million in 2018.

When it came time to replace the bridge, $4.5 billion in funding stemmed largely from the state gas tax that was set aside for infrastructure projects. By 2014, the project’s budget was increased by $250 million to account for cost overruns.

A $367 million contract was awarded to Kiewit-General Joint Venture in February 2010 for bridge pontoon design and construction; a $306 million contract was awarded to Eastside Corridor Constructors in November 2010 for the Eastside transit and HOV project; a $586.6 million contract was awarded to joint venture Kiewit/General/Manson in Augusut 2011 for designing and building the floating bridge and landings; and a $199.5 million contract was awarded to Flatiron West Inc. in July 2014 for designing and building the Montlake to Evergreen Point Bridge West Approach Bridge North project.

Noise Complaint Investigation

The Washington State Department of Transportation previously dealt with a similar expansion-joint sound issue when the second Tacoma Narrows Bridge opened in 2007. Concrete walls lined with foam were added to absorb low-frequency sounds, but the foam panels fell into the roadway, which caused a traffic hazard and subsequent removal of the panels in 2016.

Taking lessons learned from the Narrows project, engineers placed a steel-and-concrete chamber under the expansion joints of the Highway 520 Bridge; the chamber is intended to help contain noise. A different problem arose, however: Soundwaves now travel up and out.

UW scientists initially surmised that the phenomenon is akin to plucking a guitar string over the instrument’s cavity, but preliminary findings indicated the noise comes from the top of the joint instead. The new bridge also uses grooved concrete rather than asphalt, which makes the sound more noticeable, noted Per Reinhall, UW professor of mechanical engineering.

WSDOT follows the government’s guidelines in evaluating sound, implementing a 67-decibel limit. An environmental impact statement predicted that the highway’s road barriers would help control the issue. By the time the sounds reach the homes beneath the bridge, they should be less than 61 decibels, WSDOT spokesperson Kris Reitmann told The Seattle Times.

UW scientists found that the peak pressures, which occur every time a vehicle hits the expansion joints, could range from 75 to 80 decibels during the quarter of a second a vehicle passes over the joints. Reinhall has suggested adding steel pieces to reshape the 12 straight bars found in each joint into wavy bars. As a result, tires would hit the bars differentially, rather than making parallel bars resonate.

The research team is slated to discuss findings with Medina city leaders Dec. 13. WSDOT will also issue a report to lawmakers by early January, Reimann said. This round is of study and retrofitting is preliminary.

Medina Councilmember Sheree Wen noted that she hopes WSDOT will make project requests before the Dec. 12 deadline for the 2019 budget cycle.


Tagged categories: Infrastructure; NA; North America; Program/Project Management; Quality control; Research and development; Transportation

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