The greater Puget Sound area is home to four of the five longest floating bridges in the world. Seattle engineer, Homer Hadley, introduced the idea in the early 1930s. Pontoon bridges were a sound solution for the area’s deep, soft lake bed. The SR520 Evergreen Point Bridge became the longest floating bridge in the world upon opening in 1963.
A Challenging Environment, A Challenging Problem
The first floating bridge in Washington State opened in 1940 to great success. But there have been major setbacks related to our area’s occasional windstorms. In 1990 the old Lacey V. Murrow Bridge sank during refurbishment after 50 years of successful operation when a series of human errors and a holiday storm event coincided to fill the center pontoon with water and initiate progressive failure of the entire bridge. In 1979 the Hood Canal Bridge suffered catastrophic failure and sank in a wind storm with average wind speeds in excess of 80 mph.
WSDOT commissioned Glosten in 1983 to evaluate the wind and wave forces for the new I-90 bridge design and the two existing Lake Washington bridges. The analysis of floating bridges represents a unique challenge, because the wave loads and structural deformation interact. The state-of-the-art method used to predict the dynamic motions and internal structural loads is known as hydroelastic analysis.
Collaboration and On-going Improvement
Glosten collaborated with local civil engineers 30 years ago to solve this complex problem by integrating hydrodynamic analysis, wave hindcasting, and stochastic analysis with structural analysis tools. Over the following decades WSDOT periodically recognized the need to revisit and refine the bridge analyses as wind data collection and analysis methods advanced. State of Washington Bridge Division engineers still rely on Glosten’s expertise today to assess the forces on the State’s floating bridges. Our most recent projects, the wind and wave loads analysis of the SR520 Replacement Bridge and the re-evaluation of the Homer Hadley Bridge for light rail traffic, incorporate the latest wind data, 3D hydrodynamic analysis codes, and commercially available structural analysis tools.
SR520 Replacement Bridge
Washington’s SR520 Bridge Replacement and HOV system will enhance safety by replacing the aging floating bridge and keep the region moving with vital transit and roadway improvements. The State estimates construction will be completed in 2017.
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