SR-520 Evergreen Point Floating Bridge and Landings Design
Washington State Department of Transportation
Seattle, WA

The new 1.46-mile long SR-520 Bridge is the longest floating bridge in the world. It replaced aging infrastructure susceptible to wind and earthquake damage with a safer, more accessible structure that features wider shoulders, a new transit/HOV lane in each direction, a 14-foot wide bicycle and pedestrian path, and the ability to accommodate future light rail transportation. Wood Harbinger provided electrical and fire protection design engineering as well as commissioning for this iconic community project. We designed highly reliable and redundant systems while also accommodating the challenges of an over-water environment.

While the SR 520 Floating Bridge may be concrete and non-combustible, the 70,000 or so vehicles that travel across the bridge every day can catch fire. As such, there is a dedicated fire protection system on the bridge that enables more responsive and effective fire suppression, improving the bridge’s overall safety. Wood Harbinger’s fire protection design included fire hydrant replacement and connection to municipal water at the East Landing; four vertical turbine fire pumps (similar to a well pump); and a 1.4 mile-long, 8-inch standpipe system including a dozen hydrant connections on the road deck for fire department use. A single 200 HP fire pump can pump up to 1,500 gallons of lake water per minute between two fire hydrants in its zone. That water volume is similar to 30 average bathtubs (50 gallons each) dumping each minute.

The medium voltage electrical system includes 300 miles of electrical wiring and supports the East Landings roadway, Maintenance Facility, and Floating Bridge elements. We designed an innovative distribution approach using a medium voltage feeder loop in lieu of a radial feed, which saved 17,000 feet of 3-inch rigid conduit and 51,000 feet of copper cable in support of the project’s LEED Silver target. A standby generator can support the entire electrical and fire protection infrastructure. Wood Harbinger’s conduit and raceway systems support the intelligent traffic systems, bridge control and monitoring systems, and the traffic and security CCTV camera systems.

We also provided lighting for the bridge and East Landing roadways, underdeck maintenance, and navigation. The over-water environment posed some unique requirements for our lighting design. We had to take into account maximum light spillage into adjacent water surfaces specified by the project’s environmental requirements; too much light in the water could have behavioral effects on the fish species residing in Lake Washington. We chose luminaires and lighting levels that would maximize the lighting levels for safety while limiting spillover light into the water.