SR520 Floating Bridge, Landings, and Maintenance Facility Building


Washington State Department of Transportation


Seattle, WA





Fire Protection



Civic & Public

Project Team
Nick Baker
Principal—Mechanical and Commissioning
Shaun May
Project Manager
Sean Bollen
President/CEO and Principal—Electrical
Peter Lekhakul
Senior Electrical Engineer
Mike Lehner
Associate, Senior Project Manager
The Longest Floating Bridge in the World

Lake Washington is more than 200 feet deep and covers 34 square miles, necessitating that bridges over it should float. The new 1.46-mile long SR-520 Bridge 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. We provided electrical engineering, fire protection engineering, and commissioning services for this iconic update to the region's transportation plan. We designed highly reliable and redundant systems while also accommodating the challenges of an over-water environment.

Our electrical engineering scope included medium-voltage power distribution systems that support the East Landings roadway, the 20,000-SF maintenance facility, and the floating bridge systems, as well as a 2MW 12.47kV standby diesel generator that can support the entire bridge. 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.

While the SR 520 Floating Bridge may be concrete and non-combustible, there is a dedicated fire protection system on the bridge that enables more responsive and effective fire suppression, improving the bridge’s overall safety for the 70,000 or so vehicles that travel across the bridge every day. Our 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 system has built-in redundant features including backup fire pumps, a backup generator for electrical, interconnection between the two bridge fire zones, and connection to the bridge control system.

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.

Our commissioning team oversaw the bridge commissioning program and worked on the fire protection, electrical, and HVAC systems as well as a bridge’s specialty systems, such as the bridge control, cathodic protection, leak detection, emergency boater response, jib cranes, and lighting systems. We also oversaw all electrical testing by the electricians. Unique endurance tests were implemented to ensure life safety systems were operational before the bridge was opened to the public.