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January 23, 2017
Project: State Route 520 Evergreen Point Floating Bridge
Client: Washington State Department of Transportation
(KPFF for design; KGM for commissioning)
The replacement of the state Route 520 floating bridge over Lake Washington was one of Washington state’s largest transportation projects in recent history, involving many engineering and design firms, plus specialty consultants and contractors.
Wood Harbinger was part of the team, hired to provide electrical and fire protection engineering as well as commissioning services. This included lighting for the roadway, navigation, pontoons and under the deck, as well as medium-voltage power distribution systems that support the east landings roadway, the 20,000-square-foot maintenance facility and the floating bridge systems.
Wood Harbinger also engineered a standalone fire-suppression system that extracts water from Lake Washington and distributes it through more than two miles of piping. Finally, the firm provided commissioning services for the maintenance facility building and all systems controlled by the bridge-control system, including electrical, fire protection, leak detection, cathodic protection, alarms and a weather station.
The entire electrical distribution system designed by Wood Harbinger includes 300 miles of electrical wiring. Instead of a traditional conduit system, the firm used strong yet flexible CLX cable that enabled long, continuous runs even over expansion joints in the bridge.
The CLX cable was looped at these joints to accommodate expansion and ultimately required about 150 less splices, increasing the reliability of the power distribution system. A backup diesel generator ensures that all bridge equipment will remain in continuous operation with or without utility power for an extended period of time.
Wood Harbinger worked closely with the bridge-control system designers to develop a reliable communication network. The system comprises the bridge’s central nervous system, a network of electrical systems communicating over fiber optics.
The communication cabling is also a looped system connected to each pontoon, so if there is a break in the fiber loop, bi-directional communication will continue on both sides of the break. Key systems are all included in this network and can be remotely monitored and controlled from the maintenance facility.
The fire protection systems design presented some unique challenges. Because the road deck sits 20 feet above the water at midspan, and higher at each of the landing approaches, it would not be possible for a fire truck to draw water out of the lake to fight a car fire on the bridge.
So Wood Harbinger designed a system that includes four vertical turbine fire pumps, a 1.4-mile-long, 8-inch standpipe, and a dozen hydrant connections on the road deck for fire department use. When the fire-protection system is activated, the piping fills with lake water in less than 10 minutes, supplying 1,000 gallons per minute to a single fire hydrant.
The traffic-management center operator can remotely start the fire pumps to allow time for the fire-suppression system to fill while the fire department is in transit to the scene.
In its commissioning role, Wood Harbinger developed a tailored commissioning plan, witnessed system installation and startup, and developed and performed integrated functional testing for the systems. Throughout the commissioning, Wood Harbinger engineers scaled the 10-story sentinel tower stairs to access the site, climbed up and down pontoon cell access ladders, walked miles across the bridge, sometimes exposed to the cold, wind and rain, and dealt with the high humidity that kept it cold inside the concrete pontoons during the winter.