World’s Longest Floating Bridge

Completed in 2016, the new State Route 520 Evergreen Point Floating Bridge is the longest floating bridge in the world. Spanning Lake Washington and connecting Seattle on the western side of the lake to Medina on the eastern side, the 7,710 foot long bridge replaces the previous 7,578 foot bridge, which was the longest floating bridge in the world.

Two important components of the construction of the bridge were the design and construction of the dry standpipe fire systems and the mechanical wastewater removal system (to remove rainwater from the road surface and filter it before returning it to the lake).

Contractors for these systems needed grooved pipe couplings, hangers, and other piping products that would meet or exceed the needs of this unique bridge style.

Project background

The new bridge is designed with a wider span to reduce traffic congestion and improve safety compared to the former State Route 520 Bridge. The bridge features two standard travel lanes, as well as an HOV lane, a shared 14 footwide bike/pedestrian access path, and shoulders of four feet to the inside and 10 feet. on the outside. It is also designed to be better resistant to severe storms and earthquakes than the previous one.

The bridge consists of two separate decks: an upper deck for traffic and a lower deck for maintenance and operations. The bridge features remotely controlled fire protection dry standpipes systems. In addition, it features a mechanical wastewater removal system that handles the removal of rainwater from the roadway. Approximately 1.5 miles in length, the bridge is constructed much like a viaduct, with a superstructure that rests between 14 and 80 feet. above the 23 pontoons, the largest of which are 360 feet in length. The design and construction of the bridge took approximately four years.

 

Fire suppression system ensures safety

Because the bridge is too long to rely exclusively on fire trucks for fire suppression water supply, it requires dry standpipes along the length of the bridge in order for fire trucks to access water. Once they arrive, trucks can access one of the hose stations along the bridge, drawing up lake water for use in fighting fires.

The fire standpipe is split into two separate systems. Each system has two 180PSI 1250 GPM vertical turbine fire pumps and motorized valves as primary and secondary sources of water, ensuring backup in the event of a pump failure. The systems are interconnected with a motorized valve, which is a third source of water in the extremely unlikely event that the first two sources should fail. All of these systems are remotely controlled from the WSDOT monitoring station in Shoreline, WA.

The fire suppression systems includes approximately 8,000 feet of eight inch Schedule 40 piping, all of which must be joined with appropriate pipe couplings that meet the specific needs of the floating bridge: that is, they need to be flexible so that they can withstand the bridge’s motion.

Hanger assemblies were also necessary to suspend the fire system’s dry standpipe from the underside of the bridge’s upper deck and give it the flexibility to move independently of the bridge, which is critical to the pipe’s ability to withstand the motion of the bridge from the water. The couplings and components of the hanger assemblies were all hot-dipped zinc galvanized, a critical project requirement.

Mechanical wastewater removal system

Rainwater flows off bridge deck into a series of 160 catch basins on the bridge, designed to trap sediment at the bottom. From there, it filters through Schedule 40 pipe ranging from 10” down to 6” and ends up in the 44 sump wells built into the bottom deck of the bridge. These sump wells are located in the middle of the pontoons, so that the rainwater flows back into the lake, but is separated from the rest of the lake water so that oil from the roadway doesn’t contaminate the water and can be cleared off of the surface of the isolated wells periodically.

The motion of the bridge from the water was also of concern with the wastewater pipes. Just as with the fire suppression system, lightweight, flexible grooved couplings were selected for their ability to handle the pipes’ motion.

The hot-dipped zinc galvanized couplings were sent to a company in Portland, OR for an epoxy coating. This step was deemed to be necessary for ecological reasons by the bridge designers, who feared that the galvanization would flake off as a result of being in contact with water and negatively affect lake wildlife.

In total, the rainwater removal system contains approximately 15,000 feet of pipe, 3,000 pipe couplings, and 1,500 pipe fittings. It is located under the bridge’s traffic deck on both sides, and spans the length of the bridge. It took approximately 14 months to install, and was completed in March 2016.

 

 

This article was submitted by Anvil International, one of the largest and most complete manufacturers of pipe fittings, pipe hangers and piping supports systems. For more info, contact Rob D Sayers, Anvil International, commercial account manager: rsayers@anvilintl.com or Cell-778-227-2364.

 

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