Safety Through Innovation on the SR 520 Floating Bridge

by Mike Lehner, P.E.,

This article is part of Wood Harbinger’s newsletter series.

The new SR 520 Floating Bridge is an iconic update to the Lake Washington landscape and transportation plan. The 1.46-mile long bridge creates a wider and safer passage for cars and busses across Lake Washington. It also includes HOV lanes as well as bicycle and pedestrian access, which the existing bridge did not. Wood Harbinger provided the electrical and fire protection engineering for this project as well as commissioning services. We are all very proud of our involvement in this high-profile public project!

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.

Not Your Average Fire Protection System

The SR 520 Floating Bridge fire protection systems required some unique considerations and specialty systems to accommodate its height and length. The bridge height is one of the main reasons the design included a dedicated fire protection system. The roadway deck is elevated above the floating pontoons to prevent waves whipped up by winds from impeding the safe and progressive flow of traffic. The road deck sits 20 feet above the water at mid-span, and higher at each of the landing approaches. At this height, a fire truck can’t successfully draw water out of the lake to use in fighting a car fire on the bridge. A dry standpipe system is provided as a source of water to fight fires on the bridge.

The long distance of the bridge, and therefore the pipes, meant that pipe expansion was an issue we needed to address with our design. The piping system was installed on pipe rollers to allow for movement and uses telescoping expansion joints at each bridge expansion joint.

System Components

The fire protection system includes four vertical turbine fire pumps (similar to a well pump), a 1.4 mile-long, 8-inch standpipe, and a dozen hydrant connections on the road deck for fire department use. There are two zones on the bridge, separated by an isolation valve in the middle of the bridge. Each zone can operating as a stand-alone system, with two fire pumps and a fire-suppression control panel for each zone.

A single 200 HP fire pump can pump lake water to the hydrant in its zone up to 1,500 gallons per minute between two fire hydrants. That water volume is similar to 30 average bathtubs (50 gallons each) dumping each minute. The fire department uses this water—coupled with a fire engine pumper truck to further pressurize the water—to fight a vehicle fire.

The system has several built in redundant features, including backup fire pumps, a backup generator for the electrical service, and interconnection with the other fire zone. The system can be started from the bridge maintenance building, the fire hydrants themselves, and the local fire pump controller.

System Activation

The bridge is monitored 24/7 by WSDOT personnel utilizing traffic and security cameras. If a fire event occurs on the bridge, the Traffic Maintenance Center (TMC) 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. This results in a reduced wait time before the fire department can utilize the standpipe fire suppression system.

When activated, the fire pumps soft start, coming to full speed in about 5-10 seconds. The piping fills with lake water in less than 10 minutes and supplies 1,000 gpm at any one hydrant or 1,500 gpm between two hydrants. Filling two miles of pipe with high pressure water isn’t easy on the system and can cause “water hammer”—a loud noise and vibration from the pressurization caused by rapid opening and closing of the pipe valves. Without mitigation, water hammer can cause damage to the pipes. The SR 520 Floating Bridge fire protection system has slow opening valves and air release valves, which allow the air to escape in the system during initial filling. There are smaller air release valves at the end of each system, which act as a brake to slow the water down when it gets within 400 feet from the end of the piping. We used water hammer analysis software to inform our design and make sure that maximum pressures remained below about 300 psig. The system also includes rupture discs with isolation valves as a backup measure to protect the piping from system surges that might push pressures over 400 psig.

Draining for Safety

The standpipe system is normally dry when in standby mode to prevent water from freezing in the standpipe. To assure proper filling and to mitigate water hammer, the system needs to be completely drained after each use and remain dry while in standby mode. This assures there is an air cushion in the piping prior to activation. Failure to drain the standpipe system piping after usage could result in damage from freezing and/or bursting of the rupture disc from water hammer. As a life safety system, it’s crucial that the system be operated and maintained properly. Critical portions of the system include the pipe drops to the air release valves and the rupture discs.

Integration with the Bridge Control System

 The Bridge Control System (BCS), which controls all functions on the bridge, interfaces with the fire suppression system and serves as the conduit for control and monitoring of the fire pumps. Each fire pump has a dedicated fire pump controller, with a connection to the fire suppression control panel (FSCP) in each zone. Each FSCP controls the two fire pumps in the zone and connects to the BCS for inputs and outputs at the fire hydrants. The BCS provides monitoring of the fire hydrant pump start/stop switches and also monitors the pump controls in the WSDOT Traffic Management Center (TMC) and bridge maintenance building. The BCS will transmit these pump start signals to the FSCP. The motorized valves at the fire pumps are modulated to prevent large water flows colliding at the pipe tee during the initial pump start when filling the piping system. In addition, the BCS will receive status information from the FSCP and can also activate the fire hydrant indicating lights on the bridge. The FSCP then receives signals from the bridge control system for remote starting from the bridge maintenance building, remote hydrant locations and the TMC.

A Once-in-a-Lifetime Experience

While the SR 520 Floating Bridge may seem relatively simple, it is very sophisticated with many challenges that the fire suppression system had to overcome. As with all challenging projects, however, there are also rewards. For many engineers, this is a once-in-your-career type of project; as the longest floating bridge in the world, another project like this one will not come around anytime soon. As a bonus, I’m participating in the commissioning of the West Approach Bridge North project, which will connect Montlake to the floating bridge.

Toward the end of the project, the people who worked on the bridge brought out their families to see their handiwork and learn about the bridge, which was fun. It was also a pleasure to work with WSDOT and the Kiewit-General-Manson (KGM) joint-venture contractor team on this project. Now as I drive over the bridge from time to time, a feeling of pride comes over me.

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