Commissioning Horizontal Structures: SR520 Floating Bridge

by Shaun May, EIT

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

Nursery rhyme lore taught us all about the deteriorated condition of the iconic London Bridge and the many attempts to rebuild it with a variety of different materials – wood and clay, iron and steel, silver and gold, and so on. And yet, it would seem, the London Bridge kept falling down. Perhaps if the London Bridge builders had added horizontal structure commissioning to their scope of work, they might have had better luck.

Wood Harbinger’s commissioning team is currently working on a major horizontal structure commissioning project – the new SR520 Floating Bridge and the West Approach Bridge North. The new 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 also provided the electrical and fire protection engineering for this project. We are all very proud of our involvement in this high-profile public project!

As we developed the commissioning plan and conducted functional testing of the bridge’s many systems, we discovered many similarities between building commissioning (vertical structures) and bridge commissioning (horizontal structures). As expected, there are also many differences, including specialty systems and unique considerations. I’d like to share some of the highlights of what Wood Harbinger learned about horizontal structure commissioning as well as tell you about some of the SR520 Floating Bridge’s features that were most interesting to commission.


Commissioning a horizontal structure is mostly similar to commissioning a vertical structure. For both kinds of structures, we develop a unique commissioning plan, witness system installation and startup, develop and perform functional testing for the systems, track issues, and conduct regular meetings to resolve those issues and maintain project progress. Both involve common disciplines including electrical, mechanical, structural, civil, and fire protection engineering, which all include many common systems that we would commission: lighting systems, generators, cranes (we encounter these in other waterfront projects), security systems like CCTV and intrusion detection, and some HVAC systems.

Bridge Jib Crane Commissioning. Photo by Shaun May.

Bridge Jib Crane Commissioning. Photo by Shaun May.

For example, the SR520 Floating Bridge has a Bridge Control System (BCS) whereas a typical building may have a centralized Building Automation System (BAS). The BCS system comprises the bridge’s central nervous system; it is a network of electrical systems communicating over a fiber optic loop. The leak detection, bridge fire protection, generator, lighting, intrusion detection, cathodic protection, weather station, and alarm systems are all included in this electrical network. The BCS performs largely the same functions as a BAS: modulating systems; monitoring statuses; and alarms for maintenance, life safety, and operational issues.


Electrical vs. Mechanical Focus

One of the most noticeable differences from a commissioning perspective is that we’re commissioning primarily electrical and fire protection systems on the bridge, whereas in a vertical structure, the mechanical systems are the typical focus. This is due to the fact that there are simply fewer conditioned spaces on the bridge, therefore HVAC system requirements are greatly reduced.

Bridge Fire Protection System Commissioning in Action! Photo by Shaun May.

Bridge Fire Protection System Commissioning in Action! Photo by Shaun May.

More Bridge Fire Protection System Commissioning in Action!

More Bridge Fire Protection System Commissioning in Action! Photo by Shaun May.

The BCS commissioning program was very detailed. We tested each component (e.g. a float switch), then the sub-systems (e.g. the leak detection system), and then system interactions (e.g. the leak detection system triggering an alarm in the Maintenance Facility Building).

Four illuminated sentinel towers, one at each corner of the bridge, rise above the floating bridge roadway from underwater and serve as iconic landmarks for the new bridge. The architectural lighting system we commissioned is tied into the BCS; it communicates over the fiber optic network, illuminates during the night, and can be remotely controlled to change color. These massive towers guide travelers while providing bridge maintenance access. The sentinels, as artistic elements, stand for our community’s future: beautiful, bright, and sustainable.

A spiral staircase inside the Sentinel tower provides bridge underdeck maintenance access.

A spiral staircase inside the Sentinel tower provides bridge underdeck maintenance access. Photo by Shaun May.

The bridge fire protection system we commissioned fills more than two miles of pipe with lake water in less than 10 minutes. As soon as a fire is spotted on traffic cameras, the Traffic Management Center can remotely start the pumps while the fire department is dispatched. The BCS monitors fire protection equipment to assure the system is always ready. We tested backup system components and control sequences to assure that the bridge fire protection can always be ready, even in extreme conditions.

Select maintenance sheds on the bridge have some mechanical conditioning (exhaust primarily). The Maintenance Facility Building is the main space requiring typical mechanical system commissioning. This 20,000 square foot structure is occupied by maintenance staff on a daily basis. Here we find typical building HVAC systems to condition the space, as well as elevators, access control systems, and a heating water system, used for dock snow melting.

Maintenance Facility Building (MFB) Exterior. Photo by Shaun May.

Maintenance Facility Building (MFB) Exterior. Photo by Shaun May.

User Groups

Another major difference between building and bridge commissioning is the user groups and their needs. A building is generally inhabited by people on a regular basis, making user comfort, safety, and aesthetics (visual and aural) high priorities for the building structure and systems. Some spaces are inhabited by groups of people with specific needs, such as patients and care providers in a hospital or students and staff/faculty in an educational environment, which necessitates specialty equipment with different critical function requirements. Commissioning is ultimately about assuring a quality user experience, so the differences in user groups play a role in our program.

The bridge structure and facilities are, for the most part, not inhabited by people on a regular basis (trolls, maybe, but dealing with them is outside of our commissioning scope). The bridge structure is utilized by many different types of users, the most common being the people that cross it in single occupant vehicles, multiple occupant vehicles, mass transit vehicles, on bicycles, or as pedestrians. The structure and systems in place on and in the bridge are utilized, monitored, and maintained to assure their safety and continued progress. These systems includes multiple layers of lighting systems; a float switch (leak detection system) inside the floating pontoons and emergency dewatering pumps that remove water if leaks occur; fire alarm and suppression systems; the Intelligent Transportation Systems (ITS), including traffic CCTV systems and tolling systems; and cathodic protection to prevent corrosion of the bridge steel, including anchor cables.

There are also public/civic uses as well as environmental considerations that bridge systems must address. The SR520 Floating Bridge includes specialty systems for collecting tolls, a weather station operated by WSDOT, and Emergency Boater Response call boxes that provide on-water access for boaters who may be experiencing problems. These additional usages mean the needs of additional user groups needed to be considered in our commissioning program to assure quality and successful functionality.

Unique Challenges

Floating Bridge Components

The vastness of Lake Washington, at more than 200 feet deep and covering 34 square miles, necessitates that bridges over it should float. Floating bridges require additional specialty systems to function properly and safely, which led to some unique commissioning exercises.

The leak detection system is one of these specialty systems. It utilizes sensors called float switches to detect water leaking into the floating pontoon. These float switches activate an alarm and trigger the emergency dewatering pumps to remove the water from the pontoon. This leak detection system monitors the bridge pontoons in order that they may be repaired and pumped out immediately in the case of failure or damage to the concrete structure. Coordinating the electrician’s team and troubleshooting the BCS leak detection system was an important and exhaustive effort because these monitoring devices comprise a life safety system.

Float switch testing was a unique challenge. We had 1,600 float switches to test; that’s 1,600 separate pontoon cells connected in a network of catwalks and accessed via sealable hatches and ladders. Each float switch was actuated (lifted to simulate it floating for at least 3 seconds) then verified to report correctly on the BCS screen Human Machine Interface (HMI). Many of the float switches are located at the bottom of an 8-foot tube and therefore had to be lifted by a custom 3D-printed device. This device was basically a telescoping pole with a grabber at the end to catch and lift the float switch.

Electrician teams dispersed to activate float switches and our commissioning team viewed the HMI and confirmed that the correct float reported water in the pontoon cell. We accomplished troubleshooting with the BCS programmer onshore and the wiring electrician at the pontoon.

The commissioning provider in the pontoon anchor gallery, the BCS programmer onshore at the Maintenance Facility, and the wiring electrician activating the float switch had to be able to actively communicate for this testing procedure. In order to maintain radio communication, a radio repeater was transported across the bridge and stationed on the underdeck of the pontoon being tested.

The interior of the floating pontoon cells is accessible via ladders. Photo by Shaun May.

The interior of the floating pontoon cells is accessible via ladders. Photo by Shaun May.

The float switch is a vital component of the bridge leak detection system. All 1,600 of them! Photo by Shaun May

The float switch is a vital component of the bridge leak detection system. All 1,600 of them! Photo by Shaun May

The cathodic protection system is another specialty system for a floating bridge. It protects the bridge steel and anchor cables from corrosion in the lake environment. The anchor cables that stabilize the bridge would corrode and deteriorate over time naturally due to the contact between metal and water; a minute draw of electrical current is induced, similar to the chemistry of a battery. This corrosive effect can be counteracted by generating a “reverse current” battery: by injecting a small amount of direct current (DC) into the water through anode wires while grounding the anchor cables, the natural DC current is counter-balanced, the polarity neutralized, and the corrosive process prevented. This system underwent an initial calibration and is set up to self-calibrate on an ongoing basis, as monitored by the BCS, indefinitely. Therefore this bridge has a means to protect itself from deterioration by the environment.

More Ground to Cover and a Larger Team to Coordinate

Bridge commissioning is a very physical job because of the sheer scale of the project as compared to a single building. It’s more on par with commissioning a central plant’s utility distribution systems (power, water, energy) in a campus environment. Conditions can also get stormy on Lake Washington; up to 60 mph winds have forced closures of the old bridge, as they washed white-capped waves onto the road deck. The new bridge is built to outlast these storms, with the roadway built up from the water will remain open in tougher conditions and will be easier to maintain.

Throughout the commissioning program, we 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.

The commissioning team scheduled personnel and supplies across 23 pontoons stretching the full length of the 1.46-mile floating bridge. Coordinating with a larger, more dispersed team, both physically and logistically, complicated the challenge. Wood Harbinger’s commissioning team supported the larger commissioning team, which is comprised of the General Contractor (GC) and Quality Control (QC) provider, KGM; their sub-contractors who installed equipment; WSDOT’s Quality Verification (QV) team; the Quality Assurance (QA) team provided by O’Neill Service Group; and all of their consultants.

Special Tools and Safety Consideration

Safety and success go hand-in-hand in the building construction and commissioning industry. Every construction site on which I have had the pleasure to work has had Personal Protective Equipment (PPE) requirements including steel-toed boots, safety glasses, gloves, high-visibility vests, and hard hats. Floating bridges introduce a few new hazards. For this site, we added some additional gear to our PPE and toolkits. A Personal Flotation Device (PFD) was always required when working on the bridge underdeck, as a precaution in case someone went over the edge and into the lake. To work inside the pontoons, we had to complete confined space training, and we wore head lamps and used a carabiner: the headlamp as a backup in case the pontoon cell lights turned off while working inside (those pontoon cells can be quite cavernous), and the carabiner to ensure that a pontoon hatch could not be closed while occupied. We also carried a custom 3D-printed float switch actuator, aka “float stick,” used heavily during float switch testing. Each pontoon cell was checked for noxious gases, such as carbon monoxide, with a gas monitor, aka “sniffer,” before anyone entered a hatch. To travel more swiftly, the electricians often pedaled about on tricycles with their tools in the basket. If a team was lucky enough, they could even cruise around on a golf cart!

Electrician tricycling down the bridge underdeck at sunrise. Photo by Shaun May.

Electrician tricycling down the bridge underdeck at sunrise. Photo by Shaun May.


The new SR520 Floating Bridge celebrated its Grand Opening in early April 2016, and motor vehicle traffic is now running in both directions across the bridge. So far all systems that we’ve commissioned are operating smoothly! That’s always an exciting feeling for a commissioning engineer.

Commissioning the SR520 Bridge has been an exceptional experience for me, personally. I am thankful for the opportunity to work with such a talented team, from whom I have learned so much. I have loved being out on the bridge and in the elements; we have witnessed some beautiful sunrises on the lake (electricians seem to love working at the crack of dawn). Hopefully, the pedestrian/bicycle path and improved public transportation connections (via HOV lanes and transit stations) will bring more of us outside too. We could all use a little more outdoor exposure!

In this article, we briefly touched on a lot of technical systems; I hope it piqued your curiosity! If you would like to learn more please comment on this article, check out my Instagram and Twitter, Wood Harbinger’s blog, and reach out!

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