A man-made wonder: The challenges of building the I-90 rail bridge in Washington State

Written by Bill Wilson, Editor-in-Chief
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The I-90 rail bridge in Washington State presented some of the most complex problems in bridge construction history.

Orchestrating a mosh pit cannot be easy. One needs to make sure all the participants providing stabilization are in perfect sync, and even if it is performed at a high level there is still worry that an untimed arm drop could create a disaster. 

Workers on the floating bridge in Washington State must have felt like they had to execute this mass coordination day after day. Even when you are dealing with obtrusive concrete pontoons, the delicacy required is equal to walking into a sea of upright hands. 

Just the idea of a light-rail train moving across a bridge that rests on a lake is impressive enough, but the construction process belongs in the record books. 


All together the series of pontoons that form the floating bridge required approximately 3,600 ft of post-tensioning.

The floating I-90 bridge over Lake Washington is anchored greatness, with each step calling for meticulous calculations and coordinated movements.

Adding density to this enormous engineering challenge is Washington’s tragic history with floating bridges. Two have been lost during storms, the Hood Canal floating bridge in 1979 and the old I-90 floating bridge during Thanksgiving weekend in 1990. 

The fact the Washington Department of Transportation (WSDOT) was willing to take the plunge again is a feat in and of itself. However, this time it is different, and it started with the recommendations coming from a blue ribbon panel after the second bridge dropped. 

“We have had two floating bridges that have sunk, so there’s a high level of sensitivity and concern any time you are working on a floating bridge,” Sound Transit’s principal construction manager for the Seattle-South Bellevue segment of the East Link Extension, Sepehr Sobhani, told RT&S. “There were some strict guidelines that were derived from that blue ribbon panel report.”

The order that carried perhaps the harshest overtone was the fact that the vast majority of construction could only take place April through September. Before crews could gain access to the project, the fire life safety systems in the Mt. Baker tunnels, which take traffic to Lake Washington, had to be retrofitted. That project was not completed until June 2017, which essentially cut the first work window of the floating bridge in half. 

“Trying to make meaningful work in that first season was pretty difficult,” said Sobhani. “We were able to hit the ground running, though, to a certain extent.”

Injecting the project with some needed speed was an alternative contract delivery method called heavy civil GCCM. According to Sobhani, it is a hybrid between a design-build and a design-bid-build delivery. Mock-ups tested well before June also made up some time and secured certainty. One experiment was on the track attachments made specifically for the floating bridge. 

Targeted pre-planning along with the constant flow of expertise helped move the job smoothly, even during one of the world’s worst pandemics, COVID-19.

“Sometimes you went into meetings and your head was spinning when you came out,” remarked Sobhani. “It takes a lot of different subject matter experts coming together and working together to overcome these challenges.”

Pulling together

The first challenge was certainly an intimidating one—conducting the longest post-tensioning on a bridge in history. There are two floating bridges, one serving eastbound I-90 and one serving westbound I-90. The tracks are located on the inside lanes of the westbound span. Pontoons, which are bolted together, served as the building blocks of the floating structure, and each of them are 360 ft long. All together the series of pontoons required approximately 3,600 ft of post-tensioning. Twenty tendons were used, with each one carrying 615,000 lb of force. Crews had to core holes, which would serve as ducts, and build massive reaction frames to prep the bridge for the post-tensioning. Every time a hole was drilled, marine-grade plugs were inserted and tested to maintain the watertight integrity of the pontoons. 

“The floating bridge is designed with a series of cells and water-tight doors to compartmentalize the structure so you can stop progressive flooding if any one of those cells took on water,” said Sobhani.

Complexity like maintaining the watertight integrity of the pontoons was one of many reasons why a naval architect was required to prepare and review any work plans that required pontoon ballasting. 


The rail has to mimic the movement of Lake Washington, calling for the installation of eight innovative track bridges.

Each reaction frame weighed 20,000 lb and there were 10 on each side of the floating bridge. They were located on the inside of the pontoons, which called for parts and pieces of the frames to go through the 3-ft x 2-ft water-tight hatches on the bridge deck as well as the water-tight doors between pontoon cells. One of the pieces weighed as much as 3,500 lb, so carts running on Teflon surfaces were used for transport. Chains and hoists also were used to set the frames. Because each of the pontoons were unique, 3-D LiDAR scans of the surfaces were conducted and used during the fabricating of the frames so the fit was snug. 

The post-tensioning process actually shortened the bridge by about 4.5 in., or 2.25 in. on each side of the bridge. Expansion joints at the ends of the bridge accommodated the increased gap.

Moving like water

The floating portion of the bridge is essentially a living, breathing structure. Approach structures are fixed, but the transition span is the hinge that moves up, down, to the left and to the right … essentially the motion of the water underneath the floating pontoons. 

The rail also has to mimic the movement of Lake Washington, calling for the installation of eight innovative track bridges. The track bridges were specially engineered to compensate for six ranges of lake motion to enable trains to safely travel from the fixed sections of the bridge to the floating section. Almost 9,000 specially engineered and constructed lightweight concrete blocks were affixed to the bridge deck using a specialized epoxy called DexG. Rail was then set on the blocks, with steel tie bars placed between them to maintain gauge, similar to standard railroad tie systems. 

The blocks were fabricated in a precast yard and shipped to the site. Before being delivered, workers would pour a layer of elastomeric grout that contained pieces of cork. The grout served two purposes: It provided resiliency to the rail system in terms of dampening vibrations from trains crossing over, and it also prevents any kind of failure from transferring to the bridge deck itself. 

Prior to placement of the blocks, grinding of the bridge deck surface was performed to create a uniform surface that could undergo extensive survey measurements to set the alignment and profile of the rail. The contractor would then 

hang each block mimicking a top-down track installation method and begin the process of adhering the blocks to the bridge deck with the use the epoxy grout. The epoxy was injected between the small gap of the bridge deck and the blocks that hung from the rail. A steel wire was then used to screed the epoxy to eliminate any air bubbles. 

“When they work all together,” Sobhani said, “they’re able to accommodate all of those different movements of the floating bridge.”

Two prototypes were fabricated by Jesse Engineering in Tacoma, Wash., and tested at TTCI in Pueblo, Colo. The prototypes went through extensive kinematic testing to make sure the sections were able to accommodate the range of movements. Four-car trains running at 55 mph traversed over the prototypes to test the performance of the track bridges, and according to Sobhani the ride was smooth.

Adding the track to the floating bridge required the contractor to remove weight from the structure. In order to do this, a barrier on the south side of the westbound bridge was removed. The barrier, which was designed for vehicular traffic, was no longer needed, and the concrete was cut into segments and used as temporary ballast for the track. 

At press time, track on the eastbound direction had been installed and most of the track on the westbound direction was in place. Setting eastbound track was a little tricky because it included the installation of guardrail. Because the eastbound track was adjacent to the lake, guardrail was needed to prevent the unlikely possibility of trains entering the water. Workspace was the challenge when working on the westbound track. With the eastbound span completed the width of the construction zone was only about 25 ft, and 10 ft of room had to be left for WSDOT to perform operations and maintenance. 

“We were restricted to golf car access at this point,” said Sobhani. 

There were access points on either side of the bridge, and lanes could be shut down temporarily if necessary, but the entire process took a lot of coordination and effective scheduling and communication.

“If you do not sequence yourself right, you cut yourself off and essentially paint yourself into a corner,” Sobhani explained. 

Treating strays

Long-term maintenance also had to be addressed during the planning and construction phases of the floating bridge. Trains that cross the structure are dependent upon electricity, and stray current can be problematic because it can cause advanced corrosion. Cathodic protection is there to fight this issue. The elastomeric grout, track attachments, splints, etc., have all been treated, and a polyurea was sprayed on the bridge deck to prevent any stray current from entering the reinforcing steel of the floating bridge.

A current electric cable also has been installed to serve as another layer of security against corrosion. Stray current is caught in this collector cable and taken to the ends of the bridge and grounded. 

Rectifiers inside the pontoons also are part of the cathodic protection. The rectifiers are automated and actually impress current into the steel because corrosion occurs when stray current leaves the reinforcing steel so impressing a constant current can prevent this from happening. The rectifiers are constantly measuring and calibrating to make sure the right balance is achieved, because if too much is impressed into the steel you have what you call hydrogen embrittlement. 

Pandemic protection

Rehearsal, pre-planning, meetings—all and more helped construction meet any anticipated challenges, but what about the one big challenge that dictated its own timing: COVID-19? The usual protocol was followed, social distancing, temperature screening, etc., but nobody could become completely immune to the virus. Quarantining a crew for 10-14 days would put a serious hurt into a construction schedule that was already shortened to six months of each year. 


Adding another challenge to this project was trying to work during a pandemic.

The work inside the pontoon was mostly completed before the pandemic swept through the U.S., and effective scheduling helped keep workers from being on top of one another. During our interview in late summer, Sobhani said the floating bridge track crews were free of COVID-19 cases, but if one did happen there were groups of workers spread out over the entire 7-mile project, not just on the floating bridge. If necessary, workers could be pulled from other areas to complete tasks left behind by a quarantined group.

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