Light rail on I-90 bridge presents unique – and pricey – problems

Placing a light rail line on the Interstate I-90 floating bridge has always presented engineering challenges—particularly considering the potential corrosive damage incurred by the stray electrical currents that are common with light-rail systems.

Study after study proves such currents cause damage to surrounding buildings and infrastructure. In the case of placing light rail on a floating bridge, that means likely damage to the steel components of the bridge. Of course, taxpayers would be on the hook for the annual costs of fixing the resulting damage.

These potential problems came to light again earlier this summer when a Sound Transit official appeared before the Bellevue City Council on July 28, 2015. The purpose of the presentation was to provide an update on issues that have yet to be addressed from the 2008 Independent Review Team (IRT) report. The report warned of the corrosive impact and determined that light rail could run on the I-90 floating bridge if Sound Transit addressed 23 issues—which have been classified under six categories— related to the impact of the proposed installation.

Perhaps most concerning for taxpayers is the fact that Sound Transit has not conducted an economic impact study concerning what the IRT called the “life expectancy”—which it categorized as high importance—of the I-90 floating bridge given the corrosive damage expected from stray electrical currents. During the Bellevue City Council meeting, Councilmember Kevin Wallace asked a Sound Transit official whether the agency ever conducted such a study. He pointed out the importance of knowing whether or not the multi-billion dollar bridge would need to be replaced sooner due to corrosive damage. The official did not satisfactorily address the question.

WSDOT report warns of potential problems
An Independent Review Team (IRT) report conducted for the Washington Department of Transportation (WSDOT) in 2008 warns that while “methods are available to control or minimize” the impact of stray electrical currents, placing light rail on a floating bridge present unprecedented challenges. The report reads,

Since LRT has never been installed on a floating bridge, the impact of stray current needs to be analyzed with a higher degree of scrutiny. All electrical systems have an intended path of current flow. Some of the system current can leak and flow through unintended paths, and in doing so can cause corrosion of the metallic elements that are present in those unintended paths. In the case of the light rail, a small portion of the current that flows through the rails can leak into the supporting structure; i.e., the floating bridge or approach spans. This current may find its way through the reinforcement, which has low electrical resistance. To complete its electrical circuit, this current must be discharged from the concrete structure. At the discharge points, corrosion of the reinforcement will occur. On a floating structure, this corrosion can create cracks that can compromise floating bridge water tightness and ultimately its ability to stay afloat. Mitigation of this stray current is a mandatory element of the LRT installation. Long term maintenance of the stray current mitigation system is as important as installing the various stray current control measures.

Sound Transit makes light of potential costs
Back in 2008, a state project manager admitted that the “idea of putting light rail on a floating bridge” is “very new to us.” Sound Transit, on the other hand, has consistently brushed aside concerns over stray electrical currents and the potential damage caused to the bridge. One transit official called the likely incurred costs a “drop in the bucket” compared to the overall costs of extending light rail from Seattle to the Eastside—not exactly a reassuring statement considering Sound Transit’s long, failed history of project management. The Seattle PI reported in 2008,

Sound Transit’s East Link project manager, Don Billen, said his agency will monitor the current to prevent damage. Billen also said the agency is testing different methods of attaching the rail to the center portion of the bridge, to avoid pushing so far into the concrete that structure reinforcing steel is damaged. He said at the very least, radar can be used to locate steel, so it won’t be damaged if the rail blocks are attached with dowels driven into the deck, a conventional method.

Again, “testing” new methods of ensuring stray electric currents do not cause corrosive damage—especially given the report quoted above—does not exactly inspire confidence. One would hope that Sound Transit would take the potential risks and incurred costs more seriously than the blanket statements imply.

Stray electric current an expensive problem in other cities
Sound Transit’s apparent failure to fully address the potential economic impact on the I-90 floating bridge is a serious concern, especially considering the recorded damage stray electrical currents via light rail have incurred on infrastructure in cities across the country. According to a study conducted by Northwest University, corrosion caused by stray electrical currents from rapid-transit systems costs the United States approximately $500 million “despite a relatively mature technology for its control.”

Given the number of cities that actually have rapid-transit systems and the relatively small space in which the systems operate within these cities, the incurred costs is sizable. The cost nationally and the fact that our region is dealing with an entirely new set of problems associated with putting light rail on a floating bridge, it is essential that Sound Transit fully address the economic impact and present it to the taxpaying public.