I-90 bridge can handle rail
November 24, 2008 · Updated 4:30 PM
By Ruth Longoria
Results of an earlier Department of Transportation study are in and, with structural mitigation, the Interstate 90 floating bridge would be able to carry light rail, Sound Transit authorities announced last week.
Patrick Clarke, floating bridge and special structures engineer for the DOT, brought the results of the structural feasibility study to Sound Transit's regular board meeting on Thursday.
``The bridge behaved about the same for the studies as we expected,'' Clarke said. ``The live load test and the computer model gave verification that the bridge can handle the load.
The ``live load'' test was conducted by DOT in September, using eight flatbed trucks, loaded with concrete -- equal to a combined 1.2 million pounds. The trucks simulated the weight and movement of two four-car light rail trains as they would, in the future, transport light rail passengers between Seattle and the Eastside. The westbound portion of I-90 from the Island to Seattle was closed to commuter traffic for during the testing.
The tests involved 12 different load cases, including storm, wave and temperature scenarios, which would cause the bridge to move more, swell or contract, Clarke said.
In order to handle the anticipated load of light rail, the bridge would need modifications, including some redistribution of weight, Clarke said. Mitigation to the bridge would include removing one-inch of concrete overlay and replacing it with a 1/4-inch polymer (a hard plastic, epoxy mixed with gravel) overlay; strengthening the transition span, and adding more steel plates; replacing the concrete barrier with a cable rail, which would be used as a hand rail in an emergency situation if one of the vehicles broke down and commuters needed to be moved from one train to another; and removing or moving the auxiliary ballast within the pontoons, which would add weight to one side or another. ``We're being tricky and shifting weight around so the bridge floats level,'' Clarke explained.
DOT conducted the tests in order to assist Sound Transit in making a decision on what form of mass transportation would best work for commuters between Seattle and the Eastside. After months of study and public meetings earlier this year, Sound Transit narrowed possible options down to two choices, light rail or bus rapid transit, which could later be converted to light rail. Either option would take away the center HOV lanes for use by Sound Transit's mass transportation service. However, it is expected that Islander single occupant vehicles that now have use of those HOV lanes would be given mitigation, such as access to the then-planned outer HOV lanes.
Sound Transit expects to make a decision on which transit option to choose and present a draft plan by March. The plan would be taken in 2006 to the voters -- who ultimately would decide to accept or reject the plan and its subsequent taxation.
Before center lane construction could begin, an $18.9 million, four-phase plan to move the HOV lanes (called R8-A) would have to be already completed. That is expected to take about two years, Sound Transit's chief communications officer Ric Ilgenfritz said previously. If voter approved, construction on either light rail or bus rapid transit would begin in about 2008, Ilgenfritz said.
For now, Sound Transit will continue to work with DOT on specifics of the retrofit and associated costs, and also begin studies for the Rainier Avenue and Bellevue way ramps and the East Channel Bridge, Sound Transit representatives said.