Test shows I-90 bridge can take light rail

A real-world test has confirmed earlier, computer-generated conclusions that the Interstate 90 floating bridge can accommodate light rail, the state Department of Transportation said Thursday.

"The load-carrying capacity of the bridge for light rail is not a show-stopper," Patrick Clarke, a special structures engineer for the department, told the Sound Transit board.

Light rail across I-90 and Lake Washington in what is now the bridge's center, reversible roadway has been part of Sound Transit's long-range plan for a decade, but questions about whether the bridge could handle the extra weight have dogged the project for years.

Computer modeling in 2001 concluded light rail wouldn't pose a problem for the bridge. But the state DOT asked Sound Transit to fund more work to make sure.

That led to an unusual test in September. Sixty-five-foot flatbed trucks, loaded with concrete blocks to simulate light-rail cars, were driven back and forth across the bridge while instruments measured the span's reaction.

The results showed "an amazing correlation" with what the computer modeling revealed, Clarke said.

The bridge's newer, westbound span would need some changes to accommodate rail, he said. Those modifications would include:

• Replacing the concrete barrier on the span's south side with a cable rail to reduce weight.

• Removing an inch of the concrete bridge surface and replacing it with a thinner polymer overlay, also to lose weight.

• Adding steel plates to strengthen the elevated, piling-supported roadway that slopes down to the floating sections of the bridge.

• Replacing the 150-foot "transition spans" that link the floating and elevated sections.

Clarke said the cost of those changes was estimated at $31 million in 2001. Sound Transit spokesman Geoff Patrick said the agency has included those expenses in its cost estimates for rail across I-90.

The addition of light rail would not require more bridge closures during storms or shorten the bridge's lifespan, Clarke said.

Light rail across I-90 from Seattle to downtown Bellevue is one of 81 projects the Sound Transit board has identified as potential candidates for the next transit package it hopes to submit to voters for approval, perhaps next fall. On Thursday agency staff members recommended that 20 of those projects be effectively cut from the list because of high cost, low ridership, high risk or other problems.

Pierce County Executive John Ladenburg, who also is Sound Transit's chairman, said the board would decide the projects' fates next month.

Among the endangered projects are: a 1.5-mile light-rail line in Lynnwood, new Sounder commuter rail stations in Richmond Beach and Ballard, bus "flyer" stops on Interstate 405 in Kirkland and Bothell and on I-5 in Shoreline, bus-only lanes on Highway 522 near Bothell, and two Eastside "direct access" ramps that would allow buses and carpools to enter and leave freeways without mixing with other traffic.

Projects spared in the staff recommendations include a 2.5-mile Everett light-rail line, and light-rail extensions from Seattle's University District north to Northgate; from downtown Seattle east to Bellevue, Overlake and Redmond; and from SeaTac to Tacoma.

In addition to light rail, Sound Transit is considering a "bus rapid transit" system for the Eastside that could be converted to rail later. But a Sound Transit study released Thursday warns that conversion costs could make that option more expensive than rail in the long run.

Eric Pryne: 206-464-2231 or epryne@seattletimes.com