Can the Interstate 90 floating bridge be made safe enough to support light rail? Sound Transit and the state Department of Transportation...

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Can the Interstate 90 floating bridge be made safe enough to support light rail?

Sound Transit and the state Department of Transportation (DOT) intend to find out next month with an unusual test using heavily loaded flatbed trucks to simulate the weight and movements of light-rail trains.

The plan is to close the westbound span of the I-90 bridge during much of the Sept. 9-11 weekend and run a string of trucks back and forth along the span’s center car-pool lanes.

Sensors, gauges and instruments at four different points will measure the bridge’s reaction.

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“What we’re looking for is the dynamic response of the vehicle, the moving weight,” said Patrick Clarke, manager of the state DOT’s floating-bridge and special-structures group. “Floating bridges are so complex. There’s so much interaction between cables, water and wind and other things. It’s hard to capture all that accurately in computer models.”

Here’s how the test will work: Eight 65-foot flatbed trucks will drive in formations of four apiece at up to 40 mph to represent a light-rail train. Each truck will carry 148,000 pounds of concrete blocks — the weight of one light-rail car. The test will measure the bridge under different scenarios, including one “train” running and then with two “trains” running in opposite directions.

Both agencies already know Sound Transit’s proposed light-rail configuration is about 30 percent heavier than what the westbound span was designed to handle, said Rosario Revilla, the DOT’s I-90 project manager.

The state requested the test rather than simply rely on computer models because it will better show the bridge’s actual motions and stresses during operation and better guide them toward solutions, Clarke said. Sound Transit has budgeted about $700,000 for the test, roughly the same cost as computer modeling, he said.

The money would also pay for analyzing the test data combined with computer models of wind and wave action, the weight of the usual westbound car traffic, and other components. The results would determine how Sound Transit and the state could alter the span to accommodate light rail — and how much that might cost.

To compensate for the heavier load on the pontoons, the bridge could be made lighter, Clarke said. Options include removing extra pavement from where the train tracks would go, replacing concrete barriers with lighter ones of steel, and removing reserve ballast. The bridge also could be reinforced, he said.

The cities on Lake Washington’s shores, Sound Transit, the state, and various bicycle, highway and pedestrian advocates have debated for years about what type of transit should connect the Eastside and Seattle across I-90, and how best to arrange cars, bikes, people and transit on the span.

Earlier this summer, Sound Transit’s board updated its long-range regional transportation plan to connect Puget Sound communities by rail and bus. The board narrowed the transit options for I-90 to light rail or a high-speed network of buses known as bus rapid transit, or BRT. The board eliminated a monorail from consideration, saying monorail cars would not fit through the I-90 tunnels.

Literally in the middle of the debate is Mercer Island, whose residents for years have enjoyed access to the center car-pool lanes as solo drivers. Under an agreement in 2004, islanders eventually will lose that access when the center lanes are restricted to buses or trains. They say the change will make it tougher for them to access hospitals and other needs in Seattle and on the Eastside.

Results from the load test should be available in October, Revilla said.

Karen Gaudette: 206-515-5618 or kgaudette@seattletimes.com