I-90 bridge’s Link retrofit almost finished

A worker inspects a reaction frame inside a pontoon. Credit: Sound Transit

Sound Transit announced on Wednesday that construction crews are nearly done with their work retrofitting the I-90 bridge for East Link. Crews have worked for more than a year to post-tension the bridge's pontoons.

ST reinforced the bridge to help it carry the load of Link's tracks, overhead lines, and vehicles. The retrofit also improves the integrity of the bridge in heavy wind and an attendant storm surge, the likes of which sank the eastbound span in 1990.

All the Lake Washington floating bridges use a system of tensioned steel cables to hold the span in place. Construction crews installed additional cables in the pontoons of the I-90 span. The cables, which crews stressed and winched, pull the pontoons closer together, which creates greater load bearing capacity. Altogether, according to Sound Transit's Zach Ambrose, "crews installed and stressed 1,080,000 feet of steel strand and applied 41,000 pounds of pressure."

Steel cable and the frame that anchors it. Credit: Sound Transit

Giant steel structures, called reaction frames, anchor the new cables at both ends. They absorb the force from winching the steel cables, and whatever event that might stress the structure. Each of the ten reaction frames weigh about 17,500 pounds.

The structural work is finished. Crews are removing equipment from the pontoons and grouting the housing of the cables, to prevent water corrosion. After that work is done, construction of the guideway can start.

A diagram of the retrofit. Credit: Sound Transit