SDOT plans new cables and carbon wraps to keep West Seattle Bridge standing

Armed with carbon strips and steel cables, construction workers will soon begin emergency shoring of the West Seattle Bridge in an effort that could last until late October.

The Seattle Department of Transportation (SDOT), in a blog post describing the methods, said Monday that the job will begin this week.

The first step is to reopen more than 100 holes on the edges of the deck that were filled following construction in the early 1980s. These holes were once used to mount a crane-like gantry, called the form traveler, that moved the molds where builders poured concrete 16.5 feet at a time.

Traffic Lab is a Seattle Times project that digs into the region’s thorny transportation issues, spotlights promising approaches to easing gridlock, and helps readers find the best ways to get around. It is funded with the help of community sponsors Madrona Venture Group and PEMCO Mutual Insurance Company. Seattle Times editors and reporters operate independently of our funders and maintain editorial control over Traffic Lab content.

The leftover holes will be opened by high-pressure water jets, providing a place to fasten hanging platforms, SDOT said.

Workers standing on those platforms will fasten carbon wrap with epoxy to strengthen weak areas in the bridge. Carbon will also be attached inside the hollow girders. That task will begin in late July and require 10 weeks, SDOT estimated.

Rapidly accelerating shear cracks, concentrated in four areas, caused city structures director Matt Donahue to close the bridge March 23, nearly seven years after city inspections discovered initial cracking at a much smaller scale.

Carbon fiber has been used to reinforce cracked columns on the former Alaskan Way Viaduct, and to fortify the aging 15th Avenue Northeast bridge over Cowen Park.

The second major stage is to string additional steel cable across the central span of the bridge, to compress and strengthen the concrete, in a technique called post-tensioning.

The 590-foot-long mainspan of the bridge originally was post-tensioned through its middle third — but dangerous cracks formed in weaker zones just outside of where that original steel is anchored. New cables are intended to clench the weak zones.

To anchor those longer cables, steel blocks will be fastened to the concrete underside, and partly visible from both the waterway and the low swing bridge. Cables will be stretched taut inside the girders. That phase will be completed a week or two after the carbon wrapping.

Only then can the contractors, Kraemer North America, proceed to a third necessary job, to release a stuck rubber bearing atop Pier 18, on the Harbor Island side east of the river. The deformed piece thwarts normal thermal deck expansion and contraction of a few inches, adding stress to the structure.

These efforts are merely to shore up the bridge, so it will remain standing throughout either an orderly demolition, or some long-term fix.

Monday’s timeline throws into question whether the city can make a prompt decision about whether to perform full repairs to last maybe 10 years. SDOT Director Sam Zimbabwe previously said this spring the city can’t make such decisions until solving Pier 18. A cost-benefit study by engineering firm WSP is scheduled by early fall.

SDOT did not make engineers, or members of its technical panel of outside experts, available for interviews Monday, or issue cost estimates.

“Though our recent analysis indicates that repairing the West Seattle High-Rise Bridge may be possible, we still do not know how much repairs would cost, how long they would take, how many lanes could be restored, and whether repairs would last long enough to be a worthwhile investment,” a city statement Monday said.