Unless the West Seattle Bridge’s cracked mainspan can be shored up soon, it’s headed toward a “partial collapse” that could spread and destroy adjoining parts of the concrete crossing, a new report says.

The seven-page failure analysis, dated Friday, explains why the city must race against time to temporarily support the 150-foot-high mainspan. The bridge will not be fully repaired for at least two years, if ever.

The report also shows why the Seattle Department of Transportation (SDOT) said last month that it would need to evacuate a long “fall zone” from the Pigeon Point greenbelt to Harbor Island, including the low swing bridge, if the high bridge deteriorates to the brink of collapse.

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The report was requested by Mayor Jenny Durkan to improve emergency preparedness, and it was initially due by the end of April. SDOT closed the bridge March 23 after Matt Donahue, city structures director, saw that a group of cracks first discovered in 2013 had accelerated roughly two feet in two weeks.

“The previously observed acceleration in the cracking could indicate that the risk of failure is increasing, and the time to potential failure shortening,” says the report by four consulting engineers at the firm WSP.

The consultants didn’t estimate the odds of a collapse, or when that might happen.

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“This bridge’s issues are unique,” the report says, therefore worldwide failure data and case studies won’t help to provide a probability figure.

Last week, the city finished adding new measuring devices to warn if the bridge approaches failure. These include eight cameras, 16 movement sensors and 52 crack monitors that allow round-the-clock checkups, spokesman Ethan Bergerson said.

“Other actions, such as designing the temporary and permanent stabilization repairs, are just beginning,” WSP’s report says.

Without a prompt fix, there are two possible outcomes:

The first is that cracks keep progressing and then stop, while forces within the bridge stabilize.

“However, this is not likely as the bridge will continue to creep (slowly deform under its own weight) over time and thus continue to crack,” the report says.

The second outcome is called “partial collapse.”

But that scenario could actually trigger a full collapse of the 590-foot-long mainspan, and possibly adjoining portions of the 220,000-ton bridge.

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In a symmetric bridge failure, multiple zones now laced by 45-degree shear cracks would rupture together. The central bridge section would rip away from the supporting steel that runs through the bridge lengthwise, so a central chunk plops into the Duwamish Waterway. The resulting release of forces may cause the concrete stubs, still attached to the tall piers, to recoil outward and crack the adjacent spans.

In an asymmetric bridge failure, the shear cracks in one weak zone would grow until both concrete and steel rupture, causing a partial collapse.  Within moments, the opposite fragment that’s still aloft would yield to gravity and buckle — so like a teeter-totter, the drooping fragment turns the pier into a fulcrum and generates uplift that ruins the adjacent span.

A similar two-phase breakdown happened in the 1996 Koror Bridge collapse between islands in Palau.

The report recommends “temporary shoring to support the bridge in case of partial or multi-span superstructure collapse.” Added internal steel, and carbon-fiber wrap are likely methods to stabilize the bridge in mid-2020, Donahue has said.

This risk of cascading damage explains why the city depicts five spans at risk in its fall zone map released May 4. Temporary bracing can limit the extent of danger and damage until the city figures out permanent repairs or demolition.

“It’s really just managing the fall zone, so that if pieces were to fall, the fall zone would cover less area,” Alex Pedersen, chairman of the City Council’s  Transportation & Utilities Committee, said Saturday. Pedersen said he believes SDOT leaders are competently protecting public safety and making good use of outside experts to help repair the bridge.

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Even in a multi-span failure, houses on Pigeon Hill are far enough from the bridge to avoid any falling debris, the emergency-preparedness map shows. 

The city diagramed a 225-foot evacuation area on either flank of the bridge, out of an abundance of caution, although WSP’s review predicts that any broken concrete probably would fall straight down.

A collapse is not imminent, city transportation director Sam Zimbabwe said Thursday at an online West Seattle residents’ forum. 

“We have the asset, and it continues to deteriorate, and we are working as quickly as we can to stabilize it,” he said. The rate of crack growth has slowed since the bridge was closed March 23, he said.

Councilmember Lisa Herbold of West Seattle wrote in her blog Friday, “I appreciate SDOT’s candor. SDOT indicates that after a few weeks’ worth of data, they will have a clearer sense of the stability of the bridge, what work can be done.”

Zimbabwe urged residents to sign up for online notifications with Alert Seattle.

“This is especially important right now, in the event the high rise bridge moves toward failure,” he said. “Right now, it continues to be stable.”