In the 1989 Bay Area earthquake, 42 people were killed when a section of I-880 in Oakland collapsed.
Most of those people were traveling on the lower portion of the double-deck freeway and were crushed to death instantly in their cars.
While the overpasses and underpasses on I-5 in South Snohomish County aren’t double-deck freeways, a strong earthquake would make them subject to the same pancake effect that killed the people in California, said Amir Ahmadi, a project engineer for the state Department of Transportation.
“That’s what we’re trying to prevent,” Ahmadi said about the state’s work to retrofit 13 bridges on I-5 in South Snohomish County, from the King County line to 164th Street Southwest.
Crews recently started drilling and blasting to add extra steel and concrete to the understructure of I-5 where Highway 104 passes under the freeway at the Snohomish-King county line.
The bridges aren’t damaged — they came through the 2001 Nisqually quake intact, Ahmadi said. Their design, however, makes them vulnerable to a strong earthquake, a recent state survey concluded.
The state is spending $12.3 million to shore up the bridges in Snohomish County and six more on I-5 in Tukwila. The work is expected to take nearly two years, until the end of 2010.
Work to retrofit 922 bridges statewide began last year and is targeted for completion in 2015. In addition to those on I-5 in south county, 28 other bridges in Snohomish County are on that list, and their retrofit has been funded, according to DeWayne Wilson, a bridge management engineer for the transportation department.
There are 33 more bridges in the county, and many others statewide, that still await funding, Wilson said. Retrofit priorities are based on seismic risk of a site, structural conditions and route importance.
When workers are on the job, as they are now at Highway 104, it likely will barely be noticed by drivers. Cones will be placed along the road to alert drivers that workers are nearby, but disruptions will be minimal, Ahmadi said. Occasional lane closures will be needed.
In most cases, the work will take place on platforms built specifically for that purpose, above the bridges’ support columns near the girders.
Workers at the Highway 104 underpass have drilled holes into concrete crossbeams, preparing to install rebar and concrete to widen those beams by about a foot-and-a-half on each side, Ahmadi said.
When the earth shakes, it tends to make bridges move side to side, he said. In that event, the ends of the girders that sit on the crossbeams can swing and fall off, he said. This happened on I-880 in Oakland, Ahmadi said.
The girders “moved beyond the limits of the crossbeam and just fell down,” he said.
With the extra concrete, they would have to swing farther before reaching the dropping-off point.
Another measure involves simply adding concrete between the girders, along the top of the crossbeam. This will help keep the girders from moving to begin with, Ahmadi said.
In some cases, where support columns are older or there are fewer of them, steel jackets will be wrapped around them to keep them from crumbling.
I-5 was built in the mid-1960s, and many of the columns were installed back then, said Dave Schrader, chief inspector for the Department of Transportation.
Lanes and ramps have been added continuously since, Schrader said, meaning that pillars on I-5 bridges range in age from just a few years to up to 40-plus.
The section of I-880 in Oakland that collapsed in the earthquake opened in 1957 — four years after the Alaskan Way Viaduct in Seattle. Officials have chosen a $4 billion tunnel as the preferred option for replacing the viaduct.
Bill Sheets is a reporter with The Herald of Everett
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