Hollow Columns Holding Up I-5, I-405 Bridges Could Implode In Major Quake

Mar 23, 2016

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Some of Washington state's busiest bridges have a surprising design feature deep inside their massive structures.

It's air.

The concrete columns supporting these bridges are hollow, and it could mean big trouble in a big earthquake.

"If a severe earthquake hit, there's a high risk that the inside could break loose, and the column could actually implode," said DeWayne Wilson, a bridge engineer with Washington State Department of Transportation. "It would collapse on the inside. We don't know exactly the level and the mechanism. But it is a high concern."

Some of those bridges are being replaced. Construction is under way to replace bridges along state Route 520 in Seattle and on Interstate 5 in Tacoma as it passes over the Puyallup River.

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But there are no plans to prevent other hollow columns — beneath Interstate 405, Interstate 5 or elsewhere — from collapsing in a big earthquake.

Western Washington has more than 20 bridges built like this, many of them on the region's busiest highways.

WSDOT officials have known that the columns are vulnerable for at least 15 years. Engineers have known for twice as long.

Wilson said the agency is more concerned about regular deterioration on older bridges. He said the greatest risk of bridge failure in Washington and nationwide is from rivers scouring away bridge foundations.

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More than 20 bridges in Washington state – including this bridge on Interstate 405 over the Green River in Tukwila - have vulnerable, hollow columns.
Credit KUOW Photo/John Ryan

Hollow Hotbed

WSDOT is responsible for some 3,200 bridges around the state.

Compared to other earthquake-prone regions, western Washington has an unusual concentration of highways on hollow supports.

In the early 1960s, when America's new interstate highway system was growing at breakneck speed, a Tacoma company called Concrete Technology pioneered the use of a stronger material called pre-stressed concrete on highway projects. The stronger concrete meant builders needed less of it to make a column strong enough to hold up a bridge.

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Those hollow columns were light enough to be built in the Tacoma factory and then trucked to construction sites around western Washington.

Local bridge builders snapped them up, and the Puget Sound region became a hotbed of hollow-column bridges.

It made sense at the time. The hollow columns are nearly as strong as solid ones—at least when the earth isn't moving.

"Especially in the early 60s, earthquakes just weren't a thought to most bridge engineers,” Wilson said. 

On the East Coast and in other areas where earthquakes are rare, hollow-column bridges are still being built, according to Concrete Technology engineer Steve Seguirant.

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"In areas where there aren’t seismic concerns, they’re still very popular," he said.

In western Washington, Seguirant said hollow-concrete columns are still used for docks and piers. In those waterfront applications, they're used in tandem with other columns built to absorb the sideways forces of an earthquake, so the hollow columns won't have to.

Washington’s Hollow Columns. More than 20 bridges built in the early 1960s around western Washington rest on hollow concrete columns. KUOW map using data obtained from WSDOT with a public records request.

'Stop Talking And Run'

John Stanton, a University of Washington engineering professor, stood beneath Seattle’s Portage Bay bridge on SR 520, which is supported by hollow columns. He was in a grove of concrete columns in the Montlake neighborhood, with the humming highway roughly 50 feet overhead.

If an earthquake hit, he would "stop talking and run to get out from underneath the bridge very rapidly. This is not a good place to be in an earthquake," Stanton said.

Stanton's experiments in the 1980s showed that hollow columns could implode in a big quake.

"A couple of other people had noted the same thing before that, so it wasn't news," he said. "At the time, I was not aware that WSDOT had built a lot of bridges using these very large hollow columns."

Decisionmakers didn't pay much attention to WSDOT's large, hollow columns until a barge accident on Lake Washington.

One night in the summer of 2000, a wayward gravel barge plowed into the state Route 520 bridge. A column shattered. The accident helped spur the current megaproject to replace a half-dozen bridges on 520 and its various ramps.

A year later, the 2001 Nisqually earthquake generated more interest in making the region's infrastructure more resilient. But WSDOT engineers decided hollow-column bridges weren't a priority.

"Nisqually, these bridges survived very well," Wilson said. "They didn't experience any damage at all."

Wilson said he's pretty confident that most of the state's bridges can weather the moderately large earthquakes, like the Nisqually quake, that routinely hit the region.

University of Washington professor John Stanton's experiments in the 1980s showed that hollow columns could implode in a big quake.
Credit KUOW Photo/John Ryan

But the Nisqually quake was nowhere near as big as what might hit us: a magnitude 9 megaquake from the Cascadia Subduction Zone off the Northwest coast.

"If we get the big one that they're expecting, which would be the subduction zone earthquake, there's a potential for damage at different places," Wilson said. "Probably the most logical place is up where the column meets the bridge."

Geologists say there's about a 10 to 15 percent chance of a big one from the Cascadia Subduction Zone hitting us in the next 50 years. A big one could also strike from the Seattle Fault that runs right through the city.

So far, WSDOT has spent $193 million to seismically retrofit old bridges that don't have the problematic hollow columns.

The agency has focused mostly on bridges that would be critical if the Seattle-Tacoma region needed to recover from a massive earthquake. WSDOT calls it the "seismic lifeline." Its goal is to prevent key bridges from collapsing in a megaquake.

That partially completed lifeline is in long-term limbo for lack of funding.

"The big picture is, we've addressed a lot of seismic needs, but there's a lot more to go," Wilson said.

"Over the next six years, we are not funding any additional seismic retrofits," he said.

"Wow. That's unfortunate," said Jugesh Kapur, former state bridge engineer. "California and others, they've really paid a lot of attention to their retrofit program and taken care of all their major bridges." Kapur has been working as an engineer in North Dakota after WSDOT fired him in the wake of major design flaws on the SR 520 bridge project.

A man walks under the I-5 Colonnade Park, where the columns are hollow. Piping concrete into these columns wouldn't help, says a UW professor.
Credit KUOW Photo/Isolde Raftery

An Expensive Operation

There's no known way to make hollow columns resist earthquakes better.

Attempting to fill a hollow column with concrete would probably just cause it to burst from the added pressure.

WSDOT has wrapped cylinders of steel around some solid bridge columns. But such a steel jacket wouldn't stop a hollow column from crumbling apart on the inside.

WSDOT has asked John Stanton to see if there's a way out of this conundrum.

Stanton said filling columns with some sort of lightweight material seems promising, though it would probably require pouring it in through a hole in the side of each column.

"And making a hole in the side of a column doesn't seem like a very good idea if the column is already thought to be too weak," he said.

Stanton believes the columns can be strengthened, but "with great difficulty. I suspect it'll be quite an expensive operation."

Stanton said if WSDOT funds the work, he would need about a year and a half to study a possible fix. Any actual fixes would be many years down the road.

The state legislature fully funded replacing the remaining hollow-concrete bridges near the western terminus of state Route 520 in Seattle last year.

That work will take more than a decade to complete.

Even if fixing the missing links in the region's seismic lifeline would be expensive, Stanton said doing so now makes sense.

"Right now, we're in a period of very low interest rates, and this would be a perfect time to invest in infrastructure because, essentially, you can buy cheap," Stanton said. "So, why aren't we doing that?"

Of course, all levels and agencies of government, not just WSDOT, have competing demands for the tax dollars they spend.

WSDOT estimates more than $1.4 billion of seismic upgrades are still needed on the state's highways. That figure doesn't include an estimate for replacing or fixing the state's hollow columns.

"Unfortunately, it will take another seismic event for this whole program to come in the limelight again," Kapur said.