At 100 years old, the northbound span of the Interstate 5 Bridge has allowed millions of people from several generations to easily travel between Vancouver and Portland, but not without a lot of work and public investment.
“It doesn’t come cheap to maintain an older bridge of that particular design type,” said Bruce Johnson, state bridge engineer for the Oregon Department of Transportation.
ODOT operates and maintains the bridge, but the costs are split between its owners, Oregon and Washington.
To many living in the region, the Interstate Bridge is just one feature of a regular commute. But Johnson sees an old structure with lots of maintenance needs.
Steel truss bridges, such as the I-5 Bridge, are flexible by nature; that combined with the fact that the bridge is old, supports loads far greater than it was designed for, has issues with corrosion and was riveted together — which makes it prone to fatigue cracks — can cause a feedback loop of problems.
Johnson says ODOT has ways of monitoring and repairing damage as it arises. But as Oregon’s infrastructure inventory grows and more bridges age beyond their design lifespans, he worries about being able to stay on top of all the problems.
“I like to think we know the risks, but there’s a risk we might miss something,” he said.
Johnson said it costs about $1 million per year for routine maintenance and repairs to both spans of the bridge. For more substantial work, the price tag jumps significantly. For example, repainting the bridge, which has to be done every 15 to 20 years, costs $30 million to $35 million per span.
In 2019, the northbound span will close to replace two trunnions in the southern tower. Trunnions are the axles at the top of the lift towers that play a role in raising and lowering the bridge. The trunnions were installed in 1916, but one started cracking due to modifications made to it in the 1958 overhaul. Officials estimate replacing them could cost between $10 million and $15 million.
“But its too soon to tell what that number will look like,” Washington State Department of Transportation spokesman Bart Treece said. “It’s early, and we’re still two years away from design.”
The repair mirrors one made in 1997 to the north tower trunnions. Replacing those cost about $6 million.
Fault lines
When the northbound span was erected, it represented an unprecedented infrastructure investment made by men who foresaw the region’s future of growth and need for mobility. But they were blind to the dangers beneath their feet.
On the Oregon side, the bridge sits on concrete piers that stand on timber pilings. The pilings reach down between 60 and 70 feet into the riverbed’s liquefiable soils. They fall short of bedrock, which is about 200 feet below the surface.
In their final report in 1918, John Lyle Harrington and Ernest E. Howard, two consulting engineers who worked on the northbound span, acknowledged the bridge’s foundation didn’t reach solid ground, but they didn’t seem concerned. They wrote, “Even though some unusual scour should, on some extraordinary occasion, extend below the bottom of the base, the pier would still be perfectly stable, owing to the great depth of penetration of the piles and to the rigidity of their upper support.”
Today’s engineers don’t share that view.
What Harrington and Howard and everyone else in the country didn’t know until recently was that the region is due for an “extraordinary occasion” in the form of a large earthquake associated with the Cascadia Subduction Zone.
Scientists at Oregon State University believe the chances of a major earthquake — magnitude 8 or higher — striking the Pacific Northwest in the next 50 years are about 1 in 4.
Johnson, from ODOT, said there hasn’t been a definitive study on how large of an earthquake the bridge could stand. But he said ODOT officials believe a magnitude 5 earthquake at one of the two faults in the Portland area could badly damage it. Those two faults have long intervals between events, he pointed out.
Allison Marie Pyrch, an associate geotechnical engineer and president-elect of the Oregon section of the American Society of Civil Engineers, said retrofitting either span of the bridge to withstand an earthquake anywhere close a magnitude 8 would be a very daunting task.
“Between connections, moving parts and foundations, that bridge is very vulnerable,” she said, adding that the bridge already has problems because it wasn’t designed for the volume of traffic it supports.
Today, both spans are considered functionally obsolete by the federal government. The northbound span scores a sufficiency rating of 18.5 out of 100.
Johnson said there has been no detailed engineering or structural analysis of what it would take to seismically retrofit the bridge. But retrofits on similar bridges around the country were around $250 million, and other estimates to build a substructure for a new bridge ran as high as $500 million.
“Nobody’s been willing to pay for that detailed analysis,” he said.
Neither Johnson nor Pyrch fault the designers for not building to risks they didn’t understand, but they both say a new bridge, while expensive, would be safer and easier to maintain.
In their 1918 final report, Harrington and Howard said if their bridge and its approaches were properly maintained, “there is hardly any limit to the life of the structure.”
Today’s engineers would likely disagree. Most bridges they design today are meant to last between 50 and 75 years.
“It was good engineering for the time; they just didn’t have all the information and methods we have now,” Pyrch said. “Both (spans) are well past design life. In general, infrastructure only lasts so long.”
