How many Oregon bridges will hold up in a major earthquake?
New data compiled by the Oregon Department of Transportation reveals at least part of the answer. The results aren't pretty for a region facing 1-in-3 odds of a magnitude 8 or larger earthquake in the next 50 years.
ODOT data details the seismic vulnerability of bridges under state control. The data focuses on “lifeline” routes, or routes deemed critical to emergency services and necessary for “rapid economic recovery after a disaster."
The report focuses only on bridges operated by ODOT, and doesn't include nearly 4,000 county or city controlled bridges.
Of 1,232 lifeline structures ODOT identified, 713 bridges are considered seismically vulnerable or potentially seismically vulnerable. That's nearly 60 percent of state-identified lifeline bridges likely to collapse or be potentially taken out of use after a quake.
The criteria used to evaluate bridges includes age of the bridge, type of construction, and whether seismic retrofits had been completed.
The map reveals something that has been abundantly obvious to state bridge engineers for decades — more than half of Oregon’s 2,800 state bridges were built before 1970 and have no seismic design of any kind. State geologists predict Oregon can expect three to five minutes of shaking in a magnitude 8 or 9 Cascadia Subduction Zone quake. By comparison, a far smaller magnitude 6.9 earthquake was enough to collapse several bridges and overpasses during the 1989 Loma Prieta earthquake in San Francisco.
What can be done?
Oregon bridge engineers did not design for Cascadia-level earthquakes of magnitude 8 or 9 until the mid 1990s. But engineering standards for new bridges in Oregon are now among the toughest in the country. Oregon is the only state that requires bridges to be built to survive a once-a-millennium earthquake, and be serviceable within 72 hours after a quake.
But the design standards only apply to new bridges, and there's an important caveat.
New bridge design standards do not require seismic upgrades to soil under bridge approaches and roads leading up to bridge spans. Soil along rivers, in particular, may be more vulnerable to soil failure or liquefaction.
Not requiring soil reinforcement under approaches “is an economic thing,” said Bruce Johnson, state bridge engineer for ODOT.
“We believe we will be able to establish a usable road grade leading up to a bridge not too long after a quake,” said Johnson.
What hasn’t been done?
The Oregon Legislature provides enough funding to replace 1-3 bridges a year. There are 2,800 bridges in the state highway system.
Most bridges have a lifespan of roughly 50-75 years, and the most practical way to upgrade bridges for an earthquake is a retrofit. But currently, Oregon doesn't set aside specific funding to seismically retrofit bridges.
In contrast, Washington has spent over $100 million retrofitting nearly 200 bridges, or approximately 20 percent of bridges it's identified as at-risk.
California has spent 13 billion on seismic retrofits since 1989.
“The big difference," said Johnson, "is we haven’t have the big crustal earthquakes (like Washington and California) to get us more aware."