Seeing Science: Building Better Bridges

With a one-in-three chance that a massive Cascadia subduction zone earthquake— the “Big One”—will happen in the next 50 years, Portland needs to be prepared for its effects on our iconic bridges.

Peter Dusicka is on the case. A professor of civil and environmental engineering in the Maseeh College of Engineering and Computer Science, Dusicka runs PSU’s infrastructure Testing and Applied Research (iSTAR) Laboratory, which is equipped with a large shake table that simulates earthquakes. In the iSTAR lab, Dusicka and other researchers are engineering ways to make bridges, buildings, and other structures more resilient when the Big One comes to town.

Which bridges are in danger?

The Oregon Department of Transportation (ODOT) estimates 60% of Oregon’s critical bridges may be unusable following a large-magnitude earthquake. This includes most of Portland’s famous bridges. And if our bridges fail, it will be hard for Portland to recover from the quake. “Anything from food to fuel needs to make its way across this river,” says Dusicka. “And it’s not just the river. There’s a whole bunch of bridges that people don’t see that are the workhorses of the transportation network.”

Why are Portland's bridges in danger?

Many bridges built prior to the 1970s used an “elastic design” philosophy in which a structure can be deformed by a heavy load—such as an earthquake—and then return to its original shape. However, these bridges were designed before scientists realized the potential size of a Cascadia quake. “If you underestimate the demand of an earthquake, then there is potentially a catastrophic consequence—something we call ‘brittle failure,’” says Dusicka.

What can we do?

There are three options when it comes to preparing bridges for large-magnitude earthquakes: rebuild, retrofit, repair.

(Re) building a better Burnside

Multnomah County plans to rebuild the Burnside Bridge beginning in 2025 so that it can serve as a lifeline through the city following the Big One.

Retrofitting bridges

Replacing bridges is expensive and time consuming. Another option is to retrofit them. Dusicka designed and tested a brace that can strengthen existing bridges. ODOT has already installed the braces on a bridge along Interstate 5.

Repairing broken bridges

For bridges that do fail, getting them repaired quickly is essential. The iSTAR lab has developed—and tested—a method for rapidly repairing damaged concrete bridge columns by wrapping them with a steel jacket and then anchoring them to the foundation with energy-dissipating, replaceable fuses (shown right). Advantages of this method: the steel jackets could be manufactured ahead of time and the fuses help the columns withstand aftershocks, which could continue to shake Portland for months—or even years—following a 9.0 earthquake.