Reinforced Concrete Damage May Be Larger Than Expected in Seattle Quake

24 April 2019–Using ground motions generated for a range of simulated magnitude 9 earthquakes in the Pacific Northwest, researchers are testing how well reinforced concrete walls might stand up under such seismic events.

The walls may not fare so well, especially within the city of Seattle, said University of Washington postdoctoral researcher Nasser A. Marafi, who studied the phenomenon for his Ph.D. dissertation.

Buildings in Seattle. | Christopher Patterson/ Flickr

The ground motions produced by a long duration, large magnitude earthquake would be amplified in the deep sedimentary basin that underlies the city, and most buildings under 24 stories in the city are not designed to take into account the potential damage produced by such basin effects, Marafi reported at the 2019 SSA Annual Meeting.

“What we found is that the results are actually a lot more damaging than what we would expect,” Marafi said.

With a magnitude 9 earthquake, the maximum story drifts—describing the displacement between consecutive floors on a building—predicted for the reinforced concrete structures are on average 11% larger and are more variable than those used for earthquake building codes that do not account for basin effects.

Marafi’s analysis can’t always predict whether a particular structure made with reinforced concrete will collapse during a magnitude 9 earthquake in Seattle, but the study suggests that structures designed to the current minimum seismic standards may have up to a 33% probability on average of collapse, depending on their design specifications.

His project is part of a larger research effort by scientists at the University of Washington and the U.S. Geological Survey to learn more about what to expect from a magnitude 9 earthquake in the Pacific Northwest. Although there is a historic and prehistoric record of these massive and damaging earthquakes in the Cascadia Subduction Zone, there are no seismic recordings of large magnitude earthquakes in the region.

To remedy this, the researchers have used computer simulations to generate a set of ground motions that might be expected under numerous magnitude 9 scenarios in the region. “Then my work takes the ground motions that those simulations predict and asks what this means for building response. How do buildings respond to this kind of shaking that we’re predicting from this simulation?” said Marafi.

Marafi used 30 of these ground motions to test against 32 computer generated models of reinforced concrete-core-wall structures, between four and 40 stories high. Before designing the concrete models, he met with engineers practicing in Seattle to make sure that his designs were representative of how buildings are currently constructed in the city.

Seismic waves that pass through the deep and soft sediments that lie beneath Seattle slow down and pile up their energy, resulting in damaging large amplitude waves that may be trapped in the basin. Seattle buildings that are 24 stories or less do not have to be designed to withstand these basin effects, but Marafi said this is changing. The next version of the National Seismic Hazard Maps that inform building codes, for instance, will include basin effects for Seattle, and the city is likely to include some basin effects in its design codes for structures 24 stories or less by 2021, he said.

The changes mean that existing buildings in the city may need to be retrofitted and that new buildings would be built with “more steel and more concrete, so that the structure is slightly bigger and ends up being a stronger, stiffer building,” Marafi said.