Seattle is updating building codes for new skyscrapers after a shocking study revealed that the power and nature of earthquakes in the region pose a significant threat to its tall buildings, one that is worse than experts could have imagined.
The Seattle Times recently reported on results from the M9 Project, a four-year study that aimed to estimate the effects of a magnitude 9 Cascadia earthquake, revealed that the vast, sediment-filled basin under Seattle can magnify the type of ground shaking that puts high-rises at risk of collapse by a factor of two to five, which can trigger stronger surface effects than earthquakes in nearby California.
When rattled by an earthquake, the Times explained, the region’s glacial-packed soils, which can extend more than four miles underground, violently shake and convulse, trapping massive seismic waves within the basin that underlies most of the city. Intense shaking like this could last for two minutes, which is four times longer than the average quake. While shorter buildings under 10 stories can withstand the earth’s back-and-forth motion, tall buildings tend to whip back and forth under those conditions until they are on the verge of collapse.
As the Times reported, as a result of these findings, Seattle and its neighbor, Bellevue, plan on revising seismic construction standards for new buildings over 240 feet, or over 20 stories tall. These standards will require high-rises to be stronger and more sturdy than their predecessors, without the additional cost.
The plan to revise building codes has also raised concerns regarding Seattle’s older high-rises, many of which were constructed between the 1960s and 1990s, prior to when the dangers of earthquakes were fully understood. Older high-rises have a greater risk of major damage and collapse due to their fracture-prone welded joints, which are supposed to secure the steel frame, as well as their poorly-reinforced concrete supports. Seattle’s renowned Rainer Tower, for example, with its golf-tee-shaped base, was built in the 1970s and undoubtedly has fracture-prone welds. According to the study, buildings like that are up to five times more likely to collapse during an intense earthquake than a modern building.
In Seattle, where the seismic threat to skyscrapers is higher than in California because of the city’s sedimentary basin, there have still been no attempts to research and pinpoint dangerous high-rises. While the city is taking steps forward by enhancing construction standards, retrofitting old concrete and steel high-rises may be the next necessary step. This may prove costly, but taking time to fix the underlying structure of older buildings could prevent serious levels of damage that can be catastrophic to the community.