Some of the fastest-growing cities in the world sit in high-risk earthquake zones. That’s why researchers are trying to figure out how to build tall buildings using a material that’s not only plentiful and renewable, but even more resistant to earthquakes than conventional building materials.
What 7th Century Japanese Builders Knew
The perfect example of timber’s ability to ride out big quakes dates back almost a thousand years to pre-modern Japan, where builders began stacking multiple wooden levels to create high-rise temples that reached hundreds of feet into the air.
Kyoto Pagoda Toji by Kalexander2010 on Flickr/CC.
These structures used a few different methods to endure extreme shaking, including looser connections between structural members that allowed some swaying. The weight of the structure is distributed and connected between floors with loose joints, as the Economist explained in 1997: “What joints there are between the floors are loosely fitting wooden brackets that allow each storey to slither around.”
Another fascinating element of the design of these pagodas? All those swaying floors were supported by something called a shinbashira, a remarkable early version of the huge steel mass dampers you’ll find in the tallest buildings in the world today. The Economist elaborates:
What the early craftsmen had found by trial and error was that, given a hefty sideways shove, a pagoda’s loose stack of individual floors could be made to slither sideways to and fro independent of one another. Viewed from the side, the pagoda appeared to being doing a snake dance—with each consecutive floor moving in the opposite direction to the ones immediately above and below. But if a big fat shinbashira ran up through a hole in the centre of the building like a very loosely tightened bolt, each storey would then be constrained from swinging too far in any direction by banging internally against this central fixture. Better still, each time a storey collided internally with the shinbashira, it would dump some of its energy into the massive central pillar, which could then disperse it safely into the ground.
As a result, many of these temples have withstood massive earthquakes and remain standing to this day.
Kyoto Pagoda Toji by Victor Lee on Flickr/CC.
The Material of the Future Is the Material of the Past
Over the past decade, research into tall timber buildings has intensified. The US government has poured money into it, promoting timber as a renewable resource that can be used to build tall thanks to new manufacturing processes.
Last year, the White House launched a competition that rewarded designers for developing tall building designs that used a novel type of wood called Cross Laminated Timber, or CLT. “By some estimates, the near term use of CLT and other emerging wood technologies in buildings 7-15 stories could have the same emissions control affect as taking more than 2 million cars off the road for one year,” the organisers wrote.
The Wood Innovation and Design Centre, a six-story timber building in Vancouver that uses Cross-Laminated Timber.
There’s another overlooked aspect of timber that’s growing increasingly clear, too: Some types of wood construction are surprisingly good at withstanding major earthquakes, just as those Japanese builders knew–though the building technology has changed.
Take the research being done by engineers at University of Alabama, where a National Science Foundation-funded grant is helping a team led by Professor Thang N. Dao study the earthquake resistance of tall wooden buildings. They’re experimenting with combining conventional light wood frame buildings with a newer wood material–the Cross Laminated Timber, or CLT, as Global Construction Review reports. By combining these two types of wood architecture, they think they can build tall buildings that respond to shaking better than conventional steel ones.
CLT is exactly what it sounds like: A timber beam that’s been created by sandwiching layers of timber with their grains moving in opposite angles. This creates a piece of wood that’s stronger than the sum of its parts, thanks to the criss-crossing direction of the plates.
CLT blocks by the Oregon Department of Forestry on Flickr/CC
CLT is being used in tall wood buildings going up all over the world, but the engineers at Alabama want to study how well it resists earthquake shaking, a unique type of structural stress that subjects buildings to lateral, or horizontal forces. Their hybrid approach takes slabs of this super-strong, heavy CLT timber and post-tensions them (reinforcing it with steel rods that are tensioned after the fact) and combined it with standard lumber. The CLT will “self-centre the system,” and the conventional light wood framing will “dissipate the energy created during an earthquake.”
This hybrid between the heavy, rigid CLT and looser, lighter wood “should ensure main structural parts of the building remain elastic during an earthquake with no damage, making the system resilient,” they say.
A different shake test at University of Alabama, in August, 2015, via Facebook.
Similar work on post-tensioned CLT is being done across the country–for example, at University of San Diego–but the team in Alabama are focusing on high-rise timber buildings in earthquakes, which is a pretty novel concept. They’ll use the school’s Large Scale Structures Lab, where a huge shake-table will simulate major earthquakes to test how their hybrid system will work, a bit like the test above.
What’s coolest about the project to develop these new skyscrapers is that the basic concept isn’t all that different from what was being built with wood in the pre-modern world, as in Japan. We’re really just reinventing the same wooden wheel–and making it a little taller.
Lead image: MK Forty Tower, dRMM Architects, Milton Keynes by Denna Jones on Flickr/CC
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