August 21, 2013
How to best dampen the "Big One"
Construction Corner | Korky Koroluk
We hear, from time to time, that Tokyo is due - or past due for a major earthquake. But the science of quake prediction is still imprecise, so no one is sure when "The Big One" will happen or how severe it will be.
People have been saying the same thing about the Vancouver area, too, but so far, nothing has happened.
Now a Tokyo real estate developer has decided to do something that hasn’t been done before. The company’s Shinjuku Mitsui Building is 39 years old and 206 metres tall. It has survived a number of quakes during its lifetime, but the developer Mitsui Fudosan, has decided to have the construction firm, Kajima, retrofit the building with six mass dampers installed on the roof.
Mass dampers are common on tall buildings in earthquake zones now, but they weren’t when the Mitsui building was erected in the mid-1970s. Installing them now as a retrofit is expensive — about $51 million — but the building still has a long useful life ahead of it, and the developer wanted to protect it.
Tuned mass dampers, to give the devices their full name, are basically just pendulum-like counterweights — huge concrete blocks, usually—that pull a building’s mass in the direction opposite the prevailing vibrations. So when the ground under the building moves laterally, the counterweight moves in the opposite direction, taking the building with it.
To do their job, the dampers have to be heavy, of course, and each weighs about 300 tonnes.
The company’s engineers say the dampers should cut swaying in a quake by half, preventing structural damage that otherwise might bring the building down.
A rendering of the building’s roof shows three pendulums side by side on both sides of the building in derrick-like structures.
Dampers are usually installed during a building’s construction; what’s new here is that they are retrofitted on an older building. And if they work as expected, we should expect to see similar retrofits on older tall buildings around the world, wherever quakes or storms pose a potential threat.
The technology underlying dampers is well proven. Taipei 101, a skyscraper in Taiwan, needed to be able to withstand both the quakes and typhoon winds common in that part of Asia.
A steel pendulum weighing 660 tonnes was designed and installed near the top of the structure, suspended from the 92nd storey down to the 88th. During the big Sichuan quake in 2008, the building could be seen swaying to counteract the vibrations. It was sort of weird looking, apparently, but it worked.
Throughout history, people have tried to figure ways to limit quake damage in large buildings, and several techniques were involved.
The city of Los Angeles also installed a base-isolated system in its city hall, placing large rubber puck-like devices beneath each of the building’s bearing columns, effectively isolating the building’s base from the earth beneath it.
A new technique? Hardly. Cyrus the Great, of Persia, died in 530 B.C.E., and his mausoleum has a base-isolated system.
On the other side of the world, the Inca’s master masons used polished “dry-stone walls” or ashlar, with no mortar, to build the city of Machu Picchu, high in the Andes Mountains. Many of the junctions in their stonework were so perfect that even blades of grass couldn’t fit between the stones.
But the stones could move slightly, then resettle without the walls collapsing.
In Japan and in much of the Asia-Pacific region, dampers are about as good as it gets. And people living in earthquake-prone areas in western North America, might want to think about retrofitted dampers on older tall buildings as a way of getting ready for The Big One, when it comes.
Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments or questions to firstname.lastname@example.org.
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