Move over, steel: The high rises of tomorrow are "plyscrapers"
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Humans have been crafting shelters out of good old reliable wood ever since our ancestors solved the problem of chopping down trees. In everything from simple huts made of branches, to broad pavilions with large timbers set in post-holes, wood has always featured prominently.
In Europe, there's evidence of wooden structures from at least 4000 B.C. Humans were building probably well before that, given that wood typically doesn't hold up well after being buried for millennia. And the ones that are still standing today aren't mere hovels. Switzerland claims the House of Bethlehem, from 1287 A.D. The famous Urnes stave church in Norway was erected around 1130 A.D. And the great-grandtemple of them all, standing at 122 feet tall, is Japan's Horyu-ji pagoda. It was built in 607 A.D.
But wood can only build so high. So as cities grew, builders began to use new materials. The modern skyline in many cities is almost exclusively shiny glass and polished stone. The materials are supported by steel and concrete.
Yet now, wood is making a comeback in a new way.
One big boon, which architects and engineers hope will make big buildings lighter, cheaper to build and more environmentally friendly, is a material called cross-laminated timber. Just call it CLT.
"I've been here for nearly 30 years now. And during that time there've only been a few items that have generated a bit of buzz and interest. This is one of those items," said David Kretschmann, a research engineer with the U.S. Forest Service's Forest Products Laboratory (FPL) in Madison, Wis.
In use in Europe for nearly 20 years, CLT and other so-called mass timber products have been key design elements. When finished, University of British Columbia's Brock Commons 18-story building will hold the record for tallest CLT-constructed structure. And 18 stories is by no means the limit. Swedish architects have proposed a 34-story wooden building. Researchers and architects have designed an 80-story, one million square-foot giant at the Barbican in London, primarily made of wood. At 984 feet tall, it would rank as the world's 18th tallest building, just ahead of Four World Trade Center in New York City.
The benefits of CLT are numerous. As strong as any old-growth timber and nearly as strong as steel, it can be made from leftover sawmill scraps as well as new material. Panels can be used as floors or walls. By using fewer concrete or steel support structures, or none at all, the overall weight of a building made from wood is far less. So it requires much less foundation concrete.
So if it's so great, why aren't cities sprouting mass timber buildings like a fertile garden? Partly, because building laws haven't caught up yet. It's also partly because it's not a known quantity.
"It's something new to the U.S. market so there's a basic lack of familiarity with it," said David Barber. He is a fire engineer and principal with Australia-based design-build firm Arup. "Officials haven't touched it or seen it in construction, so there is a significant barrier. It's very hard to approve something when you've never seen it."
Fire also is a big question, and rightly so, given that wood is not only a building material but a fuel. Virtually every great city in history has also had one or more "great fires," including Constantinople, Rome, London and New York.
Consequently, fire testing is extensive, rigorous and therefore costly, running in the hundreds of thousands to millions of dollars, even though timbers as thick as CLTs are slow to ignite, especially when covered with drywall or gypsum board.
"It's not like putting a lot of little toothpicks up together," Kretschmann said. "These big massive panels require a lot of energy to burn."
In fact, CLTs do endow one potential advantage when it comes to fire: when enclosed within the wood structures, the steel and other fasteners are less likely to soften and fail in a fire event.
Seismic performance is another concern that's under investigation at the moment. Kretschmann's partner at FPL, Doug Rammer, is participating in several National Science Foundation-funded projects to better understand how buildings with CLTs as a core design element can be better designed to withstand earthquakes.
And here's another aspect most people probably give little thought to: wind. Understanding how truly tall CLT high-rises, 40 stories or more, behave as they intrude into gusty open skies is another question the industry is working towards answering.
So for now, as CLTs slowly gain wider acceptance, Kretschmann and Rammer said they believe they'll mostly be used in buildings of somewhat lower elevations, in the neighborhood of 12 to 14 stories.
"We see it as a wonderful, complex material," said Kretschmann.