The Sakyamuni Pagoda of Fogong Temple in China is one of the oldest wooden structures. It was built by Emperor Daozong of the Liao Dynasty at the site of his grandmother’s home – without a single nail, screw or bolt. And yet, thanks to skilful craftsmanship and sturdy design, the pagoda is still standing 900 years later. At 67m (219ft) high, it remains the tallest timber building in the world... It has survived at least seven serious earthquakes, including one in 1556 that killed nearly a million people. When the ground starts shaking, wooden structures tend to sway without collapsing.
“The tall buildings I’m talking about are made out of engineered wood – small pieces of wood glued together,” says Buchanan... "the newest material is cross-laminated timber (CLT), which is really starting to take off now.”
CLT is a true wonder material. Made from thin layers of wood criss-crossed and stuck together with fire-resistant glue, this “plywood on steroids” is claimed to be as strong as structural steel; alternating the direction of the grain offsets the weaknesses in any given plank and stops the material warping if it gets wet. The size of the panels is crucial. “If you’re making a fire, everyone knows you don’t start with giant logs,” says Thistleton. “It would take a lot to ignite them. The first thing that happens is they char and that actually protects the wood beneath.”
If you need a bit more reassurance, take a study by the Committee on Tall Wood Buildings earlier this year. When they set alight two one-bedroom apartments made of engineered wood, the fire raged until it had burnt through the furnishings, then extinguished itself. The contents were turned to ash, but the structure itself charred and remained intact.
Other than preventing the spread of a fire, one of the most important factors is what happens to CLT when it’s heated. In this respect, the material wins hands down over steel and concrete, which tend to melt and weaken. “There’s an image that we often use in our lectures. It’s a devastated fire site with a big timber column supporting a timber beam. At the top is a steel beam that’s completely melted,” says Thistleton.
Wooden buildings are so light, the main challenge isn’t keeping them up, but holding them down. As you get higher up, wind becomes faster – at the top of the tallest building in the world, the Burj Khalifa in Dubai, the wind can reach 150km/h (90mph). One way to overcome this in wooden buildings is to give them a concrete core, but some architects see this as cheating.
Jonsson agrees. “It would be great to do a 100m-tall building out of wood. We’ve done the calculations and it’s theoretically possible, but there are some major challenges. The important thing is that just one cubic metre of wood sequesters a ton of CO2. The 20th Century was the concrete age, it was all about the dominion of man over nature. Now we’re transitioning towards a different attitude, a more nurturing one,” says Thistleton. Welcome back, perhaps, to the age of wood.
http://www.bbc.com/future/story/20171026-the-rise-of-skyscrapers-made-of-wood
But can you park a B-52 on one? That's the real question here.
ReplyDelete(Context: the biggest all-wooden structure of which I was previously aware was a "bridge" capable of supporting a B-52 bomber. It was used for EMP testing during the Cold War; metal would have screwed up the simulated EMP field.)