How eighteen-story mass-timber towers stay safe

HostMost of us think of skyscrapers as these giant, cold shells of glass and steel. But lately, builders are turning back to one of the oldest materials we have to build things that reach way up into the clouds. It sounds like something out of a fairy tale, or maybe a recipe for a huge fire, but wood towers are starting to pop up in cities everywhere. How do we get wood to stand eighteen stories tall without it falling over or burning down?

GuestIt's a bit of a shock to the system when you first see it. We're so used to the idea that big buildings have to be made of rock and metal. But the wood we're talking about here isn't the kind of 2x4 you buy at the hardware store to build a shed. This is what people call mass timber. Think of it like a giant, super-strong sandwich. They take thick planks of wood and glue them together in layers. The trick is that they turn each layer so the grain runs the opposite way from the one below it. When you glue them like that, you get these massive panels and beams that are as strong as steel but way lighter. It changes the whole way we think about what a tree can do.

HostI have to stop you there because my first thought is fire. We use wood to start fires. If you have an eighteen-story building made of wood and someone leaves a candle burning, isn't the whole thing just a giant pile of kindling?

GuestThat's the most common fear people have, but it's actually one of the coolest parts of how this works. Think about what happens when you try to light a big, heavy log in a fireplace. You can hold a match to it all day and nothing happens. You need small sticks to get it going. These mass timber beams are so thick and solid that they don't just catch fire. If they do get exposed to high heat, the outside of the wood turns into char. That black, burnt layer actually acts like a shield. It protects the wood deep inside from the heat, so the beam keeps its strength even while the outside is scorched. Steel, on the other hand, starts to go soft and bend like a noodle once it gets hot enough. In a weird way, these thick wood blocks can be safer in a fire than metal.

HostOkay, the fire shield makes sense for a big log, but a skyscraper isn't just one log. It's a huge stack of weight. Wood is soft. If you stack eighteen floors of heavy stone counters, bathtubs, and hundreds of people on top of a wooden frame, why does the bottom floor not just get squished?

GuestIt comes back to that sandwich idea. When you glue those layers with the grain going in different directions, the wood stops acting like a soft plant and starts acting like a solid block of stone. It doesn't want to shrink or expand or squish. The weight of the building is spread out through these massive columns that are sometimes a few feet wide. And because wood is so much lighter than concrete, the building actually weighs a lot less overall. That means the base doesn't have to work quite as hard to hold everything up. It's a bit of a trade-off, though. Since the building is light, it can sway more in the wind than a heavy concrete tower would. Builders have to find ways to stiffen it up so people on the top floor don't feel like they're on a boat.

HostIt still feels like we're asking a lot of trees. If we start building every big city tower out of wood, are we not just going to clear-cut every forest on the planet? That doesn't seem very good for the earth.

GuestThat's a fair concern, but it actually works the opposite way if you manage the forests right. Trees are basically tubes of carbon that they sucked out of the air. When a tree dies and rots in the woods, all that stuff goes back into the sky. But if you cut the tree down and turn it into a beam for a building, you're locking that carbon away for a hundred years or more. It's like a giant storage unit for greenhouse gases. Plus, we're mostly using fast-growing trees from farms, not old-growth forests. Since wood is a lot easier to move and put together than pouring wet concrete, you also save a ton of energy during the build. You can basically ship a whole floor on a truck and bolt it together like a giant toy set. It's much quieter and cleaner than a normal construction site.

HostThere has to be a catch, though. If it's faster, lighter, and better for the air, why is every city not already full of these?

GuestA big part of it's just the rules. Building codes are very old and they were written when wood buildings were mostly small houses that burned easily. It takes a long time to prove to cities that these new wood blocks are safe. There's also the water problem. If you're building a steel tower and it rains, you just wipe the metal off. If a wooden skyscraper gets soaked while you're putting it together, you can end up with mold or rot before the roof is even on. Builders have to be incredibly fast or keep the whole thing covered, which is tough when you're eighteen stories up in the air. We're still learning the best ways to keep these giant wooden puzzles dry while we fit the pieces together.

GuestThe real test will be how these towers handle the next fifty years of rain and wind and settling.

HostThese wooden towers show us that we can build the future by looking at a forest instead of a furnace.