From seeds to skyscrapers
As more and more people move to cities every day, where are they all going to live? One of the main options is to build up. But, with steel prices climbing, engineers are looking to another resource which literally grows on trees. Wood isn't strong or stiff enough to be used in construction past a certain height. But a team at Cambridge University are working to try to change the molecular properties of wood to turn it into something suitable for building skyscrapers. Georgia Mills spoke to Michael Ramage who's heading up the project...
Michael - We're working on a project using natural materials, in particular from plants to make large buildings. The very short story is going from seeds to skyscrapers. We are hoping to be able to modify wood in such a way that it will be strong enough to last a lifetime and work at the forces that you would have in a large skyscraper.
Georgia - Why change it from steel and concrete? Why do we need to have wooden skyscrapers?
Michael - Well, we think wood is a good alternative to steel and concrete for a number of reasons. It's a carbon sink. The wood absorbs carbon dioxide while it grows and stores it as the structural material of the wood itself. There is a limited amount of steel in the world. So, we actually need to find something to replace it. there is a growing amount of wood in the world and these are crop forests. They're not virgin forests. In general, people feel very good about wood. They feel comfortable in wood, with wood surrounding them. So, we think it will make for better buildings in the long run.
Georgia - So, why don't we have wooden skyscrapers already?
Michael - We don't have wooden skyscrapers primarily because steel and concrete are pretty good materials for making skyscrapers. So, we haven't had the need to figure out how to make them with wood. Wood has worked extremely well on a domestic scale. So, people have been doing that very happily for hundreds of years. But we're reaching a point where we need to innovate more with wood and do things that it hasn't done before.
Georgia - What would happen if you try to build a skyscraper out of wood now?
Michael - What the tallest skyscraper out of wood at the moment is 10 storeys tall. We've designed a 70-storey tall building, but we know that it will collapse. Part of the reason we designed it is to figure out how and why it collapses so we have a better understanding of what properties wood needs to have in order to make it possible to build at those scales. Wood is very good in tension and compression so when you pull on it or push on it, along the length of the wood. But if you push across the length - so imagine squeezing the trunk of a tree. It's not very strong. So, in technical terms, we call that perpendicular to the grain. So, that needs to be made stronger. Wood is very flexible so we need to make it stiffer. If you think about a tree in a storm, most of the time, they don't fall over but they bend a lot in the wind. Buildings can't do that because people would get sick.
Georgia - So wood, we just find from trees. How on earth are you going to change its properties?
Michael - We're going to look at two ways of changing their properties. We'll look at a biochemical perspective and a chemical perspective. So, the biochemical approach will be looking at the cells themselves. Are there things from what we know about plants and plant science that we can genetically encourage the trees to have stronger cells. Chemically, we'll be looking at the same cells and see if there are things we can do chemically to make the cell wall stronger for example or the connection between various cells stronger. If we can do that, we can make the microproperties of the wood better and we would be looking for a translation to the large scale properties. We're looking at every single aspect of the science and the engineering at the same time. so, we're looking at the micro scale of the cells. At the same time, we're looking at the mega scale design of buildings. We end up with some very interesting conversations between microbiologists, and biochemists and structural engineers because at the outset, we speak of different language, but we all work on the same material. Those conversations we think are a new approach to thinking about the material.
Georgia - I know in some people's minds, wood is definitely linked with fire. Would these buildings pose an extra risk in terms of flammability?
Michael - That's a perception that we will certainly need to deal with. These very large wooden buildings are not flammable at all. Anyone who's built a campfire knows that we have to use small kindling to get it started and the big logs won't burn without any help. So, big wooden buildings never catch fire. They char, but we know how much they char. It's well-documented so we just make the structure a little bit larger and then the charred wood is actually an insulator for the structural material inside. There are some examples where wood has outperformed steel in very big fires.
Georgia - How long do you expect it to be before we see our first wooden skyscraper?
Michael - We would love to see one in the next 5 to 10 years. There are designs from practices around the world for 20, 30 storey buildings in timber. No one has completed one yet, but certainly, in the next 5 years, I think it would be a possibility.