Subtitles section Play video Print subtitles The world needs bridges. Have you ever thought about what it would be like not to have any? It's hard to imagine a civilization without bridges because they're so essential for growth and development of human society, but they're not just about a safe way across a river or an obstacle. They shout about connectivity -- community. They reveal something about creativity, our ingenuity -- they even hint at our identity. And when bridges fail, or are destroyed in conflict, communities struggle, development stagnates, people suffer. Even today, there are over one billion people living in poor, rural communities around the world that do not have safe, year-round access to the things that you and I take for granted: education, medical care, access to markets ... which is why wonderful organizations like Bridges to Prosperity build bridges in this kind of place -- this is in Rwanda. And they make such a difference, not only to those lives immediately around the bridge, but the impact of these bridges is huge, and it spreads over the whole community, far, far away. Of course bridges have been around for an awfully long time. The oldest ones are stone because it's a very durable material. I don't know about you -- I love to look at the development of technology to learn about what people did with the materials and tools available to them at the time. So the Pont Du Gard in the center is a wonderful example -- Roman aqueduct in the South of France -- fantastic piece of technology built using massive stones put together, dry -- there's no mortar in those joints. They're just dry stone joints -- fantastic and almost as good as new today. Or sometimes up in the mountains, people would build these suspension bridges, often across some dizzy canyon, using a vine. In this case, this is in Peru. This is using grass which grows locally and is woven into ropes to build these bridges. And do you know they rebuild this every year? Because of course grass is not a durable material. So this bridge is unchanged since Inca times. And bridges can be symbols of their location. Of course, Golden Gate and Sydney are well familiar. In Mostar the bridge was synonymous with the name of the place, and to such an extent that in the war in 1993 when the bridge was destroyed, the town all but lost its identity until the bridge was reconstructed. And bridges are enormous features in our landscape -- not just enormous, sometimes there's small ones -- and they are really significant features, and I believe we have a duty to make our bridges beautiful. Thankfully, many people do. Think of the stunning Millau Viaduct in the South of France. French engineer Michel Virlogeux and British architect Lord Foster collaborated together to produce something which is a really spectacular synergy of architecture and engineering. Or Robert Maillart's Salginatobel Bridge in the mountains in Switzerland -- absolutely sublime. Or more recently, Laurent Ney's beautiful and rather delicate bridge for Tintagel Castle in the UK. These are spectacular and beautiful designs and we need to see more of this. Bridges can be considered in three convenient categories, depending on the nature of the structural system that they adopt as their principal support. So, bending, of course, is the way a beam will behave -- so, beams and bending. Or compression is the principal way of operating for an arch. Or for the really long spans you need to go lightweight, as we'll see in a minute, and you'll use tension, cables -- suspension bridges. And the opportunity for variety is enormous. Engineers have a fantastic scope for innovation and ingenuity and developing different forms around these types. But technological change happens relatively slowly in my world, believe it or not, compared to the changes that happen in mobile phone technology and computers and digital technologies and so on. In our world of construction, the changes seem positively glacial. And the reason for this can be summarized in one word: risk. Structural engineers like me manage risk. We are responsible for structural safety. That's what we do. And when we design bridges like these, I have to balance the probability that loads will be excessive on one side or the strength will be too low on the other side. Both of which, incidentally, are full of uncertainty usually, and so it's a probabilistic problem, and we have to make sure that there's an adequate margin for safety between the two, of course. There's no such thing, I have to tell you, as absolute safety. Contrary to popular belief, zero risk doesn't exist. Engineers have to do their calculations and get their sums right to make sure that those margins are there, and society expects them to do so, which is why it's all the more alarming when things like this happen. I'm not going to go into the reasons for these tragedies, but they are part of the reason why technological change happens quite slowly. Nobody wants this to happen. Clients don't want this to happen on their projects, obviously. And yet of course they want innovation. Innovation is vital. As an engineer, it's part of my DNA. It's in my blood. I couldn't be a very good engineer if I wasn't wanting to innovate, but we have to do so from a position of knowledge and strength and understanding. It's no good taking a leap in the dark, and civilization has learned from mistakes since the beginning of time -- no one more so than engineers. Some of you may have seen this film before -- this is the very famous Tacoma Narrows Bridge collapse in Tacoma, Washington state, 1940. The bridge became known as "Galloping Gertie" because she -- she? Is a bridge female? I don't know. She was wobbling like this for quite a long time, and notice this twisting motion. The bridge was far too flexible. It was designed by a chap called Leon Moisseiff, no stranger to suspension bridge design, but in this case he pushed the limits just that little bit too far and paid the price. Thankfully, nobody was killed. But this bridge collapse stopped suspension bridge development dead in its tracks. For 10 years nobody thought about doing another suspension bridge. There were none. And when they did emerge in the 1950s, they were an understandable overreaction, this sort of oversafe response to what had happened. But when it did occur in the mid-60s, there was indeed a step change -- an innovation, a technological step change. This is the Severn Bridge in the UK. Notice the aerodynamically streamlined cross section in the center there. It's also a box which makes it very torsionally stiff -- that twisting motion which we saw at Tacoma would not happen here. And it's also really lightweight, and as we'll see in a moment, lightweight is really important for long spans, and everybody seems to want us to build longer spans. The longest at the moment is in Japan. It's just under 2,000 meters -- one span. Just under two kilometers. The Akashi Kaikyō Bridge. We're currently working on one in Turkey which is a bit longer, and we've designed the Messina Bridge in Italy, which is just waiting to get started with construction one day, who knows when. (Laughter) I'm going to come back to Messina in a moment. But the other kind of long-span bridge which uses that tension principle is the cable-stayed bridge, and we see a lot of these. In fact, in China they're building a whole load of these right now. The longest of these is the Russky Bridge in Vladivostok, Russia -- just over 1,100 meters. But let me take you back to this question about long-span and lightweight. This is using Messina Bridge as an example. The pie chart in the center represents the capacity of the main cables -- that's what holds the bridge up -- the capacity of the main cables. And notice that 78 percent of that capacity is used up just holding the bridge up. There's only 22 percent of its capacity -- that's less than a quarter -- available for the payload, the stuff that the bridge is there to support: the railway, the road and so on. And in fact, over 50 percent of that payload -- of that dead load -- is the cable on its own. Just the cable without any bridge deck. If we could make that cable lighter, we could span longer. Right now if we use the high-strength steel wire available to us, we can span, practically speaking, around about five or six kilometers if we really push it. But if we could use carbon fiber in those cables, we could go more than 10 kilometers. That's pretty spectacular. But of course superspans is not necessarily the way to go everywhere. They're very expensive and they've got all sorts of other challenges associated with them, and we tend to build multispan when we're crossing a wide estuary or a sea crossing. But of course if that sea crossing were somewhere like Gibraltar, or in this case, the Red Sea, we would indeed be building multiple superlong spans and that would be something spectacular, wouldn't it? I don't think I'm going to see that one finished in my lifetime, but it will certainly be worth waiting for for some of you guys. Well, I want to tell you about something which I think is really exciting. This is a multispan suspension bridge across very deep water in Norway, and we're working on this at the moment. The deep water means that foundations are prohibitively expensive. So this bridge floats. This is a floating, multispan suspension bridge. We've had floating bridges before, but nothing like this. It stands on floating pontoons which are tethered to the seabed and held down -- so, pulled down against those buoyancy forces, and in order to make it stable, the tops of the towers have to be tied together, otherwise the whole thing would just wobble around and nobody will want to go on that. But I'm really excited about this because if you think about the places around the world where the water is so deep that nobody has given a second thought to the possibility of a bridge or any kind of crossing, this now opens up that possibility. So this one's being done by the Norwegian Roads Administration, but I'm really excited to know where else will this technology enable development -- that growing together, that building of community. Now, what about concrete? Concrete gets a pretty bad name sometimes, but in the hands of people like Rudy Ricciotti here, look what you can do with it. This is what we call ultra-high performance fiber-reinforced concrete. It's a bit of a mouthful. Us engineers love those kinds of words. (Laughter) But what you do with this -- this is really superstrong, and it's really durable, and you can get this fantastic sculptural quality. Who said concrete bridges are dull? We could talk about all sorts of other new technologies and things which are going on, robots and 3-D printing and AI and all of that, but I want to take you back to something which I alluded to earlier on. Our bridges need to be functional, yes. They need to be safe -- absolutely.