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  • 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.