by Roma Agrawal
The engineers cut long planks of wood from tree trunks and laid them edge-to-edge on top of the taut cables. The planks were tied together and evenly covered with a layer of broken twigs and branches, after which soil was thrown on top and stamped down to create a surface that the army could walk on. The engineers also laid heavy anchors upstream and downstream of the bridge: those to the east stopped the boats being pushed down the strait by winds from the Black Sea, while the others resisted the winds from the west and the south. Fencing was installed along the sides of this wide walkway to prevent the horses from seeing the water and being spooked.
Once this bridge of boats was ready, Xerxes offered prayers for safe passage. He threw his cup, a golden bowl and a Persian sword into the straits, possibly as an offering to the sun, or possibly as a form of appeasement to the sea. The army then began to cross this monumental pontoon bridge en route to the Greeks at Thrace. It is said that it took seven days and seven nights for the Persians – including Xerxes’ elite fighters known as ‘The Immortals’ – to cross from one side of the strait to the other.
Despite this feat of engineering, the military side of the story is less epic. Xerxes was defeated at the battles of Salamis and Plataea and, after losing large numbers of men to war or starvation, he retreated back to Persia. Although he managed to subjugate Nature, Xerxes couldn’t do the same to the Greek people.
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Floating or pontoon bridges are believed to have originated in China sometime between the eleventh and sixth centuries BC, when engineers used boats with boards on top to cross large rivers. Use of the pontoon bridge continued through ancient Roman and Greek times – a notorious example was supposedly assembled by Caligula so he could show off his clothes in parades. During the World Wars, soldiers often used this technique because it allowed them to assemble and dismantle a path across water quickly and efficiently. Floating bridges are a great option when water is deep, the span is long and time is short. But storms and currents in the water affect them badly: there are many examples (such as the Murrow and Hood Canal Bridges in the USA) which have failed in strong storms. If one of the boats fills up with water, it drags down the others, until the whole line sinks. Fortunately, engineers no longer face the same fate as those that once served Xerxes.
No. 3: The Falkirk Wheel
The Falkirk Wheel: a rotating bridge.
As it bounced up and down with the waves, and sideways with the currents, walking across Xerxes’ boat-bridge would have been disconcerting. We don’t like our structures to move perceptibly: it scares us into thinking that they are not safe. But what if a bridge is designed to rotate? Many bridges allow land vessels to cross water, but one of my favourite bridges enables water vessels to cross land.
The Celtic doubled-headed axe was a formidable weapon, with a blade on either side of its shaft so that, in battle, a brave warrior could swing it right or left with equally destructive results. Unlikely as it may seem, this menacing tool is the inspiration for one of the coolest and most unusual structures in the world – the Falkirk Wheel.
The low-lying canals of Scotland were once a flurry of activity. The Union Canal, opened in 1822, went from Falkirk to Edinburgh, as a way to bring coal into the capital and feed the new industries that were setting up factories in the city. The Forth and Clyde Canal (opened in 1790) served the same purpose for Glasgow, at that time a small town that was rapidly growing into the industrial heartland of Scotland. However, once the railway network began to develop in the 1840s, these canals, like so many others, became redundant, because it was quicker to transport minerals by train. The canals gradually fell into disrepair – by the 1930s they were in such a state that a portion of the canal system was filled in. A former transport artery was sealed off for good – or so it seemed.
At the end of the twentieth century, architects and engineers conspired to reopen the canals by creating a new waterway-based link between Glasgow and Edinburgh, specifically between the Forth and Clyde Canal and the Union Canal. Making the 200-year-old waterways usable again offered environmental and economic advantages to the communities that lie along them. But doing this presented some technical challenges, foremost among them a large steep slope that had to be crossed. The traditional way canal-builders dealt with a slope was by means of a lock. Between the lower and higher sections of canal, they constructed a long, narrow, tall-sided chamber with a door (or pair of doors) at each end, which could seal or ‘lock’ in the water. Bargemen ascending the canal would manoeuvre into the chamber and close the lower doors behind them. They would then lift the ‘paddles’ (shuttered openings) at the other end of the lock, allowing water to flow in from the higher canal. Gradually the lock filled, to the point where the water level was the same as the higher section of canal. At this point the bargemen could open the upper doors and float on their way. A bargeman descending followed the same process in reverse. Originally, this journey between Edinburgh and Glasgow meant a wearying day-long passage through 11 locks, opening and shutting 44 lock gates along the way. Hardly an easy task – and in any case the locks had since been removed. So the engineers had to do some smart thinking.
Today, if you travel west from Edinburgh along the Union Canal towards the Clyde or Glasgow, you eventually reach a place where the land drops sharply away on either side, leaving you chugging along an aqueduct that thrusts out boldly into seemingly empty space. This is the end of the Union Canal. At this point, your boat is 24 metres up in the air, roughly as high as the top of an eight-storey building. To get from this elevation down to the lower basin and float off along the Forth and Clyde Canal, your boat must now enter the embrace of an exceptional piece of engineering, a modern take on the Celtic axe.
An immense vertical wheel (like a Ferris Wheel) 35m in diameter lies in front of your boat. The wheel has two axe-shaped arms that rotate through 180 degrees. Each arm houses a sort of ‘gondola’: a vessel large enough to carry two canal boats and 250,000 litres of water. A hydraulic steel gate stops the water from the high-level canal pouring out. When the wheel’s gondola is aligned with the end of the aqueduct, the gate at the end of the canal and the gate at the end of the gondola open, and the boat can manoeuvre straight into the gondola. The doors are resealed – and the arms start to rotate.
At a funfair, as the Ferris Wheel turns, you’ll have noticed that your seat also moves, so that you remain sitting vertically. As you travel from the bottom to the top and back again, your orientation stays the same. In a similar fashion, a complex system of gears and cogs makes sure that the gondolas on the Falkirk Wheel always remain horizontal as the arms swing through the air. To complete one 180-degree turn needs little power – the same amount of electricity as boiling eight kettles of water. This is largely thanks to Archimedes and his famous principle, which states that when an object is placed in water, it displaces its own weight. If, for example, you have one boat in one gondola, but no boats in the other side, the two gondolas will still weigh the same. The boat will have displaced an amount of water from its gondola equal to its own weight. So as long as the water levels in both arms are equal, you only need minimal power to overcome inertia and start the wheel rotating, and then momentum carries the balanced arms round until they’re switched off. The Falkirk Wheel brings boats from the upper basin to the lower basin (or vice versa) in just five minutes, compared to the full day required to negotiate the canal’s original system of locks.
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There are a few examples of boat lifts around the world – such as the Strépy-Thieu in Belgium, the Niederfinow Boat Lift (the oldest working boat lift in Germany), and at the Three Gorges Dam in China (now the tallest boat lift in the world, it moves boats vertically by a colossal 113m) – but there is a particular thrill to watching and travelling on the Falkirk Wheel. Perhaps this is because it taps into our childhood memories of fairgrounds. It’s an example of how engineering has an aesthetic and even nostalgic side to it that plays a part in how we respond to a structure.
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nbsp; No. 4: The Silk Bridge
The silk bridge: the longest web-bridge in the world.
One evening I was reading a book with the television on, just letting the reassuring sound of the show’s host fill the living room, but not paying any real attention. Until, that is, I heard the words ‘strong material’ and ‘bridge’ and, as you can imagine, my ears pricked up like a cat’s. The host was talking about one of the most prolific bridge builders in the world – and, exceptionally, the builder is female, and lives in Madagascar.
She’s about the size of a thumbnail, has eight very hairy legs and her body is heavily textured like the bark of a tree, which, as David Attenborough went on to explain, is the camouflage that protects her from predators. She also has a spinneret, which is the bit of her body responsible for making her the brilliant bridge engineer she is.
Darwin’s bark spider can build a bridge up to 25m long (that’s 1,000 times her own size), spanning rivers or even lakes. However, unlike most bridge builders, she’s not looking for a way to get from one side to the other. She’s looking for a way to get food.
Scurrying around in the vegetation on a river bank, she seeks out a suitable place for her project (like any professional engineer) and then releases dozens of sticky silk threads from her spinneret. They spray out – just like they do from Spider-Man’s wrists in the movies – and are caught by the natural wind currents that exist above bodies of water in dense forest. The threads are carried in a thin, almost invisible, stream across the river to catch and attach themselves to vegetation there. This line of silk – the first stage of the build – is called the bridging line. The bridging line is a catenary, the typical curve of a cable that sags under its own weight. Giving the thread a quick tug to make sure it’s secure, the spider uses the hairs on her legs, which are tiny hooks, to reel in the line a little so that it doesn’t sag too much.
She tests her bridging line by walking along it, and as she does so she uses more silk and secretions to reinforce the line, making it even stronger. When she reaches the other end, she reinforces the bridging line’s attachment to the vegetation by spinning more thread around it. It’s important that the connection – which the wind made simply by sticking the line to a branch – will be strong enough to carry the weight of the rest of the structure.
Now the bridging line has to be anchored. The spider searches for bits of vegetation, such as large blades of grass sticking up from the water, and then moves along the line until she is almost directly above them. Slowly, producing more silk, she lowers herself and attaches her anchor point to a blade of grass close to the surface of the river, creating a ‘T’ shaped skeleton for her web.
Over the next few hours, the spider effortlessly shuttles back and forth, using the T-shaped skeleton as a base on which to attach more silk threads. From bridging line to anchor thread, she keeps producing and weaving new silk, in a big circular pattern. Some of the silk is not sticky: it functions as part of the structural frame of her construction. The rest is sticky, and will be the part of the web that actually traps her food. Eventually she will have fashioned a giant orb that can be more than 2m in diameter.
Darwin’s bark spider is the only known spider that bridges water to trap its food. Its victims are the tasty mayflies, dragonflies and damselflies that zip over the middle of rivers. The large diameter of the web means that small creatures like birds or bats could also potentially get caught.
The sheer size of the web is hugely impressive, but the silk used to build it is even more astonishing – which makes sense: to build such a large structure you need an exceptional material. The bark spider’s silk has been tested in a laboratory by connecting it to hooks that are slowly pulled apart. The results show us that these tiny creatures produce silk with an incredible elasticity. This is the property of a material to stretch under a load and then recover: if, after the load is removed, a material shrinks back to its original size, it is elastically deformed; if it doesn’t fully recover to its original shape, it is plastically deformed. Tests have shown that the bark spider’s silk is twice as elastic as other known spider silks. It is also very tough. Toughness is the property of a material that measures how much energy it can absorb without fracturing. It is a combination of strength (how much load the material can resist) and ductility (how much it can deform without breaking). In fact, the silk of Darwin’s bark spider is the toughest biological material we’ve found so far – it’s even tougher than steel.
Elasticity and toughness are a great combination for building materials. Take, for example, rubber bands: if you have a thin and stretchy band, you can stretch it very far but only with a small load, as it’s elastic and ductile but not very strong. A very thick band made from brittle rubber can take more load but might snap suddenly, because it’s strong but fragile. The bark spider’s silk, on the other hand, has the ideal balance of all these properties. It can absorb large forces, and at the same time it can stretch a long way without snapping. This balance makes it the perfect material for building the world’s largest spider webs.
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I have included the Darwin’s bark spider’s bridge as a reminder that it is not just humans who create structures: in fact, as this creature demonstrates, we are still catching up with Nature. We are only now starting to build bridges that span as far as this spiders’ does compared to our own body size – the Akashi Kaikyo Bridge in Japan currently holds the record for the longest span, at 1,991m. We are already being inspired by Nature (we call this type of design biomimicry) – the ventilation system of the Eastgate Centre in Zimbabwe is inspired by porous termite mounds, and the Quadracci Pavilion of the Milwaukee Art Museum has a retractable shade inspired by the wings of birds. But I believe we can learn even more. It would be the dream of any engineer to develop a super-material like spider silk that is incredibly tough and light, and which could be launched across a river or a valley, allowing the air to carry its threads to the other side. Then we too could create a long bridge in a few hours – just as quickly as the Darwin’s bark spider does.
No. 5: Ishibune Bridge
The Ishibune Bridge: a catenary bridge.
At our hotel in Tokyo, my mum and I had been given a piece of paper on which an address had been written in a series of delicate, swirling strokes that looked like little pictures. The writing was beautiful, but illegible to us, so we simply handed the paper to our taxi driver, and hoped it was enough to get us to our destination.
It was raining so hard we could barely see where we were going, but we were aware that we had left the city and were now surrounded by steep slopes covered in dense green forest. Driving higher and higher up a narrow, winding road, we finally reached a red gate with more beautiful characters inscribed upon it. Our driver came to a halt and waved us out of the car – I hoped he would still be there when we came back. I zipped up my jacket and walked along a narrow dirt path looking for the Ishibune Bridge, the perfect example of a simple stress-ribbon bridge – a form that, until I’d planned that particular trip, was unknown to me.
Earlier in the year I had been awarded a travel grant by the Institution of Structural Engineers – my proposal had been to study a special type of bridge. Speaking to my colleagues and doing some research made me aware of the stress-ribbon bridge, a graceful, simple form of which there are less than a handful of examples in the UK. I wanted to learn more about them and understand why they are so rare. My proposal was to travel to Europe and Japan – to places where these bridges are used to great effect – and report back. I went first to the Czech Republic, where engineers showed me a huge range of structures that use the stress-ribbon technique – from bridges spanning motorways to a tunnel built using the same principles. Then, at a German university, I met researchers who had built a 13m-long prototype in a lab, and who were doing tests and experiments on it. I got to do some ‘testing’ myself – basically jumping up and down on its deck to try to make it resonate.
To make your own mini version of a stress-
ribbon bridge, use two tins of baked beans placed a metre apart to simulate bridge abutments, then lay two thick pieces of string over the tins, taping the ends to the table, which represents the ground. To turn this into a stress-ribbon bridge, it needs a deck, which you can make with matchboxes. Poke two holes through the boxes – one in each side – then lay them out on top of the strings. Thread cut pieces of rubber bands through the holes to link the matchboxes. The rubber band will stretch, compressing the matchboxes together.
If you press down on the model bridge in the middle of its span, you’ll see the supporting strings tighten (in other words, develop tension); the strings pull at the tape which secures their ends to the table. A stress-ribbon bridge works in a similar way. Steel cables are slung across the gap to be bridged. The cables are thick – with a diameter about the size of a fist – and consist of numerous thin steel wires spun together to form a strong rope, which is protected by a rubber sheath. Concrete abutments at either end support the cables, which are anchored tightly into the ground. The anchors are strong enough to take the force of the cables pulling on them even when the bridge is loaded with lots of people. Planks of concrete (equivalent to the matchboxes), with grooves on the underside, are placed on top of the steel cables and connected to them to keep them in place. The planks have holes running through them, through which smaller steel cables are threaded and tightened to tie the planks together and make the deck stiffer.
