In 1829, a competition was held to see who could design the best bridge across the Avon gorge in Bristol. Unlike today the winner was not decided by an “engineering idol” style phone-in poll, rather he was chosen by leading engineer Thomas Telford. After a minor scandal, were Telford decided that none of the entries were good enough, and judged himself to be the winner, the design was provided in 1830 by a then unknown 24 year old, Isambard Kingdom Brunel.
Brunel’s design eventually became the famous Clifton Suspension Bridge (shown in figure 1). Amazingly, although it was designed for light horse drawn carriages, the bridge is still in use now, and carries between eleven and twelve thousand vehicles every day.
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The basic design of a suspension bridge is devilishly simple, consisting of towers to hold the main cables, a main deck carried by the cables, the cables themselves, and hangers to connect the deck to the cables. The best point of this kind of design is that it means that the towers are pulled downwards but not side to side – we can say the towers are in compression, and the cables are stretched but not pushed downwards – we can say they are in tension. To use more technical terms:
The cables, are purely in tension, and are not required to be rigid, so they can be only as thick as needed to cope with the tension. The towers are almost purely in compression. Figure 2 shows a simple suspension bridge layout. The towers are shown in blue, the main cable in red, the hangers in brown, and the deck is shown in black. Look back at Figure 1 and try to spot all the parts.
Since the cables are in tension, they can actually be bendy. Imagine a rope with a weight on the end – as long as the weight is pulling downwards, the rope will be straight. Bendy materials like wire are very strong in tension, but also very thin and light, this makes suspension bridges very cost effective and strong. Most of the longest bridges in the world are suspension bridges.
The Highest Rope Bridge in the World
The Chinese writing says: “this is the highest rope bridge in the world”. This is the “Towards Heaven Tree” bridge in the rainforests of Yunnan in Southwest China. At its highest point it is 37 meters above the ground, that’s taller than a building with 11 floors, and is 120 meters long. Another sign as you approach the bridge says in Chinese: “this brige is very scary. If you are a coward don’t look down! If you are a real coward don’t even go on the bridge.”
Actually this is just another kind of suspension bridge. Figure 4 shows the view from the top of the bridge, we can see that all the elements of a regular suspension bridge are there. The trees act as towers, taking the main weight, while the wire running from tree to tree is the main cable, and the deck is made from simple wooden planks. You can also see the rope hangers, the other ropes in the picture are there to stop people falling off the bridge.
When you walk along the “Towards Heaven Tree” bridge, you do not feel very safe. The wooden deck swings from side to side and backwards and forwards as you walk across. One major problem with suspension bridges is designing a deck which doesn’t wobble. This is why suspension bridges are not often used to carry trains. If the deck moves a little while you are driving or walking, it’s no problem, but even a small movement could damage a train line.
Moving Bridges
If the bridge is very high, then cars that want to cross have to somehow get up onto the bridge, if it is low, then tall boats will not be able to pass underneath. This problem is especially true for bridges in major cities, where there will often be boats along the river, and cars trying to cross the bridges. One solution to the problem is by using a bascule bridge. This is basically a bridge which can open in the middle to let boats go underneath. There are two different kinds of bascule bridge, single and double leaf. A single leaf bridge has one part which opens, while a double leaf has two parts which can open. Figure 5 and 6 show these kinds of bridges. Figure 7 shows another kind of moving bridge – a drawbridge.
It is also possible to combine several different kinds of bridge. Take a look at figure 9 below, it is the world famous Tower Bridge in London. It is a combined bascule and suspension bridge. The middle part is a bascule bridge, while the outside parts are suspension. Along the top is a permanant walk way suitable only for pedestrians. When the bridge was built people who wanted to climb up onto the walkway had to climb a lot of stairs, so most people prefered to wait for the bascule to close and cross the normal way. In the 1900s, no one was using the top walkway so it was closed, and has been ever since.
Beam and Arch Bridges
The most simple kind of bridge in the world, must be the beam bridge. A beam bridge is just a flat piece a material held up at either end. The obvious problem is that beam bridges are not very strong and can not be very long, but the advantage is that they are cheap and easy to make. Beam bridges are often used for walkways. Take a look at the two beam bridges on the right figures 10, and 11.
Now let’s look at some maths: as you step on the middle of a beam it will bend. As it bends two things happen: the bottom of the beam is streched and the top is compressed. Most materials are good when they are being streched OR when they are being compressed, but there are not many which are good at both. This is why the beam bridge is very weak.
A slight change in the design of a beam bridge can make it much stronger. Instead of using a straight beam, how about a curved beam. This kind of bridge is called an Arch bridge. Basically, an arch bridge is already bent, so there is no need to worry.
For the arch bridges with an odd number of bricks you can see that their is a large brick in the centre. This is called the “key stone”. Contrary to popular opinion keystones are not particularly special from an engineering viewpoint, but they are very attractive and are often emphasised in bridge designs.
The problem with arch bridges is that the top of an arch bridge may not be straight and so it’s difficult to drive over. One solution is to put the deck on top of the arch bridge as shown in fig. 13, the other is to hang the deck from the top of the arch bridge as shown in fig. 14.
