Built: The Hidden Stories Behind Our Structures

by Roma Agrawal

The larger the span of a bridge, the more material you need to build it, and the heavier it becomes.

In much the same way, if you build a tower from straws and pour water on top of it, the water will stream through the different pathways it finds, dividing where more than one option is available. When planning a structure, then, it is vital for an engineer to understand where the force is flowing, what kind of force it is, and then make sure that the structure transmitting the force is strong enough for the job.

There are two main types of forces that gravity (and also other phenomena such as wind and earthquakes) creates in structures: compression and tension.

The type of force and the way it flows depends on how the structure is assembled. There are two main ways this can be done. The first is known as the load-bearing system and the second as the frame system.

The Corinthian column, with its capital decorated by intricately curled leaves, was supposedly invented by the Greek sculptor Callimachus after he noticed an acanthus plant growing through and around a basket left upon the grave of a maiden of Corinth.

Two of the ways in which a column can fail, through crushing (above left) and bowing (above right). Columns generally work by countering compression.

A ruler is wide in one direction and flat in the other: as you’ll have seen as you pressed down, it bows about its much weaker axis.

Beams work differently. They form the skeleton of the floors. When we stand on a beam, it flexes slightly, channelling our weight across to the columns that support it.

Similarly, the deeper the beam, the stiffer it is, so the less it distorts under load.

Gravity exerts a predictable pull on objects on the surface of the Earth. An engineer knows what it is, and can design columns, beams and trusses to resist it. But other, equally destructive forces are not so easily reduced to equations. One of these is the wind. Random, fluctuating, unpredictable, wind has challenged engineers throughout history, and it remains a problem all engineers have to solve if their structures are going to remain stable.

in the Roman Agora, just north of the Acropolis. Built around 50 BC by Andronicus of Cyrrhus, a Macedonian astronomer, the Horologion of Andronikos Kyrrhestes or ‘Tower of the Winds’

The Roman master builder Marcus Vitruvius Pollio (born 80 BC), who is sometimes called ‘the first architect’, talks extensively about the importance of considering wind in De Architectura, his hugely influential ten-volume treatise on the design of structures.

But when I design a larger structure, such as a skyscraper, the numbers on the wind maps no longer hold. Wind is not linear: it doesn’t change in a predictable way the higher you go into the atmosphere.

First, if the structure above ground is light, wind can make it topple over,

Second, if the ground is weak, wind can cause the building to move and sink. Think of a sailboat on a windy day. The strength of the wind pushes the boat across the water – which of course is the desired effect if you’re out sailing. But you wouldn’t want your building or bridge to move sideways in the soil as the wind hits

The third effect is similar to a boat rocking at sea. Like trees, all buildings sway back and forth in the wind, depending on how strongly it is blowing – this is normal and safe. Unlike trees, however, buildings don’t move so much that you can easily see the displacement.

But the higher you build, the stronger the wind is that you encounter. In the twentieth century, as we began to build taller and lighter structures, the force of the wind became a force to be reckoned with.

Just as a tree’s stability depends on a solid, well-rooted but pliable trunk, so a building’s stability often depends on a core, made from steel or concrete.

A core is like a spine or a skeleton, giving a building integrity from the inside, but 30 St Mary Axe is surrounded by an exoskeleton. This exoskeleton – or, to use the technical term, external braced frame or diagrid – is like the shell of a turtle.

Another spectacular example of the external braced frame is the Centre Pompidou in Paris.

The swaying in itself is not a problem: what’s important is how fast the building sways, and for how long.

When I design a tall tower, I have to make sure that the acceleration of the sway is outside the range of human perception, and that the oscillation stops quickly.

The taller and more slender the tower, the more pronounced the sway. Sometimes it isn’t possible to stiffen the structure enough to control the acceleration and how long it oscillates. So although the building is perfectly safe, it wouldn’t feel safe.

This phenomenon – an object vibrating dramatically at its natural frequency – is called resonance.

Put some rubber feet on the underside of the speakers and the effect lessens, because the feet absorb most of the vibrations. Similarly, we can install big rubber bearings at the bottom of the columns of a building, which then absorb an earthquake’s vibrations.

Earthquake energy can also be absorbed in the connections between beams, columns and diagonal braces.

I know now that there are two main reasons for this. The first is that engineers design certain buildings to resist explosions, so even if it is hit and damaged, it doesn’t collapse like a house of cards. There is a minimum standard of safety governing the design of all structures, but the more vulnerable ones – tall, iconic buildings, for example, or those with particularly large numbers of people inside – are designed specifically for a range of possible explosion scenarios.

Oddly, the explosion did not perforate Ivy’s eardrums, which suggests that its force wasn’t that large – since it doesn’t require much pressure to damage them.

There was another unusual thing about this collapse. Normally I would expect an explosion at the base of a building to cause the most damage, because there are many storeys above it which can come crashing down. In this case, however, if the same explosion had happened at the base of the building, the collapse might not have happened at all.

Even for structures built in a more traditional way, with all the concrete poured, or steel being fixed on site, it is essential to make sure that the beams and columns have robust connections.

an event like an explosion happens, then of course damage will occur, but the effect of an explosion on one storey shouldn’t propagate throughout the structure.

The taller or larger the building, the longer it takes to escape, so the deeper the steel is embedded in the concrete. Just a few centimetres make a tremendous difference.

Today, even if we use an exoskeleton to resist wind (which means we don’t need an internal core), we still often install concrete walls to safeguard escape routes.

Every brick they used, no matter what its size, was in the perfect ratio of 4 : 2 : 1 (length : width : height) – a ratio that engineers still (more or less) use, because it allows the brick to dry uniformly, it’s a handy size to work with, and it has a good proportion of surface area that can be bound to other bricks with whatever form of glue or mortar is used.

When the Roman empire fell in AD 476, the art of brick-making was lost to the West for several hundred years, only to be revived in the Early Middle Ages (between the sixth and tenth centuries), when they were used to build castles.

The next step is where the real difference between ancient and modern lies. The bricks are fired at temperatures of between 800° and 1,200° Celsius, fusing the particles of clay together so that they undergo a fundamental change. Clay turns into ceramic: more similar to glass than dried mud. This fired brick is far more durable than a dried brick, and that’s what we use to build structures today.

The ancient Egyptians used the mineral gypsum to make a plaster (also known as plaster of Paris, since it was commonly found and mined in the Montmartre district of the city).

Approximately 1.4 trillion bricks are made each year around the world; China alone manufactures about 800 billion, and India about 140 billion. LEGO, by comparison, makes a mere 45 billion or so bricks per year.

While the ancients recognised the wonders of iron, it was mostly used to make household vessels, jewellery and weapons, because the iron they extracted was too soft to build with, and they didn’t know how to strengthen it enough to create an entire building or bridge. There are nonetheless rare examples of structures that use it: in A Record of Buddhistic Kingdoms, the Chinese monk Fa Hsien wrote about suspension bridges held up by iron-link chains in India around the time the pillar in Delhi was made.

These crystals are attracted to each other, and this attraction bonds them together in a matrix or grid. However, when you heat up a metal, the crystals vibrate faster and faster until the bonds weaken. The metal then becomes malleable, and may even melt into a liquid if the temperature is high enough. Because of the flexibility of the bonds, metals are ductile, which means they can stretch and move to a limit without breaking; the process of hot-working mentioned above makes sure this characteristic is retained.

While pure iron is good in tension, it’s too soft to resist the immense loads in larger structures because the bond between its crystals is quite fluid and flexes.

The crystals that make up iron are arranged in a lattice, so scientists and engineers began devising ways to stiffen it.

Similarly, if carbon atoms are added to iron they jam the crystal lattice.

Results of their experiments include cast iron (which, being resistant to wear, is good for cooking pots, but is not used much in buildings because it’s brittle, like an Italian biscotti); wrought iron (which is not used much commercially any more, and which has a texture more like the soft, luxurious chocolate-chip cookies I used to eat as a child in America); and steel.

Noticing that they were the ones closest to the hot air, Bessemer realised that the oxygen in the air must have reacted with the carbon and other impurities in the iron – and removed most of them.

The furnace inferno was the result of an exothermic reaction: a chemical reaction that releases energy – usually in the form of heat – during the oxidation of impurities. After the silicon impurities had been quietly consumed, the oxygen in the air current reacted with the carbon in the iron, releasing a huge amount of heat. This heat raised the temperature of the iron far beyond what a coal-fired furnace was then capable of, so Bessemer didn’t need to use external sources of heat. The hotter the iron became, the more impurities burned off, which made the iron hotter still, so it burned off even more impurities. This positive loop created pure, molten iron.

scale. By 1870, fifteen companies were producing 200,000 tonnes of steel each year. When Bessemer died in 1898, 12 million tonnes of steel were being produced worldwide.

Cables can be tightened up using a jack – which is a tube with clasps on each side. Each cable had at least one break in it where a jack could be installed. The clasps each held a bit of cable either side of the break. The jack can be adjusted to pull the ends closer together (to tighten the cable) or further apart (to loosen it), therefore altering the amount of force in the cable. If you look at the cables fanning out from the tower of my footbridge you’ll see that they have connector pieces – where the cables look briefly thicker than the rest of their length: those are the points at which the jacks were temporarily connected.

The Pantheon in the Piazza della Rotonda in Rome is one of my favourite structures. Built by the emperor Hadrian around AD 122 (at about the same time as he was building a wall to divide England from Scotland), it has stood strong ever since in a variety of guises – temple to the Roman gods, Christian church, tomb – though barbarians removed what they could and Pope Urban VIII even melted the ceiling panels to make cannons.

Even now, it’s the largest unreinforced concrete dome in the world. The Romans really honed their craft, creating an engineering masterpiece from a revolutionary material they called opus caementicium