Built Page 5
Learning from disasters is fundamental to engineering: part of the engineer’s job is a constant process of improvement, endeavouring to build structures that are better, stronger and safer than they were before. Thanks to such lessons we now anticipate the removal of columns, and check in advance that a building will not collapse. The Bombay Stock Exchange tower had been built in such a way that even though the structure in the immediate vicinity of the car bomb was severely impacted, the loads it was carrying found somewhere else to go. The damaged part of the building remained stable enough because it was tied into the rest of the structure, so – unlike Ronan Point – the floors above didn’t come crashing down. The steel bars buried in the concrete walls and columns held their strength in the face of the fires that blazed after the explosion.
It was the lessons engineers learned from history, and the new way of designing for the unanticipated, which saved my dad’s life that day.
CLAY
I love baking, which is perhaps not surprising, given that it has a lot in common with engineering. I like the way you have to follow an ordered series of processes to construct a cake. I like that you work in a very patient and precise fashion, otherwise you won’t get the right shape and texture. I like the hopeful wait, that quiet period when my work is done and it slowly takes shape in the oven. Usually, I find all this incredibly satisfying. But there are moments of perplexed frustration – like the time I opened the oven door ready to slide out a delicious pineapple upside-down cake and was confronted instead with chunks of uncooked fruit swimming listlessly in a greasy sea of butter. Forget soggy bottom, this was a soggy disaster. Cursing the oven and recipe (after all, it could hardly have been my fault), I slung it straight in the bin: useless – except as a valuable reminder that in baking, as in engineering, the right choice of materials, combined in the correct way, is crucial to the outcome.
When designing a building or bridge, materials are one of my foremost concerns. In fact, different materials can entirely change the way the frame of a structure is arranged, how intrusive it feels, and how physically heavy and expensive it is. They must serve the purpose of the building or bridge correctly: I need to weave in the skeleton of the structure without it becoming obtrusive to the people using it. The materials must also resist the stresses and strains of loads that assail a building, and perform well in the face of movement and temperature fluctuations. Ultimately, my choice of material has to ensure that the structure survives as long as possible in its environment. Luckily, my engineering creations are more successful than my baking endeavours.
The science of materials has long obsessed humans, and since ancient times we have theorised about what makes up ‘stuff’. The Greek philosopher Thales (c. 600 BC) contended that Water was the primordial substance of all things. Heraclitus of Ephesus (c. 535 BC) said it was Fire. Democritus (c. 460 BC) and his follower Epicurus suggested it was the ‘indivisibles’: the precursors to what we now call atoms. In Hinduism, the four elements – earth, fire, water and air – described matter, and a fifth – akasha – encompassed that beyond the material world. Roman engineer Vitruvius writes in De Architectura agreeing that matter is made up of the same four elements, adding that the behaviour and character of a material depends on the proportions of these elements within it.
This idea – that there were a limited number of fundamental ingredients which in different proportions could explain every colour, texture, strength and other property of any material – was revolutionary. The Romans surmised that materials which were soft must have a larger proportion of air, and that tougher materials had more earth. Water in large proportions made a material resistant to it, and brittle materials were ruled by fire. Ever curious and inventive, the Romans manipulated these materials to better their properties, which is how they made their renowned concrete. They may not have had the periodic table (it would be a while before Dmitri Mendeleev published the original version of the table in 1869), but they knew that the properties of a material depended on the proportions of its elements, and they could be changed by exposing it to other elements.
For a long time, however, humans simply built from the materials that Nature provided, without changing their fundamental properties. Our ancient ancestors’ dwellings were made from whatever they could find in their immediate surroundings: materials that were readily available and could be easily assembled into different shapes. With a few simple tools, trees could be felled and logs joined to create walls, and animal skins could be tied together and suspended to form tents.
If there were no trees, humans created homes from mud. As we developed our tools and became more innovative and daring, we took this one step further – we tried to make the mud better by shaping it into rectangular cuboids of various sizes using wooden moulds. We discovered that by allowing the mud to dry in the sun (according to Roman philosophy, letting the water escape and the earth take over, using fire), the result was a much tougher unit. Humans had created the brick.
Bricks were already in use around 9000 BC in an expanse of desert in the Middle East. In the deep valley of the River Jordan, hundreds of metres below sea-level, Neolithic man created the city of Jericho. The residents of this ancient city baked hand-moulded flat pieces of clay in the sun and built homes with them in the shape of beehives. As early as 2900 BC the Indus Valley Civilisation was building structures using bricks baked in kilns. It was a process that required skill and precision: if it wasn’t heated for long enough, the shaped mud wouldn’t dry out properly. Heated too much and too quickly, it would crack. But if baked at the right temperature for just the right length of time, the mud became strong and weather-resistant.
Archaeological remains from the Indus Valley Civilisation have been found in the ruins of Mohenjo-daro and Harappa, in modern-day Pakistan. 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. At about the same time as the Indus Valley Civilisation, the Chinese were also manufacturing bricks on a large scale. But for the humble brick to become one of Western civilisation’s most used materials, we had to wait for the rise of one of its greatest empires.
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The energy and inventiveness of Roman engineering is, for me, a source of wonder and inspiration. So it was with not a little excitement that I took a train south from Naples, along the coast, to one of the most famous archaeological sites in the world. Wearing matching sandals, my husband and I alighted at our destination and put on matching safari hats to keep the scorching summer sun at bay. In great anticipation, we strode towards the ancient ruins of Pompeii.
Along the cobbled streets were shopfronts with counters studded with holes in which conical pots or amphorae were once stored. On the ground was a dramatic floor mosaic of writhing fish and sea creatures. Another showed a ferocious canine and was inscribed with the legend ‘Cave canem’ – ‘Beware of the dog’. Alongside these were well-laid-out homes, like Menander’s (a Greek writer), with its spacious atrium, baths and garden surrounded by a beautifully proportioned colonnaded walkway or peristyle. All these gave a powerful impression of what a glorious, bustling town it must have been in its heyday.
Among the things that most caught my eye, though, were the blood-red bricks. They were everywhere. They peeked surreptitiously from columns on which the decorations that originally hid them from view had crumbled away. They looked proudly on from the walls, where they were arranged in thin layers of three, alternating with sharply contrasting layers of white stone. But my favourite brick-built features were without doubt the arches.
Arches are important building components. They are curved – they are a part of a circle or an ellipse, or even a parabola. They are strong shapes. Take, for example, an egg: if you squeeze an egg in your hand
with a uniform grip, you’ll find it nearly impossible to break because the curved shell channels the uniform force of your hand around itself in compression, and the shell is strong in resisting it. To crack the shell, you normally have to use a sharp edge, such as the blade of a knife, on one side, creating a non-uniform load. When you load an arch, the force is channelled around its curved shape, putting all portions of the arch in compression. In ancient times, stone or brick were commonly used building materials – these are great under these squashing loads but not tension loads. The Romans understood both the properties of such materials and the virtues of the arch, and they realised they could bring the two things together in perfect union. Until then, flat beams were used to span distances, whether in bridges or buildings. As we saw earlier, when loaded, beams experience compression in the top and tension in the bottom – and since stone and brick aren’t very strong in tension, the beams the ancients used tended to be large and often unwieldy. This limited the length of the beams’ spans. But by using the high compression resistance of stone in an arch, the Romans could create stronger and larger structures.
Forces channel around the curve of the arch; it is all in compression all of the time.
The brick arches surrounding me had survived millennia, and made me think of the beautiful ancient Arabic saying ‘Arches never sleep.’ They never sleep because their components are continuously in compression, resisting the weight they bear with endless patience. Even when Mount Vesuvius spewed lava over Pompeii, smothering its people and buildings, the arches remained the watchers of the city. They may have been buried, but they never stopped doing their job.
The ruins of Pompeii show us that the Romans used brick in almost every form of construction in the lands they conquered. In Italy and elsewhere, legions operated mobile kilns, spreading this practice as far as what are now the British Isles and Syria. You won’t be surprised to learn that Vitruvius had an opinion on the material needed to make a perfect brick, the description for which he outlined in De Architectura. Creating a brick is much like creating a cake, so here’s my take on a recipe for The Ancient Brick, courtesy of a range of ancient engineers – one that even I would be able to follow.
RECIPE FOR THE ANCIENT BRICK
Ingredients
Clay
‘They should not be made of sandy or pebbly clay, or of fine gravel, because when made of these kinds they are in the first place heavy; and secondly, when washed by the rain as they stand in walls, they go to pieces and break up, and the straw in them does not hold together on account of the roughness of the material.
They should rather be made of a white and chalky or red clay, or even of a coarse-grained gravelly clay. These materials are smooth and therefore durable; they are not heavy to work with, and are readily laid.’
The waters of fruit
Warmth, in the form of the sun or a kiln
Method
1.Throw a lump of clay into knee-deep water and then stir and knead forty times with your feet.
2.Wet the clay with the waters of pine, mango and tree bark, and the water of three fruits, and continue kneading it for a month.
3.Form the clay, mixed with a little water, into large, flat rectangles using a wooden mould. (The Greek Lydian brick – typically used by the Romans, as per Vitruvius – is a foot and a half long and one foot wide.) Once formed, remove the bricks from the moulds.
4.Heat the clay gently and gradually. If made in the summer the bricks will be defective because the heat of the sun will cause their outer layers to harden quickly, while leaving the insides soft and vulnerable. The outer, drier layers will shrink more than the moist inner layers, causing the bricks to crack. On the other hand, if you make the bricks during the spring or the autumn they will dry out uniformly, due to the milder temperature.
5.After an interval of between two and four months, throw the bricks into water, take them out and allow them to dry completely.
Patience is key, as it takes up to two years for bricks to dry completely. Younger bricks will not have dried out completely, so may shrink over time. A wall made from such bricks and then plastered over will be seen to crack. Vitruvius alerts us to this: ‘This is so true that at Utica in constructing walls they use brick only if it is dry and made five years previously, and approved as such by the authority of a magistrate.’
Roman bricks were, in general, larger and flatter than those we use today. They looked more like tiles: the Romans favoured that shape because they realised that, with the tools and methods they used, flatter bricks would dry out more evenly – an essential feature of the ideal brick recipe. From the temples in the Forum in Rome to the Colosseum and the extraordinary triple stack of arches that make up the Pont du Gard aqueduct that spans the River Gardon in southern France, bricks formed the basis of their most impressive structures.
The Pont du Gard aqueduct, across the River Gardon in southern France, is made up of a stack of three brick-built arches.
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. During the Renaissance and Baroque periods (from the fourteenth to the early eighteenth centuries), exposing bricks in buildings went out of fashion, and instead they were hidden behind intricate plaster and paintings. Personally, I like seeing bricks on display, much as I like seeing the air ducts and escalators on the outside of the Centre Pompidou. I prefer my structures direct and honest: like my cakes, I enjoy being able to view the materials from which they are created (this has nothing to do with my complete lack of icing skills).
During the Victorian period in Great Britain (1837–1901), and between the World Wars, the use of brick peaked to its highest in recent history. One of my favourite buildings in London, George Gilbert Scott’s grand Gothic fantasy the St Pancras Renaissance Hotel, is a spectacular example of an exposed brick structure. Up to ten billion bricks were made annually in Britain. It seemed that all structures, from factories to houses, from sewers to bridges, were made from bricks, left exposed for all to see.
The brickwork of a Roman arch at Pompei, southern Italy.
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Such a timescale, stretching back millennia, is already hard to get your head around. But that’s nothing compared to the dates involved in the creation of the raw material that makes up a brick. During the filming of Britain Beneath Your Feet, a two-part documentary about the ground and what’s under it, I visited a clay mine in north-east London. There I was confronted by a vast clay cliff, sculpted by diggers from the ground on which London sits. The mine owner pointed to the top of it, which was the colour of rust. ‘That clay is new, it’s only twenty million years old.’ My flabbergasted expression prompted him to continue. He explained that the ‘newer’ layers of clay had a much higher iron content, giving them a reddish tint. The stuff at the foot of the cliff was purer, so it had a blue-grey hue – a sure sign that it was older.
By ‘older’, he meant more than 50 million years old. Long ago, igneous (volcanic) rocks were weathered and transported by water, wind and ice. While the rocks and stones were being carried along, they picked up particles of other minerals such as quartz, mica, lime or iron oxide. This mixture of rock and minerals was deposited far from its original home in layers of sediment at the bottom of rivers, valleys and seas. In these environments, plants and animals thrived then died, adding a layer of organic matter that would then be covered by more rock, and so on. Gradually, over millions of years, under the right conditions of temperature and high pressure, these layers turned into sedimentary rock. And that’s what the miners were busy digging out of the cliff face. The owner told me that, because of its incredible age, the clay is full of the fossils of tropical plants such as mangrove palms (which once flourished in British climes), and the ancestors of birds, turtles and crocodiles that no longer exist on Earth.
The mined clay is used for many things
: crafting pots, art projects in schools and, of course, making bricks. For this, it is transported from the mine to factories where it is transformed into neat, solid cuboids. The principle of heating clay to create a brick hasn’t changed from ancient times, but the method has. First, we treat the clay by adding extra sand or water to make it the right consistency: stiff but malleable. Then it is put into a machine that extrudes it through a mould or die (a bit like the hand-press in a Play-Doh Fun Factory, but on a slightly larger scale). The clay emerges in a long, rectangular column, which is chopped into brick-length pieces and conveyed to a dryer to gently remove as much moisture as possible – otherwise you end up with the cracked bricks that Vitruvius cautioned us about. The dryer is set at the relatively low temperature of 80–120° Celsius and is humid enough to stop the bricks from drying too quickly on the outside while the insides are still damp. And as they dry, they shrink.
If the process is stopped here, bricks similar to the ancient kiln-dried examples would be created. 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. A fired brick is pretty strong: if we took the four elephants that support the Earth (and cause earthquakes when they stretch) from Indian mythology, added one more for luck, persuaded them to stand on top of each other and then tip-toe onto a single brick, the brick would remain intact.