National Geographic Ancient Megastructures St Paulus Cathedral

In 1666, thousands fled as flames raced across the city of London. The wooden structure of St Paul's Cathedral was destroyed by the inferno. But from the ashes, a new architectural wonder would rise, to dominate one of the wealthiest cities on the globe. It would become an enduring feature of London's skyline. The struggle to raise its magnificent dome dominated the life of its architect. Christopher Wren confronted huge engineering challenges, as the new cathedral threatened to collapse. And he took a massive gamble, using revolutionary ideas to realise his breathtaking vision. 300 years after it was built, The dome of St Paul's Cathedral remains one of London's greatest icons. Views of the monument are now protected by law. No new buildings are allowed to obscure it. But behind this familiar image is a revolutionary structure. This dome has a secret, inspired by pioneering scientific discoveries. It owes its existence to a catastrophe. Londoners feared that 1666, ending in the apocalyptic 666, would be a year full of dread. On the 2nd of September, a fire broke out in Thomas Farriner's bakery in Pudding Lane. A violent and unseasonal easterly gale fanned the blaze, and the fire ripped through the medieval timber buildings of London. The great fire burned for four days straight, destroying 13,000 houses and 93 churches. As the firestorm engulfed London's magnificent Gothic Cathedral on Ludgate Hill, fears of an impending apocalypse intensified. St Paul's was severely damaged, a huge loss to the city. There'd been a cathedral on Ludgate Hill for a thousand years before the great fire. Author Leo Hollis has written extensively about the disaster. For him, St Paul's was not just the most important church in the city, but also a focus of national pride. It was one of the most extraordinary buildings of the age. It was certainly one of the largest buildings in Europe. It was the place for a celebration for military victories, was the place where the king and the citizens would meet. And this great building played a vital role in the daily lives of Londoners. Well, old St Paul's was the focal point of old London. Historian Andrew Lacey says the cathedral wasn't simply a religious landmark. It wasn't just a church. It was a place where people went to do business, to find a prostitute. There was a thriving book trade in St Paul's churchyard. It was very much a cultural hub, rather than just a church. Desperate to restore this city landmark, a royal commission of churchmen and city officials attempted to repair the cathedral. But the structure was a death trap. All efforts to salvage it came crashing down. There was no choice. As one of the world's most prominent cities, London needed a fitting cathedral as its centrepiece. In deciding who should design and oversee the building, the royal commission took a risk. It appointed an astronomer. His name was Christopher Wren. It would seem very strange to us now that Wren, whose background was in astronomy and experimental science, should be chosen to design a building. But as architectural historian James Campbell points out, professional architects didn't exist at that time. In the 17th century, architecture was perceived as a branch of mathematics. And Wren, thought of as one of the leading mathematicians of the age, was thus a natural choice. 35-year-old Wren was one of a new breed of scientists unlocking the secrets of the universe. They established the Royal Society, which broke with tradition, basing its thinking on deductive logic without influence from religion or philosophy. When he turned to St Paul's, Wren also broke with architectural tradition. His pioneering designs caused conflict with his employers, delaying the start of construction until almost nine years after the Great Fire. The first stone of the new cathedral was not laid until the 21st of June, 1675. But there was much more at stake than mere architecture. In an age of intense religious conflict, where the wounds of the English Civil War had barely healed, even the shape of a church held huge significance. In the previous 500 years, the church had favoured the Gothic style for the great cathedrals of England. But this was a problem for Wren. Like most educated men of the 17th century, Wren would have considered Gothic architecture to be rather rude, rather barbarous. It was associated with the Middle Ages, a product of a less civilised time. As a mathematician, Wren also disliked the odd shapes and proportions of Gothic architecture. He believed that only certain regular geometric shapes and mathematical symmetry could create true architectural beauty. He describes two types of architectural beauty. The first is that of architectural style, architectural decoration. But the second one was more fundamental, and that was mathematical beauty. The proportions of a building, division of it into cubes or spheres, which gave it its fundamental beauty. For Christopher Wren, the classical buildings of ancient Greece and Rome embodied a superior geometric beauty to Gothic architecture. In Italy and France, Baroque architects also looked to classical architecture for inspiration. Wren was enthused by their designs and their domes, but his passion was not shared by his employers. The new architecture flourishing in Paris and Rome was sponsored by the Catholic Church. England's Protestant clergy, represented on the Royal Commission, was deeply hostile to Catholicism. To sell his classical vision to the Royal Commission, Wren gambled and commissioned a hugely expensive scale model of his design. But if he could not persuade the doubters, his dream of a classical cathedral would crumble before it was built. In an upper room of St Paul's Cathedral stands a huge wooden model of the church. It is one of the most intricate architectural models of all time, and to build it would cost hundreds of thousands of pounds in today's money. For Leo Hollis, this so-called great model design was Wren's definitive vision of his new church. This is the great model. In 1672, Wren came up with a brand new vision of what St Paul's could be. Everyone could see Wren's vision. It was detailed and carved both inside and out. Reigning above the cathedral and above London itself was the dome, the heart of Wren's plans. This was his dream. It was heavily inspired by recent French and Italian designs, such as St Peter's Basilica in Rome, and it had support in high places. Even the current king, Charles II, is an enthusiastic supporter of Wren's design. But it wasn't the king's money that was to pay for the cathedral, and Wren's grand vision was about to suffer a serious setback. The Royal Commission was manned by Protestant churchmen, wary of any traces of papist or Catholic influences. Wren's hugely innovative plan was a step too far for his employers. The clergy didn't like it because it didn't look like a traditional English cathedral. Also, they probably thought that the building looked too foreign, and in the 17th century, to be accused of being foreign almost inevitably meant that you were a papist as well. Traditionally, cathedrals have been built in parts. You start at the east end and build sections of your cathedral, which means you can get into the east end, the choir, and start holding services, whilst the rest of the building is still a building site. And you can't do that with the great model design. It's a truly classical building which has to be built as a whole. It was a great blow to Wren. It's said that it's the only time he was seen to weep in public. It was a devastating setback, but Wren persevered. Bowing to the Royal Commission's demands, he decided to play safe. He offered a more traditional design, shaped like an elongated cross. It could be completed in stages, as the Commission required. It was a fudge of two different styles, a long way from Wren's original classical vision. But the Commission approved the plan and gave the design the Royal Warrant. Wren was able to start his building. The warrant design was an unusual set of plans. He found himself having to compromise, to reconcile the differences between Gothic architecture and classical architecture. You see it. The dome has been flattened out completely, and there is a strange, bizarre sort of spire that rises up through the centre of the building. For Wren, the success of this fudge design was a bittersweet victory. It was not the design he favoured. Could he defy the Commission and go back to his original plan? He needed to make an immediate decision, because he had to lay the foundations for whichever of the designs he was going to build. Wren knew that without sound foundations, a huge building can lean dangerously. When you look at the ground, actually, it looks like something firm that won't move, but that's a delusion. Civil engineer Ed McCann explains how the lack of sound foundations can lead to catastrophic failure. Actually, the ground is more like a mattress with an infinite number of variable stiffness springs. So if I push down hard in one place, I'll go down so much. If I push down hard in another place, I'll go down so much more. And when you put big, heavy buildings on top of this giant mattress, it's inclined to go down more or less in different parts. Now, clearly, that could be quite catastrophic for a building, because if that bit goes down a lot, that bit doesn't go down at all. We can get a situation where it breaks its back or towers lean over too much or buildings separate. Wren knew his huge building would need a stable base. He had a difficult choice to make. He could save time and money if he built on the old foundations, which had already compressed the ground. But his new design was a different shape, and he would have to adapt it, which, for Wren, was out of the question. Wren decided to dig new foundations. The old foundations would have to be broken up. Any hard areas left behind might affect the cathedral's stability. He examined the ground and found layers of clay and sand. He decided to build on the clay, believing it would offer a more stable base than sand. Wren didn't yet know it, but his choice of foundations was to have dramatic consequences for his cathedral. Nearly ten years after the Great Fire, the rebuilding began. Wren decided to tailor the cathedral plans to something close to the original Great Model. The Warrant Design, approved by the Commission, became architectural history. Wren's new plan was another compromise, using the Great Model as an inspiration, but like the Warrant Design, it could be built in stages. Making any changes to the Warrant Design was a gamble, but Wren believed he could create room for manoeuvre, and through a mix of cunning and proud defiance, he would build his cathedral. Keeping his plans under wraps would be difficult. Wren needed to hide an entire building. Like any large construction project, St Paul's needed a vast framework of scaffolding. Wren could hide his architectural rebellion behind a screen of timber and canvas. But there was another problem. He was worried that the money might run out before he could complete his masterpiece. After the destruction of the Great Fire, rebuilding both the city and its magnificent new cathedral was a massive financial burden. Most of the money would be raised through a tax on the main domestic fuel, coal. The Royal Commission wanted the cathedral, but the coal tax was set by Parliament, so it held the purse strings. Wren feared Parliament wouldn't stomach the cathedral's costs and might halt construction before it was complete. Building the cathedral in sections, Wren knew the money could run out before he realised his dream of crowning it with a glorious dome. He gambled with a bold and simple plan and committed to building the whole cathedral. Long before he completed other sections of the building, Wren had already laid the foundations for his visionary dome. He calculated that Parliament wouldn't call a halt to the half-built cathedral, leaving an eyesore in the heart of the city. They would have to complete the cathedral as he planned. But behind the timber curtain, Wren quickly ran into engineering problems. Rejecting the Gothic style in favour of his classical vision created a structural challenge. To prevent them from collapse, high-walled buildings need extra support. In Gothic buildings, this job is done by the flying buttress. Freestanding buttresses allow thin walls to rise to extraordinary heights. Flying buttresses presented a particular problem for Wren because they were, in essence, Gothic. And if he was to design a cathedral where they could be seen on the outside, the building would not look classical. And thus, he had to devise a way of hiding them. Wren had an ingenious plan to save his classical vision. Where the old cathedral had thin walls supported by buttresses, Wren built massive walls, two and a half times thicker. But Wren was aware these colossal walls could destroy the building's elegance. So to make his walls appear lighter, Wren devised a clever disguise. The walls are massive, over 16 and a half feet thick. Now, normally, if you make walls that thick, the internal space is going to be very dark. But Wren's great move is to carve out this great niche. So the windows are set on the outside wall and the space is flooded with light. Well, the masonry on either side acts as great internal buttresses, strengthening the wall. Despite these reinforcements, Wren still needed additional support for the upper stories. He had a brilliant solution. An aisle was to run along each side of the building. Wren's idea was to put buttresses inside the lean-to roof over this aisle. They would be invisible. But it wasn't enough. Wren soon calculated that his hidden buttresses wouldn't be up to the job. The load on the walls meant they would need to be supported much higher up, so the buttresses needed to be steeper and more visible. It was a disaster. The engineering would be on show, just as it was in old Gothic cathedrals, precisely what Wren was determined to avoid. Another quick fix needed to be found. Wren devised an original design to hide the problem, with a fake wall. Under the dome and at the west end, the cathedral is two stories high. Wren decided to make it look as if the rest of the cathedral would also be two stories high by using false screen walls all the way round. This is what those screen walls look like from the inside, creating this enormous space which conceals these flying buttresses, these features that are essential for holding up the upper half of the cathedral. This enabled the inside of the cathedral to be built without any ornament, cheaper and easier. But despite Wren's skill, lower down the project hit an embarrassing setback. To save money on laborious stone cutting, Wren built the cellar vaults out of bricks. The builders constructed the vaults over a wooden frame. Once the brick vault was complete, they backfilled the space above with rubble and removed the frame. When the wooden formers were taken away, it was a disaster. The vaults collapsed. We're not exactly sure what happened, but we think it was a problem with not adequately backfilling behind the vault. If we imagine this is a model of the vault, the floor of the cathedral would come here. Now, the backfill was rubble that was poured in to this triangle left over. Now, with the weight of the floor on top pushing down, it tends to push up these elements of the arch. Without the backfill, they would lift, causing the arch to collapse. Even more embarrassingly for Wren, the vaults collapsed not just once, but twice. Wren couldn't afford delays. Parliament demanded results. The 58-year-old architect needed Parliament to renew the coal tax to keep the money flowing. He had to finish the east end, the choir, so ministers could hold services again. But once that was complete, there was a chance that the Royal Commission would call a halt to the rest of the building, which would leave Christopher Wren with little chance of living to see his dome completed. One of the crown jewels of St. Paul's Cathedral is the choir. Elegant and ornate, it is a showpiece of 17th-century craftsmanship and a fitting interior for the great cathedral. In the late 1690s, Christopher Wren, now in his 60s, feared that this was all he would ever see of his classical cathedral. The money was running out. In its time, St. Paul's Cathedral was the most expensive building in England. Its total cost was more than £804,000, around a billion pounds in today's money. 20 years into construction, a tax on coal still provided the finance. But Parliament held the purse strings. Before his paymasters would vote for more money for the project, Wren would have to complete the east end of the cathedral, the choir, so they could hold services again. But this presented a dilemma. Wren had been working on the entire building at the same time to ensure that his cathedral was crowned with a magnificent dome. Now, even when the money was short and he needed to concentrate on the choir, Wren still continued with his gamble. He refused to slow down work on other parts of the cathedral. Wren's dilemma is that if he finishes the choir, they might lose interest in the rest of the building. Even while he's working on the choir, he is issuing contracts and commissions to masons to begin work on the great piers that are going to support the dome. So he's hoping that if work is underway on all parts of the building, it will be harder for Parliament and the commission and the clergy to lose interest in the building once they've got their choir. At the same time, Wren couldn't save money by cutting corners on the choir. It had to be an architectural triumph that would win over his critics and keep the cash flowing. The work continued at a furious pace. Wren used several teams of masons to increase competition and speed up the work. But he had to feed them a continuous supply of building stone that needed to be transported from 300 kilometres away in Portland on the south coast. Most of the stone from Portland came round by sea. It was so much easier because the roads were so poor. This contorted route caused delays. Many ships were held up by storms or even captured by the French navy. No architect wants their workmen to be standing idle on a building site because the materials haven't arrived. You've got to keep the flow of materials arriving so that they're in the right place at the right time for the project to move forward. Wren was discovering the art of project management. He faced similar problems with the roof timbers. Vertical king posts were to hold up the horizontal tie beams that would span the roof. But Wren's building was to be so wide that his tie beams would need to be massive. Wren chose a king post roof structure which gave him the low profile on the outside and a long span so that you could barely see the roof from outside the cathedral. And it relied on these tie beams. You could make this out of two pieces of wood and join them together in the middle. But, as in everything at St Paul's, he didn't want to cut corners. He wanted to make something simple that would last a long time. Wren wanted timber that could span 14 metres. But he didn't just need one. He needed 48. And to make them, he needed 150-year-old oak trees. A duke offered oaks from his estate but they were 250 kilometres away. Transporting them by cart over rutted tracks and then by ship took weeks. When the timbers finally arrived Wren completed the roof. The east end was nearly finished but was still open to the elements where it would join the planned dome. Wren asked his carpenters to craft a huge wooden wall to keep out the rain. Only when the choir was weatherproofed could Wren call on master craftsmen from all across Europe to decorate the interior. In 1694, Wren could finally turn his focus onto the decoration of the choir. He wanted only the best craftsmen and for his carving, he went to Grinling Gibbons, by far the most extraordinary carver of his generation. Right at the beginning of the process, they sat down and decided that because they were not able to do the Catholic tradition of saints and of New Testament characters, they would reach for nature. They would go for the abundant harvest of the natural world and they divide a series of beautiful panels which include cornucopia, bursting pea pods of heads of corn in order to represent the beauty of the natural world. They also came up with a series of cherubs to run along both sides of the stalls. The story goes that Grinling Gibbons based all of the cherubs on the faces of his two twin children. But in fact, every single one is unique and an extraordinary testament to England's premier carver, perhaps the greatest carver in British history. In December 1697, the choir was finally finished, 31 years after the old cathedral burnt down. The East End of St. Paul's was ready, just in time to become a renewed focus of national pride. The war against France was over and the people of London came out to celebrate. It would have been a very important occasion. Ludgate Hill was thronged with people and carriages eager to take part in the service and to see their new cathedral, part of their new cathedral, because most of it's still a building site. To Wren's relief, when the choir was finished, Parliament didn't halt the construction. Finally, he could turn to the most ambitious part of his vision, the Great Dome. Except his moment of triumph was turning into a nightmare. His half-built cathedral was sinking. The foundations he had laid so meticulously two decades earlier couldn't even support the huge walls, let alone the dome. If Wren was to go ahead with his massive stone dome, the cathedral could quite simply collapse. The most iconic element of St Paul's Cathedral is its dome, surmounted by a gold cross standing 111 metres above the ground. It weighs 65,000 tonnes and is supported by eight awesome pillars. The most ambitious dome ever built in England is in many ways Christopher Wren's greatest achievement. But the design hides a unique piece of engineering based on a remarkable mathematical discovery made by another leading scientist of the time. The war against France was over, but Parliament didn't increase the coal tax which was funding the building of the cathedral. In 1697, it reduced it. They also withheld half of Christopher Wren's salary until the cathedral was completed. He calculated that there would not be enough money to complete the cathedral. Christopher Wren's vision of a great classical dome seemed doomed. By the end of the 1690s, Wren was forced to go back to Parliament and to ask for more money to complete the cathedral. He said that the two major things left were the west front and the dome. He also put in his own personal plea that he wanted to see the dome completed. He was absolutely paranoid that if they started on the west front, that they would get a building that they would be happy with and they would cut off the money before the dome was complete. But Wren had been dreaming of this dome for over 40 years. Wren won the support of the Royal Commission, but just when it looked as if the construction of the dome would happen, he faced an even more formidable obstacle. It became obvious to Wren that the entire building was sinking. The new foundations he had laid 20 years before were giving way under the weight of the massive walls. And the colossal piers, which would support Wren's great dome, were sinking faster than the rest. If a solid stone dome weighing thousands of tons was added, the cathedral could crumble under its weight. Wren was not about to give up on the dome he had dreamed of for 40 years. There was nothing he could do about the foundations without knocking the building down. If he could somehow reduce the weight of the dome, the foundations might hold, but it would be a huge gamble. One solution would be to construct a double-skinned dome. In double domes, a slimline stone inner dome supports a larger, more impressive outer dome built from timber. The weight would be reduced, but this solution was not for Christopher Wren. He was determined that his dome would carry an impressive stone lantern to rival St Peter's in Rome. Weighing 850 tons, it was far too much for a timber dome to support. Wren embarked on the almost impossible task of designing a supremely strong dome with a vastly reduced weight. In its ingenuity, Wren's solution was both audacious and groundbreaking. When we get inside the dome, it's a complete surprise. It's not what we expect, a single dome, nor is it the double dome that we find in France or Italy at the time. It's something completely new, something entirely of Wren's invention. Wren's solution was to build an invisible third dome. Which was also a completely different shape to the others. The inspiration for this ingenious structure came from one of Wren's principal collaborators, one of the most important scientific thinkers of the age. Robert Hooke was already famous for work in chemistry, physics and the development of microscopes. He had wrestled with the problem and found that the answer lay remarkably in chains and the shapes they make. The result was one of the most original pieces of engineering ever attempted. Hooke hangs chains up and he finds that if all that happens in a chain is it acts under its own self-weight, hasn't got anything on it, then it will form this form called a catenary. In this form, every single element is being pulled. Everything is in tension, like this, all the way around. He goes a step further and he realises if he flips that thing over so that everything is now into compression, being pushed all the way around, that this would make a very effective and efficient compression structure. Inspired by Hooke's discovery, Wren rethought the idea of double domes and created a revolutionary triple dome, unprecedented in architectural history. Once again, Wren had opted for architectural sleight of hand, creating two structures for show and another hidden structure to do the work. The inner dome is the one on view inside the cathedral. It fits the proportions of the internal space and forms a hemispherical ceiling. Geometrically, it resembles the arc of an unweighted chain. It is self-supporting, but doesn't hold up any other part of the dome structure. The outer dome is also there simply for show. It achieves the strikingly dramatic profile that Wren wanted to impose on the London skyline. Again, it reflects the shape of an unweighted chain, but its massive profile is deceptive. It, too, carries hardly any weight. The thing that makes the impact on the London skyline, that makes St Paul's so imposing, is this, this timber structure covered by planks and lead, which is the outer dome, which gives St Paul's its grandeur. It's the illusion of being a huge dome without the weight of the economy of timber. The heavy work is done by the third dome, hidden between the inner and outer domes. It provides the secret support for the cathedral's 850-tonne stone lantern. This revolutionary third, middle dome, is a different shape to the others. This is the third structure, Wren's greatest invention. Entirely secret, concealed from both outside the cathedral and inside it. It is a brick cone designed to support the lantern above, that great stone structure that bears down on it, and as it does so, pushes out. So Wren embeds within the brickwork iron chains, which resists those hoop stresses. Even though it's a cone rather than a hemisphere, once again, it's Hooke's experiments which hold the key to its shape. Hooke found that with his hanging chain, when he hung something off the bottom, the sides got steeper, it formed a different shape. It wasn't a catenary anymore, it was a different shape. And he realised that this might be useful to his friend Christopher as well, because when this was turned over, this thing was very much like the lantern that he had to support St Paul's. Wren made the middle dome the shape of a heavily weighted chain, making an incredibly strong structure. Through Hooke's brilliance and Wren's design, St Paul's would have a stone lantern to rival St Peter's. To Wren, St Paul's dome was the epitome of perfect architecture. All the engineering that would carry its massive load was cleverly concealed, leaving only airy space and elegant form on display. If only it could be seen. The trouble with Wren's revolutionary structure was that the innermost dome inside the cathedral would be very dark. He needed to find a way around this, and his solution would rely on another piece of architectural trickery. The inside of the dome of St Paul's Cathedral needed natural light to reveal its splendour. With a bold stroke, Christopher Wren created a solution. You hear a lot of people tell you that if you take the keystone out of an arch, an arch will fall over. Of course, this is true. If you imagine an arch like this, and the keystone there, and I pull it out, well, clearly the arch falls down. It's also true of every other bit of an arch as well. There's nothing magic about the keystone. If you take out any part of an arch, it will fall over. Now, domes are a bit more resilient than that. You can take bits out of the dome and they won't fall down. If you imagine a dome like this, and you imagine loads increasing on it until it fails, what it wants to do is fail by the top middle part going in, and the bottom part kicking out. It fails inward like that. It doesn't fail like this. It fails like this. You can imagine how it can't push in if you've got any rings built into the top. And so you can easily imagine cutting a hole and that that hole would not close up, provided there was enough compression strength in the ring that went around it. Wren took advantage of this principle. He removed the central portion of the dome and punched windows through the outer and middle domes to allow light to pour in from the lantern above. His final touch was to create an optical illusion. What looks like a masonry outer dome is just a painting on the inside of the brick cone. The building of St Paul's had taken 35 years. Christopher Wren had survived to see his vision completed. In 1708, at the age of 76, Wren watched as his son was hauled up to the top of the dome to lay the final stone of the cathedral. Single-handedly, Christopher Wren had designed a classical masterpiece to compete with anything in Europe. What he had done in the last 50 years is not only just transform St Paul's itself, but transform the idea of what an architect should be, as well as transform the idea of what British architecture should be. St Paul stands as a symbol, as perhaps his greatest work. Here is a building that could compete with St Peter's in Rome, could compete with any other capital in Europe. Wren outthought and outlived his original opponents on the Royal Commission. His cathedral bore more than a passing resemblance to his original great model, rejected over 30 years previously. Historian Andrew Lacey is in no doubt that Wren's cathedral quickly became the iconic symbol of one of the world's pre-eminent cities. It very, very quickly becomes the feature of the London skyline. There's the Canaletto paintings of the London skyline from the 1740s, and it's a forest of spires and towers of Wren city churches, and in the middle is the great stately dome of St Paul's. So I think very quickly Londoners took St Paul's to their hearts. Three centuries after its completion, St Paul's Cathedral faced its biggest challenge. During World War II, German bombers pounded London and threatened to destroy its greatest landmark. I think in 1940, there was a very real sense that if St Paul's and its dome survives, then somehow Britain will survive. Incendiary bombs rained down on the cathedral, some lodging in the dome itself. But while London was once again consumed by flames, the cathedral itself survived. It's fitting that the symbol Wren chose for St Paul's was the phoenix. The mythical bird which lives for eternity and is repeatedly reborn through fire. For the man who spent more than half of his working life on this building, the project ended as it began, in a battle with the bureaucrats of the Royal Commission. A dispute broke out over the finishing touches, and the 86-year-old architect was sidelined. He died five years later. Sir Christopher Wren was buried in his cathedral with one of the most famous epitaphs of all time. It says in Latin, Reader, if you seek a monument, look around you. St Paul's Cathedral is a monument to the power of science and the ambition of its designer.