http://www.bbc.com/future/story/20160808-will-the-skyscrapers-outlast-the-pyramids
Will the skyscrapers outlast the pyramids?
Egypt's pyramids were the skyscrapers of their day – and they are still standing 5,000 years later. Do today's tallest buildings stand a chance of outlasting them?
The cracks first emerged in April. By 29 June 1995, a vast network of fissures spanned the entire fifth floor ceiling of one of Seoul's busiest department stores. Hours later, loud bangs could be heard coming from the roof. The cracks widened.
An emergency board meeting was called but the chairman flatly refused to evacuate, citing lost profits. Then he fled the building.
At 5pm the fifth floor ceiling began to sink. Shopping continued as usual, until the alarms were finally sounded nearly an hour later. But it was too late. The roof went next, followed by the building's main support columns, sending the entire south wing crashing into the basement. 1,500 people were trapped – including the chairman's own stepdaughter – and 502 never made it out of the building.
The Sampooning collapse is an example of how fragile modern engineering can be. Even with materials, equipment and an advanced understanding of physics, the building didn't last five years, let alone 5,000.
The Sampoong department store collapse was the deadliest peacetime disaster in South Korean history (Credit: Kwangmo/ Wikimedia Commons)
Meanwhile the Egyptian Pyramids have been drawing crowds for millennia. Unfazed by earthquakes, erosion or vandalism, they've endured the collapse of the civilisation that built them and the transformation of the Sahara from lush grassland into today's vast desert.
Of these, the Great Pyramid of Giza – completed in 2540 BC – is unrivalled, with superior materials, engineering and design to any built before or since. Ancient Greek tourists would travel thousands of miles to gawk at its towering limestone steps, which were so highly polished they were said to glow; their names can be found carved into its walls to this day.
Remarkably, Cleopatra lived closer in history to today's tallest building – the Burj Khalifa – than she did to this monumental tomb. When the last mammoths died out, it was already 1,000 years old.
Constructions must withstand immense forces just to stay upright, including regular lightning strikes and spiralling 100-mile per hour winds
It was the Burj of its day, towering above every other building until the spire of Lincoln Cathedral was completed around 700 years ago. "The ancient Egyptians were creating a – I hate to say it – launching pad for the deceased, so they could get up there with the Sun and the stars," says Donald Redford, who has been studying the pyramids for over four decades.
Fast-forward to 2016 and we're piercing the heavens with ever-taller buildings, from looming clock towers to 20-storey robots, to the possibility of the first mile-high building – though it's not yet clear if the latter is even possible. We're entering the age of the skyscraper, as people move out of the countryside and pour into ever-crowded cities.
The constructions must withstand immense forces just to stay upright, including regular lightning strikes and spiralling 100-mile per hour winds – not to mention the constant effect of gravity. In some areas you can add major earthquakes to that list. What was the pyramids' secret? And do today's skyscrapers stand a chance of outlasting them?
Originally the Great Pyramids at Giza were white – it's thought the valuable outer casing stones were stolen (Credit: Getty Images)
In fact, the impressive age of the pyramids is no accident. The ancient Egyptians believed the afterlife would last forever and took great pains to ensure their tombs would too. Pyramid design evolved over thousands of years, as they experimented with the materials and architecture that would live up to their ambitions.
"They were always saying this is a construction 'for eternity'; 'for ever and ever' creeps into their vocabulary constantly," says Redford, who currently works at Penn State University, Pennsylvania. They were so confident in their abilities, many pyramids were named with the suffix "of millions and millions of years".
They designed these buildings to last forever – nowadays that's not a priority – Roma Agrawal, structural engineer
Despite their efforts and hyperbolic claims, the Egyptians didn't really know what they were doing – and this may have been a distinct advantage. To make up for gaps in their understanding of the laws of physics, early pyramids were heavily over-engineered. They knew about columns, for example, but didn't know that they could support a roof. They always added extra walls just in case.
Another explanation is sheer size. Take the Great Pyramid. It's less a building than an artificial mountain, made of nearly six million tonnes of solid rock. Five millennia is no time whatsoever when you consider the limestone had been lying in the ground for 50 million or so.
Modern skyscrapers, in comparison, are positively flimsy. It took just 110,000 tonnes of concrete and 39,000 tonnes of steel to construct the Burj, which is more than six times the height of the Great Pyramid. "They designed these buildings to last forever – nowadays that's not a priority. We're designing practical buildings to be lived in," says Roma Agrawal, a structural engineer who worked on the Shard in London.
Just like early pyramids, the earliest generation of skyscrapers may be the most robust. When a B-25 plane crashed into the Empire State Building in 1945, the building was reopened in a matter of days. "Back in the early 20th Century they were still calculating everything by hand, so they always added extra steel just in case," Agrawal. Though the Empire State Building is less than half the height of the Burj, it weighs two-thirds as much.
In addition to all the usual risks, building into the clouds carries some entirely of its own. To make it to 7,000 AD – roughly equivalent to the lifespan of the pyramids today – skyscrapers must run the gauntlet of thousands of years of rain, wind and storms.
"The dominant factor with tall buildings is the wind," says Bill Baker, the structural engineer behind the Burj. As wind rushes past a streamlined object, such as a tree or a lamppost, it whirls into a single organised gust, which circles round to the left, then back to the right, then left again, pushing from alternating directions as it travels and causing the object to sway. In high winds the Burj can sway up to five feet in either direction.
Important buildings like the Burj Khalifa are designed for events which occur just once in nearly two millennia
The trouble is, the higher up you go, the faster the wind gets. To stop skyscrapers falling over – and those at the top getting seasick – they are designed with irregular shapes which disrupt the wind and stop it becoming organised. It may look like an architectural flourish, but the distinctive jagged profiles of the Burj and the Shard are more for safety than artistry.
Even a hurricane wouldn't push them over. "If it's a normal building you might design it to withstand the kind of wind storm you'd expect to happen once every 700 years," says Baker. Important buildings like the Burj Khalifa are designed for events which occur just once in nearly two millennia.
Then there's lightning. The United Arab Emirates, home to the Burj, experiences about 10 lightning storms per year. With up to a billion volts of electricity in a single bolt, skyscrapers are at risk of being hit by a force with more power than a nuclear reactor. "I've been in Dubai during a lightning storm and the Burj is like the lightning rod for the entire city – every minute or so it will get hit," says Baker.
Luckily there's a workaround. During construction, the building's steel "skin" is wired together – every steel bar, every window frame, – right the way down to the foundations. It works like a giant Faraday cage, a protective enclosure similar to the wire mesh on microwave ovens, which keeps its contents safe by restricting electricity to the outside. "I've talked to the maintenance people after some big storms and they don't pick up any damage," says Baker.
Just like bending a paperclip back and forth, if steel is tested too many times, eventually it will snap
Even in an earthquake, skyscrapers are extremely sturdy. In fact, the faster the shaking, the better. It's all down to a phenomenon known as resonance. If the ground is shaking at a frequency which matches the rate at which a building sways, it will keep swaying faster and faster and might eventually collapse. "Slender buildings take a long time to sway back and forth – for the Burj it's about 11 seconds – so it will move but it won't break," says Baker.
It's not entirely failsafe: just like bending a paperclip back and forth, if steel is tested too many times, eventually it will snap. But the recent blockbuster in which a catastrophic Magnitude 9 ripples through San Francisco – sending skyscrapers toppling to the ground – would in reality have made far less compelling viewing.
In fact the real perils are as pedestrian as it gets. "It's mainly about keeping the water out," says Baker.
Fourteen people died when a bomber flew in to the side of the Empire State Building in 1945 (Credit: Getty Images)
Back in the 1930s, 96 of the world's 100 tallest buildings were made of steel. Today most cityscapes are built from concrete reinforced with steel, which combines the tensile strength (ability to resist stretching), of metal with the compressive strength (ability to resist squeezing) of rock.
When kept dry, reinforced concrete is a wonder material which may last forever. But in areas of high rainfall, mild acids in the water will slowly react with the limestone in cement and wear it away – turning steel to rust and riddling the building with holes.
"The fact that the pyramids are in an arid environment is extremely important," says Michel Barsoum, a materials scientist at Drexel University, Philadelphia. Even in the sun-baked Sahara, early pyramids crumbled under the destructive power of frost, which expanded as it formed on chilly desert nights and prised open gaps in the limestone blocks.
Even with a double layer of glass, without regular maintenance most windows aren't likely to last long
For years it was thought that the Egyptians had eventually figured this one out by learning to carve blocks with a tighter fit, though how exactly remained a mystery. Then in the early 2000s someone finally peered at the rocks under a high-resolution microscope. It was Barsoum, who had been tipped off that the rocks weren't natural limestone at all, but had been cast from an early form of cement.
Though he was a ceramics expert – Barsoum had never studied pyramids in his life – he couldn't resist the challenge of finding out. Deep inside the ancient blocks, he found tell-tale signs: microscopic algae, called diatoms, whose hard shells had been partially dissolved by the alkaline cement. "Around 90% of the pyramid is carved stone – but the rest is cast," says Barsoum.
The Egyptians made their rocks from four main ingredients; limestone, lime, water and mud. These reacted together to form a chemical glue. The neat part is that as it ages, the glue reverts back to its raw ingredients, turning the cement back into rock. "It looks smells and tastes like natural limestone," says Barsoum.
But if the main concrete shell of a skyscraper may be relatively resilient, the fate of the their windows is less straightforward. Glass has the weight of granite and the stiffness of aluminium; it would take 10 tonnes of pressure to crush a single 1cm cube. Even in the sea, it can take up to 50 years of tumbling to wear into the colourful frosted pebbles found on beaches. And yet it can spontaneously fail. "It will shatter just sitting there. No one knows why," says Baker.
Even with a double layer of glass, without regular maintenance most windows aren't likely to last long. "Glass isn't much affected by the environment, but eventually windows will break away after the consequences of time due to wind vibrations, storms, etc," says Konstantinos Tsavdaridis, a materials scientist at the University of Leeds.
Finally, would the glass eventually flow towards the bottom of the frame? The idea is based on medieval windows – which appear to be thicker lower down – and the myth that glass is actually an extremely viscous liquid: over hundreds of years, glass was thought to flow to the underside of the frame.
Back in 1998, this popular idea was decisively disproven by a team of physicists, who calculated that it would take a number of years "well beyond the age of the universe" to produce any noticeable change at room temperature. Instead the uneven thickness of antique windows is thought to be entirely accidental – making flat panes of glass wasn't easy a few hundred years ago.
The densely packed island of Manhattan is the birthplace of the modern skyscraper (Credit: Getty Images)
So will today's skyscrapers last as long as the pyramids?
Bill Baker thinks there's a good chance they might. "The structural materials are good for pretty much ever. Yes if they maintain them and no if they don't," he says.
Agrawal agrees. "It depends. If they are looked after I don't see why not," she says.
In the end, most skyscrapers are more likely to be torn down than fall down
According to Konstantinos, concrete structures will last longer, since rust sets in long before concrete begins to crumble. Either way, Redford is not convinced. "I very much doubt it. They are functional structures for sure, but they will last only as long as there is a need for that function. And then they will simply be abandoned," he says.
In the end, most skyscrapers are more likely to be torn down than fall down. In fact the Great Pyramid was not the only impressive building around 4,500 years ago.
One of them, known as The Labyrinth, was reportedly even more extraordinary. "When the Greek historian Herodotus saw it, it took his breath away. He could not describe the size and the weight of some of the huge blocks that went into construction," says Redford. Try and find the building today and it's been completely levelled. The rocks were plundered and used as building materials elsewhere. "If you wander the streets of old Cairo and look at the foundations of old buildings, you'll sometimes see Hieroglyphic inscriptions which come from this very building," says Redford.
If we don't tear down any skyscrapers in say, New York – and they don't fall over – then at the current rate of construction there will be 10,000 buildings over 160 metres tall in the city by the year 7,000. Perhaps it wouldn't be so bad if some of them went the way of the Labyrinth after all.
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