1 jun 08 | Focus

The rise and rise of the superscraper

By Ian Taylor

Chicago, 1956. Frank Lloyd Wright stands in front of an 8m-tall drawing. The canvas behind him shows the master architect's latest grandiose vision, a skyscraper called the Illinois. Wright didn't actually like skyscrapers that much - he once referred to them as mantraps of monstrous dimensions - but he also believed they were necessary to stop cities from sprawling ever -outwards into the countryside. Never one for half measures, Wright's skyscraper was unlike anything the world had seen. A slender spike of a building, the Illinois would house 100,000 people. It would have atomic-powered elevators. And it would easily dwarf what was then the world's tallest building, standing one mile high - three times taller than the Empire State Building.

Of course, the Illinois was never built. Wright believed it could have been, even with 1950s methods and materials. But more than half a century later, no structure has come close to reaching the mile-high target. That's not to say that bragging rights to the world's tallest building have become any less appealing. Today's record holder is still a construction site, but its scale, ambition and engineering ingenuity look set to herald a new age of 'supertall' structures that could well reach the immense heights that Wright (and many others since) envisioned.

The Burj Dubai, which bears more than a passing resemblance to Wright's sketch for the Illinois, became the tallest building in the world when it edged past Taiwan's Taipei 101 (509m) last July. The tower's final height is a closely-guarded secret, but it is rumored that when the Burj Dubai is completed in 2009 - spire included - it will top out somewhere above 800m.

"The Burj has totally changed the playing field," says David Scott, chairman of the Council on Tall Buildings and Urban Habitats (CTBUH) - the body charged with holding the tape measure against record-breaking skyscrapers. The council defines four key criteria to measure: the height of the structural top, the highest occupied floor, the top of the roof and the highest point of any spires. When it's finished, the Burj will hold records for the lot.

"For a long time there were small incremental steps in the height of buildings," says Scott, who is also a principal at the international engineering firm Arup. "With the Burj, people are realizing that the things limiting them before were not actually that real."

The Burj Dubai will be around 50 percent taller than Taipei 101. The tip of its spire will be visible from 95km (60 miles) away. Laid end-to-end, the steel rods that reinforce the structure, all 31,000 tons of them, would stretch a quarter of the way around the world. And if you still don't understand why the Burj is such a beacon, then take one of its express elevators up to the observation deck on the 124th storey. From there you'll be able to see panoramic views that stretch 80km (50 miles).

Make it big

But it wasn't always going to be quite that impressive. When Emaar Properties, the building's developers, first hired architectural and engineering firm Skidmore, Owings & Merrill (SOM) back in 2001, the design was a diminutive 550m - still the tallest building in the world, but only by a piddly 49m.

"I was quite happy to be a meter taller than the world's tallest building," says Bill Baker, SOM's chief structural engineer and the man tasked with building higher than anyone has before. "One has to be careful not to promise more than you can deliver. But the client kept pushing - they wanted to be taller."

Baker was able to accommodate his client's ever-climbing ambitions - even after the foundations were dug 50m into the ground - because of several features about the building's design and the materials used to build it. First of all, the Burj retreats as it climbs - it much wider at the lobby than the penthouse. "If you're standing up and someone is trying to push you over, you spread your feet apart," Baker says. "If you look at this tower, it's clearly spreading its feet as it goes down."

Then there's the concrete. Excluding the foundations, the Burj will consist of roughly 230,00m³ of concrete slabs, walls and columns. It will weigh around 500,000 tons. But every square centimeter of its high-performance concrete is able to withstand an incredible 800kg. Baker predicts that over time, the deadload might sink five or six centimeters into the sedimentary rock, but no more. "It's not going anywhere," he says.

For such a solid structure, the Burj is not actually that heavy - at least compared to a conventional skyscraper. One of the limiting factors in building supertall buildings is the sheer amount (and cost) of concrete and steel required. The higher a building goes, the larger its frame work needs to be to stop the whole thing from tumbling over in the wind. The Burj Dubai doesn't have this problem because its design is based on a revolutionary method of construction known as the 'buttressed core'.

The brainchild of Baker, this sees the building's footprint shaped like a propeller or tripod, with three 'wings' set around a central hub. "The wings are set 120 degrees from each other," Baker explains. "When the wind blows on two of the wings, the third one is resisting the force. So you use the strength of one wing to buttress the weakness in the other two."

At the centre of the building, a hexagonal concrete box anchors the whole thing. "It acts like an axle and stops the building from twisting," Baker says. "You really don't want this thing to spin. Not only do you get very undesirable structural things going on, but there's also a problem with motion perception. If the building is twisting and you look out the window, the horizon moves from side to side. And when you 160 storey's up, that can be really quite unsettling."

The buttressed core not only prevents unwanted inhabitant nausea, it makes life easier for the building's developers by maximizing the retable floor space and the number of windows. It also allowed the structure to go up quickly - at its peak the Burj was gaining a new storey every three days - because while it's a novel engineering concept, it relies on conventional components that Baker knew most contractors could handle.

"The elements are those you might see in any residential high-rise," he says. "They're conventionally dimensioned but put together to form this buttressed core. We took conventional elements and arranged them to make a new system."

Yet for all the Burj Dubai's technical innovations, safety is still a major issue. When building so high, there are four main threats to worry about: wind, fire, earthquakes and, since 9/11, terrorist attacks. The buttressed core does a fine job of minimizing the risk from high winds, while 'fins' on the structure's perimeter also help to deflect prevailing Arabia gusts.

Baker also rotated the original design by 120degrees to minimize the effects of the highest winds blowing in from one direction. And after extensive wind tunnel tests, the building's developers could address other safety concerns.

"Fire is the historic issue with tall buildings," Baker says. "Concrete kind of protects itself but steel has to be fire-proofed. And one of the major issues with tall buildings is exiting."

The Burj will be the first supertall structure in which certain elevators can continue to run in the event of a fire. Part of an integrated system called 'Lifeboat', the lifts will carry cameras that will send images to an emergency command centre. That way, fires marshals will know where the most dangerous areas of the building are and when it's safe to use elevators to evacuate people.

There are also stairs, of course, but the massive scale of the building means anyone using them to get out in a hurry would quickly get exhausted. So the Burj also features 'refuge floors' essentially open spaces - off the main stairwell where people can rest if they need to. These might also be used during an evacuation in the event of a terrorist attack on the scale of 9/11.

Dubai has never been a terrorist target, but in building such iconic structures, it pays to read findings from the investigations that followed the collapse of the World Trade Center. For instance, the US National Institute of Standards and Technology found that the initial impact from the hijacked planes was so intense that fireproofing on the tower's steel infrastructures was completely blown off. Some 10,000 gallons of jet fuel then spread fires that weakened the trusses in each building. The floors sagged and the columns bowed, pulling the towers' outer walls inwards and causing the two structures to collapse, one after the other.

Against such a horrific attack, there's only so much you can do, and for security reasons, Baker, who sat on the FEMA investigation into the World Trade Center in 2002, doesn't go into details. "On all of these buildings, we have a series of confidential discussions with our clients and tell them their options," he says.

When the earth moves

Like terrorism, seismic activity is not a traditional problem in Dubai. Nevertheless the Burj is, according to some claims, designed to withstand earthquakes that measure up to 8.5 on the Richter scale. But for some geologists, it's not just resistance to earthquakes that engineers need to think about - supertall buildings might actually trigger them as well.

Scientists at the National Taiwan Normal University are concerned that the huge vertical pressure exerted by Taipei 101 has reopened an ancient fault. In recent years, there have been quakes measuring up to 4.2 on the Richter scale near the building. Other researchers are not convinced, saying Taipei 101's foundations are not deep enough to bother fault lines, but the theory does have historical precedent.

"The triggering of earthquakes by loading is a well-known phenomenon," says Bill McGuire, Director of the Benfield Hazard Research Centre at University College London. "It is commonly seen when reservoirs are filled behind dams. In the 1960s, the filling of the Koyna reservoir in India triggered a magnitude 7-plus quake that killed around 150 people."

Baker falls into the skeptic's camp, claiming that seismic activity around Taipei 101 can be explained by a nearby fault. He also believes that building tall in the 21st century is not only safe, but necessary to prevent cities from sprawling into green belts. "Urban density is good," he says. "Everyone uses public transportation, people walk to lunch. Look at the Sears Tower in Chicago. It has 4.4 million square feet - that's 100 acres on one city block."

The Burj Dubai also promotes a densified (if luxurious) mode of living. Combining apartments, shops and a hotel, it is part of a growing trend for skyscrapers to be much more than high-rise office space. The Chicago Spire, for instance, a 609m tower due for completion in 2011, will be the world's tallest all-residential building. Some claim this kind of vertical living can be part of a green 21st century. Others are not so sure.

"Generally, a skyscraper is not particularly environmentally-friendly because they use about a third more energy and materials to build than a conventional medium- or low-rise building," says Ken Yeang, considered to be the world's leading architect in ecological design. "Obviously, in instances where the building is next to a transportation hub or something like that, its impact is reduced."

For the last two decades, Yeang has pioneered an approach to architecture that puts ecological concerns at the forefront. His 'bioclimatic' skyscrapers use external conditions to control their interior climates, and Yeang believes low-energy buildings generating their own power will soon be the norm.

"The pressures for building to produce their own energy using solar panels, wind turbines or underground heat sources are already growing," he says. "In the future, if a building does not produce its own energy within 10 years, it probably won't get planning approval. This will radically change the approach to building design. It will mean that we can't just design anything we want."

So what does that mean for supertall structures out to top the Burj Dubai? One thing is for certain: the Burj won't be the world's tallest building for all that long. In the next decade, the Mubarak Tower proposed in Kuwait will break the 1000 meter mark. And just down the road from the Burj Dubai, a skyscraper called the Al Burj will go higher at 1050 meters. However, as Focus went to press, speculation was mounting that something even higher could soon be achieved - something that would finally realize Frank Lloyd Wright's dream of a mile-high tower.

The UK's Hyder Consultancy, an engineering company whose portfolio includes the Burj Dubai, is reportedly designing a tower that will be twice as high as the Burj itself, putting it somewhere in the region of 1600m tall. The Mile High Tower, proposed for Jeddah in Saudi Arabia, would cost £5bn to build. It's rumored that helicopters would be used to lift materials and builders to the highest floors. And when completed, it would offer views of the Middle East, the Pacific Ocean and Africa.

If the project does indeed establish a new mile-high club, it will raise one more big question: just how high could a building theoretically go? Until now, financial concerns have held engineers back as much as technological limitations. That said, pumping concrete a mile into the air is hardly child's play and when a building exceeds a mile in height, its elevators will begin to cause real problems.

Firstly, there's the weight of the cables: to support lifts at such heights, the cables would be too heavy to actually raise them. Even in the Burj Dubai, there isn't one elevator that travels from the ground to the top of the building. Even if lighten materials become viable - and there are plenty of researchers working on that problem already - there's something altogether more biological to get around. A person who gets in a lift at ground level and travels a mile upwards will be subjected to enormous changes in air pressure. What's the use in living at the roof of the world if you pass out before you get to the front door?

Yet even this hasn't stopped the most ambitious of blue-sky urban planning. The X-Seed 4000 is a proposed city that would have its foundations in Tokyo Bay in Japan and its head truly in the clouds. The ultimate in urban densification, the project would pack an entire city into one 4000m-high building shaped like Mount Fuji. For now it remains little more than an artist's impression. But according to David Scott, it's not impossible.

"I don't think there is an absolute limit," he says. "Mount Everest is essentially a pile of stone. You don't need a lot of technology to create it - just a lot of money."

Ask the Expert Adrian Smith
Designer of Burj Dubai, the tallest building in the world

Burj Dubai isn't finished, yet it's already an astonishing sight to behold. How would you describe it?
For me, the building is very much about its place, geographically and historically. I drew inspiration from patterns prominent in the region and traditional Islamic architecture, including onion domes, pointed arches and the indigenous flowers. But the tower is very much a modern interpretation of these forms.

What technical developments have enabled us to build skyscrapers so high?
There has been a significant advancement in high-strength concrete that has enabled structures to go much higher without a premium in material cost. There are also advanced construction technologies in areas such formwork (systems for molding concrete) that are enabling supertall buildings to be built more efficiently. Elevator technology is slow to catch up.

How tall can these buildings get in the future?
It's not so much a question about 'how tall they can get', but about how tall they can get and still make sense as habitable buildings'. You have to consider the technological limits of things, like elevator systems and floor transfers - and also a building's footprint. Taller buildings require larger footprints, and the wider the building footprint the less natural light you'll have in the building's interior.

Is Burj Dubai vulnerable to high winds?
Form and shape are critical in how a building responds to high winds. We conducted extensive wind tunnel tests to ensure the tower would perform in response to weather conditions. The Burj Dubai's shape actually sheds the negative forces of the wind moving around the building.

Why have residential high-rises recently become more popular?
Part of the reason is urban densification. Cities are growing at an unprecedented rate across the globe. High-rise structures accommodate larger populations on smaller land footprints, creating walkable neighborhoods, encouraging the use of public transport, improving quality of life and addressing environmental concerns.

Anatomy of a superscraper: inside the Burj Dubai

Concrete pumping
For some of the upper levels, engineers have pumped concrete more than 600m through vertical pipes - a world record. The concrete is only poured at night, and even then ice is added to the mix to keep it cool. If it was poured during the day, the high temperatures could weaken the mix and cause it to set unevenly.

At its peak, the building will require 36MVA of electricity - about as much as 360,000 100-watt light bulbs all going at the same time.

When the wind blows on two of the wings, at least one in the buttressed core resists the force.

The building's foundations sit in sedimentary rock that contains groundwater more corrosive than seawater. To guard against chlorine and sulphates, the steel foundations are coated with a cathodic-protection system that provides a corrosion-resistant surface. There is also a sacrificial layer of concrete to ensure there will always be a protective layer over the steel.

In the sweltering heat of Dubai, the tower requires the equivalent of 10,000 tons of melting ice per day to cool it down. However, the building's height means that the air is cooler outside the upper levels. For this reason, many of the building's air conditioning units are place up high so the system can harness the environment's natural cooling.

Window cleaning
Three permanent, track-mounted machines carry the window cleaners who have the daunting task of washing the building's façade. Housed on the uppermost level, the machines feature telescopic cradles that reach down to level 7. It will take up to four months to clean the whole building, while a separate system is responsible for the spire.

83,600m² of reflective glass coats the exterior of the building.

The combination of Dubai's high temperatures and the building's air conditioning systems means a lot of condensation will collect on the tower. This will be collected and piped to a holding tank in the basement car park. Some 15 million gallons of water - equivalent to 20 Olympic-sized swimming pools - will be collected every year and then used to supply the building's irrigation system.

The tower will feature 56 double-decker elevators, all housed within the central concrete hub. Each one will carry up to 42 people at a time at speeds of up to 10m/s (22mph). None of the lifts travel from the ground to the top of the building.

The building includes 160 flights of stairs protected by reinforced concrete in case of fires. Pressurized, air-conditioned refuge areas are included every 25 floors so that evacuees can rest wile exiting the building.

Buttressed core
The tripod-shaped footprint of the building steadies the entire structure. Its three wings are anchored to a hexagonal concrete block running up the centre of the tower.

192 steel cylinders descend 50m into the ground. These are joined together by a 3.5m-thick 'mat' on top of which the entire structure sits. The foundation also contains 45,000m³ of concrete, weighing 110,000 tons.

Skyscraper safety: protecting tall buildings against…

Most skyscrapers can sway a few metres in the wind without compromising their structural integrity - it's just the people inside who feel uncomfortable. But the very tallest structures require sturdy central cores to anchor them. The Empire State Building and other skyscrapers of that era feature steel beams around their elevator shafts. The Burj Dubai has a 'buttressed core', consisting of a concrete hub at the centre with three wings extended off like a tripod. When wind blows against two of the wings, the third wings, the third wing supports them.

A skyscraper's resistance to fired depends on its materials. Concrete is good at protecting itself, but metal structures have to be fireproofed with materials such as asbestos. A modern way of fighting fires is knowing how it behaves. The FireGrid system developed at Edinburgh University uses sensors around a building to gather situational data which is relaxed to a central computer that plots what the fires will do next.

Modern skyscrapers - especially in places of high seismic activity, such as Japan or California - are subject to strict building codes to protect them against earthquakes. A common solution is the use of tuned mass dampers seismic shock absorbers that stabilize a building. These feature giant springs or hydraulic systems that move in the opposite direction to the earthquake's oscillations.

Since the collapse of the World Trade Center in 2001, numerous recommendations have been made to better protect buildings from terrorist attacks. While even the most high-tech skyscrapers are unlikely to withstand the impact of hijacked passenger jets, many built since 2001 are more fire-resistant and feature improved evacuation routes, with systems brought in to track emergency personnel's movements inside a building.