After a tsunami hit Japan in 2011, a 27-metre (89-foot)-long boat was left perched on the roof of a two-storey building. Although almost every other nearby structure had been flattened, this particular building had survived both the wave and the weight of the vessel on top. It was a hostel in the town of Otsuchi, made of concrete blocks with a flat roof. When the tsunami struck, the water swept through the ground floor foyer and knocked down some of the walls, but the supporting corner pillars survived and, as a result, the building stayed up. The houses around it were made of timber and the wave simply ripped them from their foundations.
In this modern version of The Three Little Pigs story the house with the best design is the one that stays upright. But in the 21st century, buildings have a lot more to contend with than hungry wolves.
There are now nine buildings in the world that are over half a kilometre tall with more planned or currently under construction.
At that height, winds cause skyscrapers to sway from side to side by up to two metres (6.6 feet) on the top floors. From below, earthquakes can vibrate the ground to such an extent it turns to quicksand, causing buildings to pull loose from their foundations and topple clean over. Fortunately today’s architects have more than straw, sticks and bricks at their disposal…
Concrete has been used since Ancient Roman times, but the modern version comes in a lot of exciting new flavours. Concrete can be made extra light, extra dense, springy, translucent and even self-healing, while glass can be shatterproof, load bearing and heatproof. And there are totally brand-new materials too…
Magnetorheological fluid normally behaves as a liquid, but in a magnetic field it stiffens to become solid. Pistons filled with this wonder fluid can act as dynamic shock absorbers with great strength and lightning-fast responses. Previously this was the preserve of high-tech vehicle suspensions, but engineers are now starting to use magnetorheological dampers to control earthquake vibrations in tall buildings.
Halochromic paints change colour if the underlying metal begins to rust. This tech is still being trialled for use on aircraft, but one day could warn if a bridge needs repainting.
Fire is a threat to all buildings but the danger is particularly acute in skyscrapers. However many storeys you stack on top of each other, everyone still has to evacuate via the ground floor. The Burj Khalifa has over 160 floors and so taking the stairs all the way down just isn’t practical. Instead the elevators feature water-resistant equipment, redundant power supplies and drainage sills to keep water from the sprinklers out of the lift shafts. If you do need to take the stairs, there are pressurised, air-conditioned refuge areas every 25 floors to allow evacuees to rest and the stairwells are built from highly fire-resistant concrete.
In 1956 the architect Frank Lloyd Wright proposed the Mile High Illinois Sky-City. A steel-framed building 1,600 metres (5,250 feet) tall would have swayed far too much using the construction techniques of the time, and the lift shafts would have taken up all the space on the upper floors, so the project was scrapped.
However, materials, techniques and technology have all come on leaps and bounds since then and a lot of the practical problems have now been solved. The Burj Khalifa is already more than half the height of Lloyd Wright’s science-fiction design and human ingenuity shows no signs of slowing down.
Counteracting the wind
Because they are anchored at the bottom and free at the top, tall buildings sway in the wind. Skyscrapers can defend against this by making themselves stiffer, but only up to a point as stiffer materials are more prone to cracking. Sometimes it is better to design the building with some flexibility and to avoid harmonic frequencies that could exaggerate the movement. Dubai’s Burj Khalifa uses a deliberately irregular, stepped shape to break up wind vortices, while others like the Taipei 101 use tuned mass dampers – giant hydraulic pendulums hung near the top -that swing to counterbalance sway from the wind. Low-rise buildings aren’t safe either. In a hurricane, pitched roofs act like an aerofoil as wind passes over them, sucking them upwards. Hurricane-proof houses use steel struts or cables that run through the walls to bind the roof to the foundations.
Staying steady in an earthquake
Most office buildings and skyscrapers are built with floors and roofs resting atop wall pillars. Their strength comes from the huge weight pressing down. But this strength is a vulnerability in an earthquake as the floors collapse in on themselves. For medium-sized buildings, the best way to quake-proof them is to cut down on the weight.
Lighter roofs and floors lower the peak stresses during an earthquake, while constructing concrete floors by pouring them in situ bonds them to the walls.
Some skyscrapers have huge roller bearings in the foundations that allow the whole building to slide without cracking. Tuned mass dampers can also be used to counter quakes.
Sensing disasters before they happen
Sensors are very cheap compared to the cost of a skyscraper or a suspension bridge, but their valuable information could save lives. Accelerometers provide the raw data to control the swing in the mass damper pendulums of some skyscrapers. But even when the building can’t react immediately, sensors are still vital. The strain gauges on a bridge can detect dangerous harmonic oscillations before they get out of control. This allows the bridge to be shut and helps engineers find tiny cracks that might otherwise be missed. Sensors don’t operate in isolation. Wired and wireless networks connect them to computers that analyse patterns. If sensor A records a movement and milliseconds later sensor B records the same movement, it shows a vibration passing through the building. This data can even be passed from one building to another, allowing smart structures to interact and send out early warnings.
Concrete is getting clever
Building smart structures isn’t just about attaching microcomputers; sometimes the technology is embedded in the actual building materials. Reinforced concrete is strengthened with steel bars, but steel isn’t the only thing you can add to concrete. Adding plastic fibres with a special nonstick coating makes concrete as springy as wood. Alternatively adding optical glass fibres that run from one side to the other lets enough light through to make concrete translucent; that’s not just attractive – translucent concrete can let you spot cracks deep within a block. But the ultimate building material doesn’t just reveal cracks, it repairs them. A team in the Netherlands is developing concrete which has tiny capsules of special bacterial spores embedded in it (pictured). Any water that seeps in through hairline cracks reactivates the dormant spores. As they reanimate, they produce limestone as a by-product, which seals up the cracks.