Extreme machines

From buildings that can withstand earthquakes to nuclear flasks that can survive impact from a 100mph train, extreme engineering is prime business.

Earthquakes. Intense fires. Pressures of 400 atmospheres. These are daily considerations for engineers who work in extreme engineering. They are the designers and builders of technology that has to survive at 11,000m deep in the ocean or in collision with a freight train travelling at 100mph.

Their success in developing equipment and structures that can overcome these tests could be thought of as man overcoming nature, but the truth is somewhat different. Extreme engineering is actually about directing these forces to your advantage. Rather than being overcome, nature is, in a small way, being harnessed.

That includes one of the most powerful natural forces: earthquake energy. From Afghanistan to San Francisco, earthquakes strike developed and undeveloped nations alike. They set up vibrations in the ground that cause sideways forces on buildings, making them sway backwards and forwards.

Andy Thompson, a Californian who experienced earthquakes in his youth, now lives in the UK and works as a senior engineer in the advanced technology department of UK consulting firm Ove Arup. He regularly travels to earthquake zones as part of teams Arup sends out after every large event. These have included Bhuj city in the Gujarat province of India in 2001 and the Northridge quake in Los Angeles in 1994.

The Northridge quake measured almost seven on the Richter scale, unleashing greater destructive energy than the Hiroshima atomic bomb. In the quake 57 people died, with a further 1,500 injured. But the Gujarat quake, of similar size, destroyed 145,000 homes and killed 30,000.

Though the tall buildings in cities like San Francisco might be expected to be more vulnerable in an earthquake, Thompson says smaller buildings in India are far more susceptible. ‘A quake, coming up through the bedrock, has a pretty high frequency of vibration. If the structure naturally shakes at the same frequency, then there is resonance and you have a catastrophic event.’ Tall buildings have a lower natural frequency, so there is no resonance. The real problems occur in buildings with three to six storeys. This is made worse if a building has a ‘soft storey’, for example a shop front, which causes a weak point.

Buildings can be strengthened against earthquake forces by incorporating, for example, suitable bracing. Other effective approaches involve isolating the building, so inhibiting earthquake forces from being transmitted into the structure in the first place, or adding dampers to absorb forces that would otherwise go into shaking the building.

To isolate a building rubber bearings may be placed at the base of its structural columns. Consisting of layers of rubber separated by thin steel plates, these allow vertical loads to be transmitted to the foundations, but they offer only weak resistance to horizontal loads, allowing sideways movement within the bearing but not transmitting it to the building proper. So the sway induced by the vibrations in the ground is much reduced.

Examples of mechanical dampers include friction dampers, consisting of a set of steel plates bolted together but with elongated holes. They slide over each other in an earthquake, absorbing energy through friction, and can be specially treated to increase friction.

Viscous fluid dampers are like huge shock absorbers on cars but fitted diagonally across an entire storey of the building between the beams and columns making up the structural frame. As the earthquake tries to distort the frame the piston of the damper moves in and out of its cylinder, again absorbing energy through friction.

But some weak points can actually be designed into buildings to improve their resistance, to allow the energy from the quake to damage part of the building in a predictable way. Examples can be as simple as a beam whose destruction is acceptable; diagonal bracing between columns can also be allowed to distort, unless it is critical to the structure, in which case extra struts specifically designed to distort and fail are included.

But while these self-destructing parts are used now, the seismic research labs have a new idea up their sleeves. The concept is to design buildings in separate vertical sections capable of moving independently and linked only by a ‘crumple zone’ designed to absorb the damaging power of the earthquake.

Earthquakes are a distant threat in the UK (though our nuclear power stations are designed to withstand a theoretical one in a 10,000 years quake). But something of far greater public concern is the prospect of an accident involving highly toxic nuclear waste.

In 1984, in a vividly remembered demonstration for TV, BNFL set out to prove the safety of the flasks it uses to contain the waste by staging a crash between a flask and a freight train. Roger Norman, BNFL Magnox fuel and waste operational team leader, was a test engineer involved in the preparations for the stunt.

The flasks, still used today, contain Magnox fuel rods, consisting of uranium encased in manganese, water and nitrogen to inhibit electrolysis.

Until the 1970s the flasks were made from five square steel sections. ‘Then forging equipment became available to ram a chunk of metal into the basic flask shape,’ says Norman. After the forging crudely created the flask shape, made from carbon manganese steel, it was machined to the final dimensions.

The new monolithic flask held up well against the legally required tests. These included being exposed for 30 minutes in temperatures of 800 degrees C, withstanding pressures at a depth of 200m of water and surviving train impacts of 30mph. So why ram it with a freight train travelling at 100mph?

There was still the question of how well the lid would seal. And research into what the public perceived as the worst possible accident possible led to the decision to carry out the crash test. The engineering brief was to manufacture a flask that had to open and close, with valves for removing or adding water or nitrogen, yet could survive massive physical force.

The solution was to build in corner crumple zones so the metal, when deformed, flows and seals itself to ensure the flask remains closed. To ensure this happened dozens of tests were performed with various scale models and full-sized flasks.

The biggest test involved a full-scale flask being dropped in a quarry in Cheddar, Somerset. The bedrock of the quarry was covered in concrete and a 50mm metal plate and steel bolts were driven through the bedrock and then encased in concrete.The flask design has remained basically unchanged since, though new seals, designed to contain waste better in case of a fire or impact, were fitted between 1992 and 1995.

Being hit by a freight train travelling at 100mph bears no comparison, however, with the loads a flight recorder is expected to cope with. A crashing aircraft is typically exposed to acceleration of 3,400g. Withstanding a 2,000kg crushing force for five minutes is just one of the tests ‘black box’ flight recorders, which are actually bright orange, have to endure to make sure the information they store is kept intact.

The latest flight recorders record the last 25 hours of flight data and up to the last 120 minutes of cockpit voice recordings. The recording medium used to be magnetic tape and before that metal foil. But now solid state memory devices, similar to those found in home computers, are standard. This has also allowed a size and weight reduction in the recorder – in regional jets they can be just 230mm long and weigh about 4kg.

Despite these developments the box itself, which may be reclaimed from wreckage or the depths of the sea, is not expected to survive intact. Only the memory unit itself needs to be recovered. This is stored in an armoured and insulated capsule made of stainless steel or titanium.

Samples of the casing are tested by firing them from a pneumatic cannon into a barrier of specified hardness to stimulate the high g-forces. Another test entails dropping a 225kg weight with a hardened 6mm steel pin protruding from it on to the recorder from 3m, to test penetration resistance. Crushing forces of 2,270kg are applied along each axis. The casing has to withstand fire tests consisting of one hour at 1,100 degrees C to replicate a jet fuel fire and 260 degrees C for 10 hours to recreate a smouldering wreck. It also has to survive immersion in various fluids for up to 30 days.

Falling 30,000ft is a long way but there are other machines that have to descend further, into more dangerous environments: the oceans.

The Marianas Trench in the Pacific is the deepest point in the seas at 11km deep. Fortunately 95 per cent of the ocean is somewhat shallower at around 6,000m or less. But, even that far down you would be subjected to pressures of 600 atmospheres. In the 1960s when explorers began descending to these depths, and even to the bottom of the Marianas Trench, spheres of steel and aluminium with windows 10cm thick were the answer.

Manned descent continued into the 1990s but today robots in the form of remotely controlled mini-subs are the voyagers to the deep, and they are using liquids to keep the water around them at bay. The robots are launched underwater to find sea-floor thermal vents and to monitor the condition of the eight nuclear submarines that were lost at sea during the Cold War.

Because liquids are incompressible, silicon oil protects the submarine’s electronics and other control systems. They are housed in plastic containers filled with the oil. The need for a hugely strong hull designed to keep out the water pressure is eliminated.

Not everything that has to operate in extreme conditions is designed to survive, however. Dramatic rocket explosions on launch pads have always been a feature of space exploration. But, despite the progress that has been made in materials science, today’s launch pads are not designed to withstand catastrophic launch failures.

The space port used by the European Space Agency is Kourou in French Guiana. Bernard Brande, a spokesman for the launcher directorate of the French space agency CNES, is under no illusions about what has to be done in the event of an Ariane rocket exploding: ‘It is not resistant at all. It is in fact easier to rebuild it afterwards, within a reasonable timescale, and that was the decision that was taken.’

Similarly, a report last week by the US Federal Emergency Management Agency into the September 11 attacks on the World Trade Centre concluded that ‘it may not be technically feasible’ to design structures to resist aircraft impact. Instead, many engineers argue, efforts should concentrate on improved evacuation routes and fire protection to enable more people to escape.

The reality is that accepting that there are some forces engineering just cannot cope with is a strong element of today’s extreme engineering. No resistance or little is a key factor. Absorbing, damping, crumpling and deforming are the methods used to endure the extreme conditions that humanity faces as it continues to extend the boundaries of where people live and work.

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