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Page last updated at 15:39 GMT, Friday, 22 August 2008 16:39 UK

How is a cooling tower demolished?

The Magazine answers...

Towers being demolished
A campaign failed to save the landmark
After 70 years of dominating Sheffield's skyline, the Tinsley cooling towers have been blown up. So how do you safely explode two 76m (250ft) -high structures within sight of a motorway?

Once part of a set of seven, the two Tinsley cooling towers that were demolished on Sunday had been left standing after the demolition of Blackburn Meadows electricity generating station in the 1970s.

They had become a local landmark, dubbed the Gateway to Yorkshire by passing drivers. And thousands of signatures were collected in a petition to save the structures - thought to be the only surviving example of pre-1950 hyperbolic cooling towers in the UK.

But site owner E.ON, which is developing a biomass power station, said the towers must go. And recent tests had shown the towers, made of a concrete shell internally reinforced by steel, were structurally unsafe.

Select areas for explosive charges
Delay between detonations
Spray surface with water to minimise dust

The contractor responsible for the explosion would not speak to the BBC. But explosives engineer Sidney Alford, speaking days before Sunday's demolition, said the key factor in ensuring they fell to the ground safely and completely in a matter of seconds, was in placing the charges in the correct locations.

Over the past few weeks crews have been making preparations for the demolition. Up to 3,000 holes would have been drilled in two areas of the towers - a section of the legs at the base of the tower - and in horizontal rows higher up.

These would be filled with explosives.

Dr Alford said the charges would be detonated in two stages - probably within a second of each other. The first detonation would provide an initial "jolt" and the cooling tower would tilt downwards; the second would destroy the integrity of the structure, causing it to break up and collapse.

Crews prepare Tinsley Cooling Towers for demolition
Wire mesh is used to help prevent the projection of concrete fragments

"As the concrete legs are blown, the tower hits the base, and the slots which have been blown [above] cause it to fold… like a bit of wet cloth," said Dr Alford.

But with the towers said to be three times stronger than those built to current standards, would the demolition face any unforeseen difficulties? And what about the risk to the nearby M1 motorway?

If the shell is thicker, then they would simply drill longer holes and use correspondingly more explosive, said Dr Alford. A cooling tower is "inherently strong" but "once it buckles, it falls".

And unlike an industrial brick chimney, which would tend to topple over as it is demolished, the strength and shape of a cooling tower - it has a wide base and gets much narrower as it goes up - should mean it collapses within the boundary of its 50m bases, said Dr Alford.

Nonetheless, he acknowledged things could go wrong.

"If the slots cut in the shell do not collapse - if part of the explosives do not go off - one of the worst things that could happen is that all the legs collapse and the thing just sits down on itself.

"It is exactly the same as it was before, just about 10ft lower."

Then you would have "a problem", he said, because it would have lost "the stability that the architect has designed into it and it would be much a more hazardous process to climb up again and drill more holes in it to bring it down."

Dust cloud

The demolition crews would consider the possibility that fragments of the towers could be projected hundreds of metres when the explosion takes place.

They would cover the areas where the explosive charges have been placed with wire mesh or rubber mats to catch any bits of concrete. The towers would also be sprayed with water before detonation to mitigate the effect of the dust cloud caused as they fall, he adds.

The nearby M1 viaduct will be closed while the demolition takes place.

And after the event?

E.ON maintains the demolition poses a very little risk of damage to the road, which has been strengthened over the years, but are promising a "rigorous programme of inspection and testing" before its reopening.


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