Critical Sequences in Demolition
Tuesday, March 30th, 2010 | Great Sites
Introduction
The definition of the word ‘demolish’ is deliberate and controlled collapse of a structure. In the late 1970’s Fred Dibnah became the most famous steeplejack in Britain when his work on demolishing tall chimneys without the need for explosives was shown on Television. Fred was an interesting character and became famous as a specialist on repairing tall chimneys, so when it came to taking them down he knew just how to do it. He would cut an alcove at the base of the chimney, and then place wooden props to support the structure. By carefully positioning the props, the chimney would collapse in the right direction when the wood was set alight and burned away. Once he was nearly crushed because he miscalculated somewhat when demonstrating how dangerous demolition work really is.
The actual word to demolish only came into being in 1570, and was used after that to mean the deliberate taking down or destroying of a building or structure. Although in fact the process of demolition, in some form or other, has been occurring for thousands of years. Buildings have been destroyed either by natural disturbances of the Earth’s crust or during battles or attacks on towns for as long as there have been structures. The remaining ruins would be removed and rebuilt or used for other buildings nearby and must be early examples of recycling.
Preserving the Past - Preparing the Future.
To preserve Britain’s history of agriculture the land between the built up areas of the villages and towns is protected from growth of urban sprawl. This land is known as Green Belt and is preserved for farming which is vital to the continuing economy of the country. By protecting the Green Belt, urban areas are being squeezed to provide housing, so developers have to be much more creative to find suitable sites.
Britain is a country with a very dense population per square mile. The amount of land available for construction is limited compared to other countries. Therefore there is always a high demand for space on which to build. Taking down old and unused buildings and replacing them with new is one way to counteract this shortage.
Lands that may be disused industrial and commercial sites, but could be contaminated with hazardous waste or pollution, are called Brownfield sites. Once the land has been cleaned up they are potentially valuable for redevelopment. Both Glasgow and South Wales created gardens out of old industrial sites and put in Shopping centres to attract visitors and generate income.
Greyfield sites are distinct from Brownfield sites in that they do not have the environmental concerns of toxic waste. The term greyfield comes from the large areas of asphalt which had once been car parks of commercial urban properties. Their value is in the fact that the infrastructure such as roads, electricity, water, sewage, and gas is already in place. These urban areas which become under-utilised or abandoned and are valuable because require very little remedial work to be renovated. Cities such as Leeds and Manchester have had a massive programme of converting the old warehouses or factories into apartments, shops and restaurants, retaining the original shell and refurbishing the interior into modern accommodation.
The Demolition Process
A skilled building demolition company contracted to do the work will have taken the time to perform an initial basic review of the following.
• The construction type and physical size of the building.
• What items have re-use value?
• What will the cleared site’s re-use be for?
• Where are the waste by-products going to be disposed?
• What considerations need to be made with regards electricity, water, sewage and gas mains.
Hydraulic excavators and bulldozers can be used to undermine the walls at the base, so that the structure will topple; at the same time controlling the manner and direction of the fall. Safety issues are paramount, and clean-up strategies are also taken into account when determining how the building will be demolished.
Traditional Demolition
At one time the traditional way of demolishing a building was very crude. Once all the services were disconnected, the men and machinery would go in and just knock down the walls. Then the whole structure would collapse and the resulting wreckage would be piled onto lorries and be dumped in landfill sites. Concrete foundation would be broken up by pneumatic drills and the site would be cleared of rubbish ready for the new work to begin. However today demolition practices are subject to strict planning, safety and monitoring regulations and are highly controlled by the local authority. The way a building is constructed means that one section is dependent on another for stability and strength. For instance the roof of a building is not really fastened down other than fixing tiles with nails to stop them slipping. The roof stays in place with its own weight as the beams are laid on the top of the walls rather than fastened into them as they once were for much older houses. The cross beams and purlins then tie the whole thing together to secure the frame of the roof. To this slats are laid as a frame for laying on the tiles or slates. Other sections of a building are also interdependent so when it comes to demolition these factors are taken into account in the sequence of deconstruction. In a sense, like Fred Dibnah you need to be an expert in construction to know how to do the deconstruction.
Deconstruction and Recycling
The new approach to demolishing buildings is known as deconstruction - a green approach. Landfill sites are in short supply so the aim when demolishing a building is to reduce the amount of waste remaining.
Small structures such as two or three storey houses can be dismantled quite easily. The work may be a painstaking task of dismantling by hand - brick by brick - or beam by beam but by going carefully costly materials are preserved for re-use. The value of deconstruction is that 90% or more of waste is saved from going into landfill sites and reclaimed materials can be re-used and recycled for future buildings. The farmer opposite my house sold his barns in the farm yard for housing; when the builder demolished the barns he cleaned up the bricks and reused them for the wall around the farmhouse thus retaining the character of the farm.
Modern techniques and machinery allows demolition companies to efficiently segregate waste types on or off-site. Construction materials are recycled and re-used whenever possible in the new structure making significant savings in project costs as well as being good for the environment.
Concrete can now be rapidly broken up with a huge guillotine.
Instead of rubble from the demolished building being taken to a landfill site for disposal it can be crushed on site to become 6f2 sub-base material or 6f2 crushed material, for the foundations of the new building, or it can be sold on and transported to other sites. Crushed material from the demolition can be used as a sub-base for the new building foundations or used on other sites where construction is underway.
Copper pipes, lead, roof tiles or slates, floor tiles, wiring and doors, and wood panelling are valuable items that are saved for recycling to be re-used. Many specialist firms offer reclaimed old or antique building items in most towns and cities.
Tall Buildings
Tower blocks and chimneys are the type of tall buildings that may need to be demolished. The demolition of tall buildings necessitates expert techniques. The tallest building to be demolished lawfully was in 1967/8 of the Singer Building in New York. The collapse of the World Trade Centre after the 9/11 attack in 2001 demonstrates the terrible devastation that occurs if the demolition is uncontrolled and haphazard.
In the demolishing of tall buildings and large structures a wrecking ball on a crane can be used, but is rarely practiced because the swinging ball is rather uncontrollable. The proximity of other buildings is a determining factor which prevents the use of explosives to implode a tall structure. So ‘High Reach’ demolition excavators are used where other methods are not possible to demolish the top part of a tall building. Once it is down to a manageable height demolition can continue in the usual way. The various methods of demolishing tall buildings are by implosion using explosives, controlled collapse and piecemeal. To control the dust produced in demolition, water hoses and spray equipment are sometimes used and then it is called a wet demolition.
Explosions
The use of explosives in demolition is very specialist work and getting it wrong would be catastrophic. If for instance there is atmospheric pressure from low cloud above the implosion site, the shockwave may spread outwards instead of upwards causing the wave of energy and sound to break windows. If an implosion is not prepared correctly the danger may be damage to surrounding buildings where flying debris may cause injury to spectators.
For many people when they think about demolition they may have in mind the use of explosives in the dramatic collapse of a tall building. This process is actually called implosion using explosives. Implosion is essential for dense urban areas as it brings down a tall building so that the surrounding environment is damaged as little as possible. The collapse takes only seconds for the building to fall into its own footprint.
Because of the dangers of working with explosives they will only be used when other methods are too costly or impractical. Where there is a partial collapse of a building and there are still primed explosives that failed to go off, workers are in great danger because the remaining structure is highly unstable. At the same time the demolition has to continue to secure the safety of the site.
Health and Safety
The work of demolition is a much more technical and complicated process than most people would appreciate. The job is highly dangerous and requires experienced and skilled operators to carry out the work. It is essential that personnel working in the industry are adequately trained. Health and safety awareness is crucial in demolition services so it is advisable for operatives to have gained a Certificate of Competence in Demolition to ensure safety for both workers and public alike. All demolition work is regulated by the Construction, Design and Management Regulations.
Prior to any dismantling occurring, your demolition company must demonstrate they have adhered to a similar process as the above in order to gain demolition planning permission.
Sequence of Demolition
An incorrect sequence of dismantling will result in unplanned collapse of a building because the stability of any structure is reliant on the interdependence of its component parts. Think of a house of cards and what happens if one of the supporting cards is removed.
There is a strict sequence of events before any demolition can take place. Councils throughout the country will have their own specific list for planning approval in their area, but a typical order would be as follows:-
• Provision of Information
Information must be provided about the construction of the structure to be demolished. Details of its previous use and the appropriate demolition methods to be used, including disposal of hazardous substances, have to be submitted by the demolition company.
• Survey of Demolition
A thorough survey of the site to identify any structural problems, as well as risks associated with hazardous or flammable substances, will need to be discussed in detail with the authorities. (E.g. A disused garage where petrol has been stored is a potential fire hazard so preventative measures will need to be taken).
• Preferred and Safe Method of Work
A reputable demolition company will be able to select the appropriate method of disposal showing the outline dismantling process. Planning is essential for assiduous monitoring. The authorities will require a detailed statement of the safety procedures to used, and all parties involved need to agree the methods before any demolition can take place.
• Preparation and Planning
Issues such as asbestos abatement, rodent baiting, dealing with hazardous substances, disconnecting utilities, and making safe any electric, gas or other services have to be shown in the planning stage. There is a lot of preparation to be done before even starting work on demolishing the building itself.
• Protection of the Public
Safety cannot be compromised so where there are heavily populated areas around the demolition site the protection of the public is paramount. Any health hazards will need to be assessed and temporary services arranged, and peopleaffected will have to be informed.
Conclusion
The aim in demolition is to eliminate an unwanted structure as safely and quickly as possible and in our modern environment efforts are made to recycle or re-use most of the old material. This is not a new idea although the word itself is relatively modern. On the borders between England and Scotland after the Romans left, a large part of Hadrian’s Wall was hauled away and use was made of the beautifully dressed stone to construct the new buildings in the towns and villages nearby and some are still standing today.
Demolition work by its nature is a very hazardous business and demolishing any building is a complex and skilled process. Next time you see demolition work occurring on a building give a thought to the people who work in a dangerous situation daily and how much is involved in the meticulous planning, regulations compliance, care and skill that goes on to carrying out the project to clear the way for our future.