Demolition

General Demolition is a dangerous and complex activity. Requires sound knowledge of various legal procedures, construction technique & their associated demolition methods. Construction (Design and Management) Regulations 1994 requires demolition to be considered during the design stage of a project & fed into the O&M manuals. General sequence of demolition is from top down.  Approval is required from the local authority when works involve any structural alterations.  Criteria for a Successful Demolition Safety - may require decontamination works prior to work - keep flying debris to a minimum. Thorough investigation - prior to demolition to identify potential problems such as weak sections, hazardous materials... Good neighbour policy - adjacent owners/occupiers should be notified at all stages of the demolition process. Avoid damage & operational problems - protect other parts of the building and adjacent properties through temporary screening. Proper disposal - dispose in proper bags and appropriate/licensed sites. Maximise recycling - utilise materials that are directly useable (windows, roof slates, doors, sanitary ware, glass...) Reasons for Demolishing a Structure Structural failure Progress of society - change in use, new legislation, redevelopment. Economic factors - repairs costs, land use & end of useful life. Two Options for Demolition Demolish part of the structure. Completely demolish the structure. Considerations Before Demolishing a Building Ownership & legal matters. Freehold & leasehold interests. Rights of way & other easements. Boundaries. Trees Basements Footpaths & access to the site. Services and drainage. Noise, dust & other nuisance. Planning Initial Procedures for Demolishing a Structure Obtain necessary statutory consent, licences, permissions... Disconnect or cap off redundant services. Protect remaining elements. Safety/security check to ensure no on is inside the building. Soft stripping - removal of non-load bearing parts of the building and salvageable materials first (partitions, windows, doors...) Pre-weakening - cutting holes in walls or other elements prior to using a pusher arm to topple the remaining elements. What Determines the Method of Construction? Type of structure Type of construction Location of site Structural Calculations Many buildings that are adapted need to have their structural capacity improved or altered in some way.  Major demolitions will require temporary support in some form. Undertaken to ensure the proposed design is sufficient. Fundamental objective is to ensure the 'permissible stresses and loads' exceed the 'actual stresses and loads' by the appropriate factor of safety. The factor of safety must be greater than 1 to prevent catastrophic failure of the structure. Factor of Safety = Ultimate Load (Load to Cause Failure / Working Load (Actual Load on Structure) FOS can range from 1.5 for steel structures to as high as 5.5 for masonry.  Methods of DemolitionCategory 1 - Piecemeal By Hand Using a Sledgehammer - least damaging & disruptive option (commonly used in adaption work) Pusher Arm - suitable for low-rise redundant extensions. Mechanical Demolition - use of crane with ball & chain - limited use due to the risk of collateral damage. Hydraulic Splitter - used to pre-weaken or remove masonry or concrete elements. Thermal Lance - used to remove difficult concrete elements. Category 2 - Deliberate Collapse Controlled Using Non-Explosive - used to pre-weaken walls & burn timber wedges & other temporary supports. Controlled Using Explosives - limited use in adaption work due to risk of collateral damage. Techniques for DemolitionBy Hand Used when inaccessible to machines, high level work or materials need to be retained. Tools include: dis-cutters, crow-bars, pneumatic drills... Time consuming & labour intensive method. Wire Rope Commonly used for masonry & brick structures. Structural members are removed and a wire rope is positioned around a building & pulled by a tracked vehicle.  Not to be used on buildings over 20m high. Length of rope must be at least 2x the height of the building.  Demolition Ball Used for large brick and concrete structures. Steel or concrete ball weighing 0.5-3 tonnes is dropped or swung into a building. Requires high level of supervision as crane driver has restricted view of the building.  Usually used in conjunction with hand demolition to separate it from the neighbouring structure. Pusher Arm Used on low level masonry structures. Excavator with a steel pad connected to the arm demolishes walls from the top down by a hydraulic action or by driving the excavator towards the wall. Deliberate Collapse Used when certain structural members support the whole building. These members are removed in a methodical & controlled manor.  Dangerous method & care must be taken to ensure site personnel are a suitable distance away. Explosives Used on large structures where materials don't have to be retained. Structure will collapse and break upon impact with the ground. Fast & economical method but there's safety implications. Hydraulic Burster Used for reinforced concrete structures. Holes are drilled into the concrete & a steel tube is placed into each of them. Hydraulic pressures force pistons out the tube to break the concrete along each tube line. Slow but quiet method. Gas Expansion Burster Used for demolishing mass concrete. Same as hydraulic burster but steel tubes contain chemicals which expand when triggered by an electrical charge.  Care must be taken against structural collapse & falling masonry. Thermal Lance Developed in 2nd World War - primarily used for reinforced concrete structures. Steel tube with internal steel rod has its end heated before passing oxygen down it. Resultant 3,500 degree temp. melts away concrete & reinforcement. Disadvantages - can damage surrounding members through expansion & hot slag is produced. Other Methods Hydraulic Concrete Hammer - one of the proprietary methods available. Water Jets - high pressure cuts through rock & concrete. The 'Grab' - bites out sections of the structure.  Thermal Reaction to make structural steel become plastic to easily collapse.  Demolition Sequence of OperationsAn incorrect sequence of dismantling can lead to premature collapse. Typical Sequence of Events for a Demolition to Take Place: Provision of Data on Structure - previous use, appropriate demolition method to be used, disposal of hazardous substances. Survey of Demolition - identify structural problems, risks regarding hazardous or flammable substances... Preferred & Safe Methods of Work - detailed statement of safety procedures to be be used & an agreed demolition technique between all parties. Preparation & Planning - asbestos statement, handling hazardous substances, disconnecting utilities, making safe services have to be shown. Protection of the Public - assess health risks, arrangement of temporary services & informing of surrounding community. Suitable Demolition Methods for Various Building ElementsMasonry Demolition ball, hand demolition or pusher arm is usually applied.  Demolished masonry usually used for hard-core. Roof Trusses Structure usually requires temporary support before demolition. Trusses are lifted out and lowered by a crane after removal of the purlins. SteelworkCranes usually used to lower steel members to ground.Reinforced Cast In-Situ Concrete Expensive to demolish as equipment required is costly.  Slabs usually cut out with hydraulic bursters in small sections. Reinforcement is cut at one end and the beams are lowered to the ground. Columns are supported by ropes then pulled over after reinforcement is cut.  Pre-Stressed Concrete Structures Most difficult form to demolish. Mistakes may cause reinforcement bars to shoot out the beams. Expert advice should be sought. Support to Adjacent BuildingsBefore or after demolition, temporary or permanent support may be required. Shoring Shoring is a supporting framework used to provide temporary support to structures. Can be constructed out of timber or steel & always have a timber wall plate to distribute load from the wall onto the shore. 2 types of shoring: Raking & Flying Shores: Raking Shores - shores that lean against the exposed wall of the building requiring support - timber wall plate distributes the load to the shore. Flying Shores - shores that span between two close buildings up to a distance of 9m - allow free working spaces across the demolition site. Buttresses - these are used as a means of permanent support - brickwork is bonded to the flank walls to form piers.  Site Safety During Demolition Falling materials, unintentional collapse, fires, explosions & people falling all cause deaths & serious injuries each year. Accidents are often blamed on poor supervision. Operatives are not expected to understand the structural implications of what they're removing.  Notice must be given to all statutory bodies.  Stripping Out Glass should be removed but window frames left in to maintain wall strength. Openings should be boarded up. Lift shafts and areas with no flooring should be boarded up. Projections such as balconies should be removed & cleared. Timber removed from the building should have it's nails removed.  Rooft trusses should be lifted down. At the end of the stripping out process - only concrete, masonry & steel should remain. Safety Implications for Each Demolition Method Hand Demolition - rubble shouldn't be allowed to build up as it can cause lateral pressure causing a wall to collapse. Wire Rope - operatives to remain well back from the building - pusher arm should be used if building does not collapse. No person should sand further than 3/4 of the rope length away from the pulling force because of the danger of whip lash if the rope fails. Demolition Ball - cab of the crane should be able to withstand the impact & have laminated glass - crane should be on firm, level ground - operatives should stay at a safe distance and the ball never allowed to spin or be trapped in the wreckage. Pusher Arm - steel pad must be placed within 600mm of the top of the wall. Excavator should work from level ground & have a robust cabin. Deliberate Collapse - all people should be at least 2x the height of the building away - method not be used on confined sites. Explosives - requires specialist demolition firms - only people over 21 can use explosives - escape routes & warning systems should be arranged - people & animals kept well back and protection against debris provided. Milli-second delay fuses should be used to reduce noise & flying rubble. General - hoarding to be used to protect 3rd parties - fans used around perimeter on buildings of 2 storeys or more - scaffolds with protective sheeting should be use on restricted sites to stop falling debris. Demolition Site Provisions Plant & Equipment - regularly serviced & only operated by skilled operators. Protective Clothing - specialised clothing where asbestos, lead paint, dust, fumes or chemicals are present. Shoring & Underpinning - demolition contractor who has a legal obligation must ensure temporary supports are in place for unsafe areas. Working Areas - signposting & clear warnings required of demolition taking place. Debris - sections of the building must not be overloaded with debris. Weather Conditions - walls must be supported to prevent strong winds causing collapse. Flooding - measures must be taken to protect against build-ups of water. Overhead Cables - crane heights must be checked against the height of surrounding overhead cables. Scaffolding & Hoarding - must be constructed & illuminated to relevant building regulations. Security - site must properly secured against entry. Dust - should be kept to a minimum by spraying with water when required. Noise - suppressors & silencers should be used to minimise noise levels. Demolition Method StatementComplete a detailed H&S method statement before work starts - should include a full risk assessment. Should be easy to understand & agreed by all levels of management & supervision. Must include: Sequence and method of demolition or dismantling.  Details of personnel access, working platforms & machinery requirements. Design of any temporary structures required. Details of any pre-weakening required & areas to be pulled down or demolished with explosives. Arrangements for personnel protection & public and the exclusion of unauthorised people. Details of areas outside the site boundary required control to improve safety during critical stages. Details of removing/making safe services.  Details of temporary services available and required by contractor. Details & methods for detailing with flammable materials & gases found. Details of methods to identify hidden or other hazardous substances, the methods for disposal & what protective equipment is required. Arrangements for the control of site transport to be used for removing demolition debris. 

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