Monday, March 17, 2014

Sustainable Demolition

Concept ~ to reduce waste by designing for deconstruction.

Linear (wasteful, non-sustainable) process ~

Sustainable Demolition

Closed-loop (near zero waste, sustainable) process ~

Closed-loop (near zero waste, sustainable) process

Demolition -- Methods

Generally ~ the reverse order of construction to gradually reduce the height. Where space in not confined, overturning or explosives may be considered.

Piecemeal ~ use of hand held equipment such as pneumatic breakers, oxy-acetylene cutters, picks and hammers. Care should be taken when salvaging materials and other reusable components. Chutes should be used to direct debris to a suitable place of collection.

Pusher Arm ~ usually attached to a long reach articulated boom fitted to a tracked chassis. Hydraulic movement is controlled from a robust cab structure mounted above the tracks.

Wrecking Ball ~ largely confined to history, as even with safety features such as anti-spin devices, limited control over a heavy weight swinging and slewing from a crane jib will be considered unsafe in many situations.

Impact Hammer ~ otherwise known as a ``pecker''. Basically a large chisel operated by pneumatic power and fitted to the end of an articulated boom on a tracked chassis.

Nibbler ~ a hydraulically operated grip fitted as above that can be rotated to break brittle materials such as concrete.

Overturning ~ steel wire ropes of at least 38 mm diameter attached at high level and to an anchored winch or heavy vehicle. May be considered where controlled collapse is encouraged by initial removal of key elements of structure, typical of steel framed buildings. Alternative methods should be given preference.

Explosives ~ demolition is specialised work and the use of explosives in demolition is a further specialised practice limited to very few licensed operators. Charges are set to fire in a sequence that weakens the building to a controlled internal collapse.

Some additional references ~

BS 6187: Code of practice for demolition.
The Construction (Health, Safety and Welfare) Regulations.
The Management of Health and Safety at Work Regulations.

Demolition - Buildings

Demolition ~ skilled and potentially dangerous work that should only be undertaken by experienced contractors.

Types of demolition ~ partial or complete removal. Partial is less dynamic than complete removal, requiring temporary support to the remaining structure. This may involve window strutting, floor props and shoring. The execution of work is likely to be limited to manual handling with minimal use of powered equipment.

Preliminaries ~ a detailed survey should include:

• an assessment of condition of the structure and the impact of removing parts on the remainder.
• the effect demolition will have on adjacent properties.
• photographic records, particularly of any noticeable defects on adjacent buildings.
• neighbourhood impact, i.e. disruption, disturbance, protection.
• the need for hoardings, see pages 89 to 93.
• potential for salvaging/recycling/re-use of materials.
• extent of basements and tunnels.
• services † need to terminate and protect for future reconnections.
• means for selective removal of hazardous materials.

Insurance ~ general builders are unlikely to find demolition cover in their standard policies. All risks indemnity should be considered to cover claims from site personnel and others accessing the site.

Additional third party cover will be required for claims for loss or damage to other property, occupied areas, business, utilities, private and public roads.

Salvage ~ salvaged materials and components can be valuable, bricks, tiles, slates, steel sections and timber are all marketable. Architectural features such as fireplaces and stairs will command a good price. Reclamation costs will be balanced against the financial gain.

Asbestos ~ this banned material has been used in a variety of applications including pipe insulation, fire protection, sheet claddings, linings and roofing. Samples should be taken for laboratory analysis and if necessary, specialist contractors engaged to remove material before demolition commences.

Determination of Temporary Support Members - Buildings

Temporary Support Determination ~ the basic sizing of most temporary supports follows the principles of elementary structural design. Readers with this basic knowledge should be able to calculate such support members which are required, particularly those used in the context of the maintenance and adaptation of buildings such as a dead shoring system.

 Stability check using the example from previous page ~

The grade stress and slenderness ratios are used to provide a modification factor (K12) for the compression parallel to the grain. The following table shows some factors adapted from BS 5268-2:

By interpolation, a grade stress of 867 and a slenderness ratio of 20 indicates that 7.5N/mm2 is multiplied by 0.57.

Ref. BS 5268-2: Structural use of timber. Code of practice for permissible stress design, materials and workmanship.

Friday, March 7, 2014

Shoring Systems - Buildings

Shoring ~ this is a form of temporary support which can be given to existing buildings with the primary function of providing the necessary precautions to avoid damage to any person from collapse of structure as required by the Construction (Health, Safety and Welfare) Regulations 1996.

Shoring Systems ~ there are three basic systems of shoring which can be used separately or in combination with one another to provide the support(s) and these are namely:-

1 . Dead Shoring † used primarily to carry vertical loadings.
2. Raking Shoring † used to support a combination of vertical and horizontal loadings.
3. Flying Shoring † an alternative to raking shoring to give a clear working space at ground level.

Dead Shores ~ these shores should be placed at approximately 2000 c/c and positioned under the piers between the windows, any windows in the vicinity of the shores being strutted to prevent distortion of the openings. A survey should be carried out to establish the location of any underground services so that they can be protected as necessary. The sizes shown in the detail below are typical, actual sizes should be obtained from tables or calculated from first principles. Any suitable structural material such as steel can be substituted for the timber members shown.

Raking Shoring ~ these are placed at 3„000 to 4„500 c/c and can be of single, double, triple or multiple raker format. Suitable materials are timber, structural steel and framed tubular scaffolding.

Flying Shores ~ these are placed at 3„000 to 4„500 c/c and can be of a single or double format. They are designed. Unsymmetrical arrangements are possible providing the basic principles for flying shores are applied.

Unsymmetrical Flying Shores ~ arrangements of flying shores for unsymmetrical situations can be devised if the basic principles for symmetrical shores is applied (see page 156). In some cases the arrangement will consist of a combination of both raking and flying shore principles.

Scaffolding Systems

Scaffolding Systems ~ these are temporary stagings to provide safe access to and egress from a working platform. The minimum legal requirements contained in the Construction (Health Safety and Welfare) Regulations 1996 applicable to traditional scaffolds apply equally to special scaffolds. Special scaffolds are designed to fulfil a specific function or to provide access to areas where it is not possible and or economic to use traditional formats. They can be constructed from standard tubes or patent systems, the latter complying with most regulation requirements are easy and quick to assemble but lack the complete flexibility of the traditional tubular scaffolds.

Birdcage Scaffolds ~ these are a form of independent scaffold normally used for internal work in large buildings such as public halls and churches to provide access to ceilings and soffits for light maintenance work like painting and cleaning. They consist of parallel rows of standards connected by leaders in both directions, the whole arrangement being firmly braced in all directions. The whole birdcage scaffold assembly is designed to support a single working platform which should be double planked or underlined with polythene or similar sheeting as a means of restricting the amount of dust reaching the floor level.

Slung Scaffolds ~ these are a form of scaffold which is suspended from the main structure by means of wire ropes or steel chains and is not provided with a means of being raised or lowered. Each working platform of a slung scaffold consists of a supporting framework of ledgers and transoms which should not create a plan size in excess of 2500 x 2500 and be held in position by not less than six evenly spaced wire ropes or steel chains securely anchored at both ends. The working platform should be double planked or underlined with polythene or similar sheeting to restrict the amount of dust reaching the floor level. Slung scaffolds are an alternative to birdcage scaffolds and although more difficult to erect have the advantage of leaving a clear space beneath the working platform which makes them suitable for cinemas, theatres and high ceiling banking halls.

Suspended Scaffolds ~ these consist of a working platform in the form of a cradle which is suspended from cantilever beams or outriggers from the roof of a tall building to give access to the facade for carrying out light maintenance work and cleaning activities. The cradles can have manual or power control and be in single units or grouped together to form a continuous working platform. If grouped together they are connected to one another at their abutment ends with hinges to form a gap of not more than 25mm wide. Many high rise buildings have a permanent cradle system installed at roof level and this is recommended for all buildings over 30„000 high.

Cantilever Scaffolds ~ these are a form of independent tied scaffold erected on cantilever beams and used where it is impracticable, undesirable or uneconomic to use a traditional scaffold raised from ground level. The assembly of a cantilever scaffold requires special skills and should therefore always be carried out by trained and experienced personnel.

Truss-out Scaffold ~ this is a form of independent tied scaffold used where it is impracticable, undesirable or uneconomic to build a scaffold from ground level. The supporting scaffold structure is known as the truss-out. The assembly of this form of scaffold requires special skills and should therefore be carried out by trained and experienced personnel.

Gantries ~ these are elevated platforms used when the building being maintained or under construction is adjacent to a public footpath. A gantry over a footpath can be used for storage of materials, housing units of accommodation and supporting an independent scaffold. Local authority permission will be required before a gantry can be erected and they have the power to set out the conditions regarding minimum sizes to be used for public walkways and lighting requirements. It may also be necessary tocomply with police restrictions regarding the loading and unloading of vehicles at the gantry position. A gantry can be constructed of any suitable structural material and may need to be structurally designed to meet all the necessary safety requirements.