Idea Transcript
OSH5063EP(PRS3607) Workplace and Work Equipment Hazard
Session 16 Demolition Works
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Demolition
There will always be a need for demolition as buildings deteriorate with age to the point of instability and where existing buildings must be replaced in the cause of redevelopment.
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Demolition
Demolition is one of the most dangerous activities undertaken in the construction industry.
An accident on demolition work is much more likely to be fatal and serious than on other construction work.
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Demolition
There are various causes attributing the accidents in demolition works:
structure to be demolished without adequate background information; incompliance with planned work method; site management staff and operatives acquire insufficient training to identify the hazards, etc.
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Demolition
Generally, demolition works should be planned in similar details to other building operations or works of engineering construction.
To ensure safety and health of workers, a safe system of demolition work need to be estabished and implemented carefully.
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Pre-demolition survey
Prior to demolition taking place, it is necessary to collect data as well as on-site surveys on the building to be demolished, in order to establish the safe system of work.
The system will be determined by a pre-demolition survey undertaken by a competent person, appointed by the employer.
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Pre-demolition survey The pre-demolition survey should identify:
The nature and method of construction of the building;
The arrangement of buildings adjacent to that for demolition and the condition of this adjoining property;
The location of underground services, e.g. water mains, electricity cables, gas pipes, drains, sewers, telephone cables, etc; 7
Pre-demolition survey
The previous use of premises, e.g. for the storage of inflammable substances;
The presence of dangerous substances, e.g. asbestos lagging;
The method of bonding of the main load-bearing walls;
The system of shoring or other supports necessary during demolition;
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Pre-demolition survey
The presence of cantilevered structures, their form of construction and the nature of the danger;
The presence of basements, cellars, vaults or other spaces affecting the structure of adjoining properties;
The potentially dangerous effects of removing superstructure stabilizing loads from an old basement or vault retaining walls;
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Pre-demolition survey
The presence of storage tanks below and above ground, and the nature of their contents; and
The actual sequence of operations, which should generally take place in the reverse order of building erection.
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Pre-demolition
Before the demolition begins, a risk assessment should be conducted to assess the condition of the demolition works, to find out any foreseeable hazards and to devise and adopt suitable safety measures.
The use of a demolition hazard checklist is recommended.
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Structural Design
To study the design of the building structure is a very important step in the survey.
If special design such as arches, pre-stressed reinforcement, reinforcement under tension, curtain wall or thin shell is observed, special shoring and demolition method should be adopted.
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Structural Design
It has to find out whether the structure is capable to withstand the increasing load as the process proceeds.
Temporary support and suitable method could be devised to cope with the additional loading.
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Structural Design
The structural survey should note variation to the original plan due to structural alterations and additions.
Special precautions should be observed to handle structural alteration such as external metal cages, water cooling towers and canopies, etc.
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Structural Design
Buildings or structures may have seriously weakened after heavily used for many years or exposed to adverse environment.
The protective element of reinforced bar may be corroded or the concrete may be weakened by weathering. The structural integrity of the building may not be as safe as its original design.
It is important to determine the structural condition as serious deterioration may impose restrictions on the demolition method. 15
Hazards associated with the previous use of the building
The previous use of the building may have produced hazards requiring detailed surveys to be carried out.
In buildings where chemicals may have been stored and used, such as factory buildings and hospitals, hazards associated with flammable material and substances hazardous to health should be identified.
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Hazards associated with the previous use of the building This survey should be conducted by experienced and competent persons and should observe:
Dangerous goods stores may co-exist with boilers or other naked flame processes in hospitals or industrial undertakings.
Storage of hazardous substances may exist. Large quantities of toxic or harmful chemicals may be used in some of the manufacturing processes.
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Hazards associated with the previous use of the building
Hazardous substances may also exist in building material or industrial installations, e.g. asbestos may exist in heat insulating material in boiler rooms, lead may exist in paints, chemicals may exist in pipes radio-active material may exist in equipment.
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Building Services
The survey should cover also the installation of building services such as electricity, gas, water pipes, drainage, telephone, radio an TV lines.
Written confirmation should be obtained from the relevant authorities before commencement of the demolition works to ensure that all known supplies have been cut off from the site except those required for the execution of the work.
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Safe working method statement
The precaution to combat the identified hazards, protect workers and remove hazardous material together with the preferred demolition procedure, should be set out in a method statement approved by the relevant authorities.
A detailed method statement, produced before work starts, is essential for safe working. It should identify problems and their solutions, and form a reference for site supervision.
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Safe working method statement
The method statement should be easy to understand and suitably illustrated by diagrams; should be agreed by and known to all levels of management and supervision, including those of the sub-contractors.
Working method should be revised to cope with unforeseeable hazards during the demolition process and any changes should be made know to all parties concerned.
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Safe working method statement
The method statement should include controlling measures to ensure that the sequence and method by which the elements of a building are to be demolished must be decided before work starts.
At each stage of work a conscious appraisal of risk of collapse must be made, the precautions worked out and workforce properly controlled to ensure that the precautions are put into practice.
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Safe working method statement A comprehensive method statement should include the followings:
Work schedule and the sequence of demolition, especially to the sequence of cantilever structure demolition. Design suitable demolition method to tackle with special structures, e.g. chimney, steel structure, pre-stressed concrete, electric cable tower and water-cooling tower.
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Safe working method statement
Selection of suitable machinery and equipment.
Temporary shoring method and other precautionary measures to safeguard the stability of any adjoining and adjacent buildings.
Details of safety procedures to remove or cut off the existing utility installations such as electrical, gas, drains and other services.
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Safe working method statement
Details of temporary services required for execution of work.
Details of methods for dealing with flammable material and gases.
Details of methods to be used for the disposal of substances hazardous to health and any necessary protective equipment.
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Safe working method statement
Arrangement for the control of site transport used for the removal of demolition debris.
Special precautions for entering confined space in demolition site.
Arrangement for the protection of personnel and public and the exclusion of unauthorized person.
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Safe working method statement
Measures to protect the environment, e.g. controlling the emission of excessive noise and dust from demolition.
Contingency plan for typhoon, storm, flooding, fire, explosion and industrial accident.
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Demolition
Where possible, demolition should be carried out in the reverse order of building erection.
No isolated freestanding wall should be left unless judged to be secure by the competent person in charge.
Scaffold working platforms should be used, all refuse and debris being removed from these temporary structures on a regular basis to avoid overloading. 28
Demolition
Debris which has accumulated behind walls should also be removed.
Independently supported working platforms over any reinforced concrete slabs should be demolished.
Support for members of framed structures must be provided before gutting, along with temporary props, bracing or guys to restrain remaining parts of the building. 29
Demolition
On no account must operators work from the noor being demolished, and site control must ensure that all personnel are kept at a safe distance from the scene of operations, when pulling arrangements, demolition balls, pusher arms and/or explosives are being used.
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Demolition
An on-going system of inspection must be maintained during demolition to detect further hazards which may result from the demolition process, e.g. loosened materials, overloaded floors.
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Working at height
Whenever the demolition work has to be conducted at heights, suitable working platforms or scaffoldings should be erected.
They should be designed by a professional engineer; erected, altered or demolished by adequately trained and experienced workers under the close supervision of a competent person.
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Use of machinery
If the building is to be demolished by excavator, it must be ascertained that the structure can withstand the weight of the excavator and the accumulated debris. Suitable propping should be provided if found necessary
The machine must be maintained in a safe working condition in accordance with the Regulations.
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Use of machinery
The cab of all machines should be strong enough to protect the operator against the fall of debris.
Machinery operator shall be properly trained in the skill of demolition as well as safe practices so as to maintain their level of awareness.
The excavator operator must take care not to damage the beams and columns supporting the excavator.
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Debris and waste disposal
No part of the building under demolition shall be allowed to be so overloaded with debris or material as to render it unsafe to workers employed.
Special attention must be paid to the debris at ground level as lateral force created by the debris and the machinery under use would cause the side wall to collapse.
Debris and rubbish shall be cleared regularly to avoid overloading. 35
Debris and waste disposal
Debris may be allowed to fall through floor openings but must be closely supervised. Under no circumstances should debris be allowed to fall outside the building.
Large pieces of debris shall be lowered properly by cranes.
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Safe working on Demolition sites
All precautionary measures are in place before commencement of demolition works. Pay special attention to the demolition of special structures, cantilever, chimney, and pre-stressed structure.
Ensure that a professional engineer has inspected the structure/adjacent structure and a competent person is in charge of the operation.
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Safe working on Demolition sites
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Safe working on Demolition sites
Pay attention to the presence of flammable or hazardous substances such as asbestos in insulation, lead in paints and radio-active substances in laboratory equipment.
In buildings where carcinogenic chemicals may have been used or stored such as factory and hospitals, associated hazards shall be identified by an experienced competent person.
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Safe working on Demolition sites
Check and ensure that existing floors planned to be used are not overloaded. Otherwise, shoring should be installed to support the floors.
Provide sufficient shoring to prevent premature structural collapse or damage to adjacent property.
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Safe working on Demolition sites
Ensure that the site is properly enclosed. Scaffold with screen and catch-fan, and hoarding or covered walkway shall be erected.
Erect warning and information notices.
Ensure that all ladders, cranes, cables and other equipment are in good order.
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Safe working on Demolition sites
Ensure that suitable personal protective equipment is provided and used during operation. Suitable anchorage points for safety belts and safety harnesses shall be identified and assessed by competent persons or Professional Engineers.
Adequate protection and safe access for the public and workers are provided at all times, including protection from dust and noise hazards.
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Safe working on Demolition sites
Provide sedimentation/treatment system to prevent silty / dirty water discharging into water courses or public drains.
Properly plan the daily work. Complete the scheduled work and check the stability condition of the structure under demolition prior to leaving the site at the end of each working day.
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Q&A
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