1. References

The College of Estate Management, 1995, Technology - Module 4, London: The College of Estate Management

2. Learning Objectives

'An ability to carry out the demolition work in a safe and economical manner."

3. Preliminary Procedures

At this moment, there are no legal requirements in Hong Kong for plans and constructional details of buildings to be kept and handed over with the building once constructed. Consequently, details of the specification and constructional elements have to be established largely by survey, tests and careful inspection To carry out demolition safely and economically needs a great deal of experience and skill and an excellent knowledge of building construction.

3.1 Preliminary Investigation
Demolition is a highly skilled and dangerous activity in terms of damage to life and property and there are certain basic factors to consider before a contract is placed:

• The demolition contractor should have ample experience of the type of work to be offered;

• Fully comprehensive insurance against all risks must be maintained at all times;

• An experienced supervisor should be continuously in charge of the work;

• The contract price should include all safety precautions included in the relevant building regulations;

• The completion date should be realistic, avoiding and need to take risks to achieve the date.

3.3 General Site Provisions

1. Plant and Equipment

   Must only be operated by skilled operators and must be regularly serviced.

2. Protective Clothing

   Buildings where chemicals have been stored or where asbestos, lead paint, dust or fumes may be present will require specialized protective clothing, e.g. respirators, helmets, goggles, footwear, gloves, etc. Projecting nails, pieces of metal, etc. resulting from demolition can cause accidents.

3. Shoring and Underpinning

   The demolition contractor has a legal obligation to show technical competence when carrying out the work. When removing sections of the building which could have leave other parts unsafe, adequate temporary supports and shoring etc. must be provided.

4. Working Areas

   These will need to be well signposted and clear warnings given that demolition work is in progress. This may include the necessity for some kind of lighting.

5. Debris

   Sections of the building must not be overloaded with debris either on suspended floors or against party walls.

6. Weather Conditions

   These can affect safety. Strong winds or drifting snow against unsafe walls, suspended floors etc. which are unpropped may lead to collapse.

7. Flooding

   The build-up of water can sometimes be hazardous.

8. Overhead Cables

   An crane heights etc. must be checked against the height of any surrounding overhead cables to avoid damage and cutting off supplies etc.

9. Scaffolding and Hoarding

   These must be constructed and illuminated to the relevant building regulations.

10. Security

    The demolition site and any partially demolished buildings must be properly secured against entry.

11. Dust

    Should be kept to a minimum by spraying with water when necessary.

12. Noise
    Suppressors and silencers, particularly on compressors etc., should be used to keep noise levels to a minimum.

3.4 Supervision of Demolition Work
A method statement showing how the demolition work is to be carried out should be prepared and the contractors should appoint a "competent person" to supervise the demolition work.

4. Demolition Processes

As an intrinsic part of the construction process, efficient demolition of structures is an important factors deserving careful consideration in the evolution of any redevelopment project.

Modern emphasis is on reduction of construction periods to ensure economic redevelopment, coupled with increasing town centre regenerating calling for careful demolition on constructed and restricted site, have resulted in more consideration being given to demolition as part of the process of construction and redevelopment than was typical in previous times.

4.1 Developing a Demolition Strategy
The strategy will need to take into account the method of construction used for the original building and its proximity to other buildings, structures and the general public. These factors, together with location, the cost and availability of tipping and disposal and the desirability and economics of reuse, must be taken into account in the development of an appropriate strategy for the demolition of a structure.

4.2 Building Information
Information on buildings in terms of "as built" drawings and structural details may often be unavailable or unreliable, and consequently some investigative site and desk work may be necessary, both to ascertain the way in which the building was originally constructed, and to identify the stresses and strains which exist within it.

In order to plan the most efficient method of demolition, it is important to have a full understanding of the method of construction and the stress patterns imposed upon the building. Failure to do so may result in risks to the safety of both those involved in the demolition and those in close proximity to the site.

4.3 Selecting Appropriate Techniques
Majors factors to be considered in selecting an appropriate technique include:-

4.4 Preliminary Aspects Prior to Site Demolition Work
Considerations should be given to:-

• Conducting a site and building survey, with a structural bias;

• The examination of drawings and details of existing construction where available;

• The preparation of details and drawings from site survey activities where no such information is available;

• Establishing previous use of premises, especially with regard to flammable substances or substances hazardous to health or safety;

• Programming the sequence of demolition work;

• The preparation of a Method Statement.

4.4.1 Method statement

• A detailed health and safety method statement, produced before work starts, is essential for safe working. It should include a full risk assessment, identify problems and their solutions, and form a reference for the site supervision.

• The method statement should be easy to understand, agreed by and known to all levels of management and supervision, and should include such matters as:-

• The sequence and method of demolition or dismantling of the building or structure with details of personnel access, working platforms and machinery requirements;

• Details and design of any temporary supporting structures to be used during the demolition process;

• Specific details of any pre-weakening on structures which are to be pulled down or demolished with explosives;

• Arrangements for the protection of personnel and the public and the exclusion of unauthorized persons, with details of areas outside the site boundaries that may occasionally need to be controlled to improve safety during critical aspects of the work;

• Details of the removal or making safe of electrical, gas and other services and drains;

• Details of temporary services available or required for the contractor's use;

• Details of the methods for detailing with flammable materials and gases which may have been retained or deposited as residue in process machinery, pipework or storage;

• Details of methods to establish the presence of hidden or other substances that may be hazardous to health, the methods to be used for their disposal, and any necessary protective equipment;

• Arrangements for the control of site transport used for the removal of demolition debris.

5. Demolition Methods

In many circumstances, buildings and structures should be demolished in the reverse order to their erection, although where partial demolition is involved a more careful evaluation of the nature of the effects of the demolition is necessary.

Normally, the demolition contractor is able to adopt a method of work which:-

• gradually reduces the height of the building; or

• arranges the deliberate controlled collapse of the building or structure so that work can be completed at ground level.

5.1 Demolition Technique Selection
The choice of demolition technique will depend on the nature of the building or structure and its environment. Risks to the public, operatives involved in the demolition process and adjacent structures and buildings should be considered.

Demolition techniques may be categorized as:-

• Piecemeal demolition, using hand-held tools or machines, to reduce the height of the building or structure gradually;

• Deliberate controlled collapse, demolition to be completed at ground level.

5.1.1 Piecemeal Demolition by Hand

Demolition of buildings or structure by hand-held tools such as electric or pneumatic breakers, sometimes as a preliminary to using other methods, should be carried out, where practicable, in the reverse order to the original construction sequence. Lifting appliances may be necessary to hold larger structural members during cutting and for lowering severed structural members and other debris. Chutes may be used to discharge debris into a vehicle or hopper. Foundations would normally be grubbed up by excavation machines.

Simple roof structures supported on wall plates should normally be demolished to the level of wall plates by hand, but if this may involve unsafe working, then demolition totally by machine may be appropriate.

Where a building that is to be demolished by machine is attached to another structure, the two properties should be separated by the use of hand methods before the main demolition process begins.

When any part of a building is being demolished by a balling machine, pusher arm or similar equipment, only the machine operator and banksman should be allowed close to the working area. The cabs of all machines should be strong enough to protect the operator against the fall of debris. In particular, the windscreen and rooflight should be of shatterproof material and guarded by a grille of steel bars or a substantial mesh.

1. Balling Machine
   Balling machines generally comprise a drag-line type crawler chassis fitted with a lattice crane jib. The demolition ball, with a steel anti-spin device, is suspended from the lifting rope and swung by the drag rope.

   Balling should only carried out by skilled operatives under the control of experienced supervisors using well maintained machines adequate for the proposed duty and standing on a firm, level base.

   The manufacturer should be consulted before a machine is used for balling to establish any restrictions on the type or length of jib or the weight of the ball.

Balling operations subject cranes to dynamic stresses and wear, and the ball chosen should have the minimum weight necessary for effective use. In many cases, demolition balls of quite light weight will be adequate.

Floors should be demolished by dropping the ball on the highest remaining floor and allowing the debris to fall inside the building. The debris should be removed regularly to prevent excessive weight accumulating on the lower floors.

Walls or columns can be demolished either by swinging the ball in line with the stationary jib, using the drag rope, or by slewing the jib. The ball should not swung by derricking the jib.

Figure 2: Demolition ball

  

2. Hydraulic Pusher Arm
   Articulated, hydraulically-powered pusher-arm machines are normally mounted on a tracked or wheeled chassis, and have a toothed plate or hook for applying for applying a horizontal force to a wall. The machine should stand on a firm level base and apply force by a controlled movement of the pusher arm.

Figure 3: Hydraulic pusher arm

3. Explosives
   If explosives are to be used for demolition, the planning and execution, include pre-weakening, should be under the control of a person competent in these techniques. For large demolition, the competent person is likely to be an experienced explosive engineer; for smaller work, a shot-firer may be sufficient.

When the use of explosives is contemplated, it is usual to employ a technique that will ensure the total demolition of the whole building by staging a controlled collapse. The explosive charges are set and fired in a sequence that will weaken the structure in such a way that the building collapses in upon itself.

Although we tend to think of explosives as devices producing spectacular bomb-like explosions, the use of non-explosive "explosives" is now at an advanced stage. These non-explosive techniques are essentially expanding charges that achieve the same results as explosives but without the noise and initial devastating blast.

4. Overturning - Wire Rope Pulling
   This method is the application of a horizontal force at a high level by pulling with wire ropes attached to winches or vehicles, and allowing the impact on overturning to demolish the building or structure. An adequate steel cab or cage should protect the winch or the pulling vehicle and the operator.
   Building over 21m high should not normally be demolished by rope pulling.

5. Impact Hammer and Nibblers
   Impact hammers normally have a track- or wheel- mounted chassis, an articulated boom, and a heavy duty pick vibrated by hydraulic or pneumatic power to demolish concrete or masonry.

Nibblers use a rotating action to snap brittle materials such as concrete or masonry. In either case, material should be removed from the top of walls or columns in courses not greater than 600mm in depth, steel reinforcement should be cut separately as necessary.  

5.2 Pre-Weakening
Buildings and structures normally have structural elements designed to carry safely the loading likely to be imposed during their life.

As a preliminary to a deliberate controlled collapse, after loads such as furnishings, plant and machinery have been removed, the demolition contractor may be able to weaken some structural elements and remove those new redundant. This pre-weakening is essentially a planned exercise and must be preceded by an analysis of its possible effects on the structure until it collapses, to ensure that the structural integrity of the building is not jeopardized accidentally. Insufficient information and planning relating to the structure may result in dangerous and unsafe work.

5.3 Deliberate Controlled Collapse
The deliberate collapse of the whole or part of a building or structure requires particularly high standards of planning, supervisions and execution, and careful consideration of its effect on other parts of the structure or on adjacent buildings or structures. A surrounding clear area and exclusion zone are required to protect both personnel and property from the fall of the structure itself and debris which may be thrown up by the impact.

The collapse is usually achieved either by removing key structural elements (e.g. with explosive charges) or by wire rope pulling at a high level to overturn the structure. The possible modes of failure must be studied to ensure that the method selected will produce the required pattern of collapse. If the operation is not successful, the remaining structure may be extremely dangerous for the completion of the demolition.

Demolition by deliberate controlled collapse is not usually appropriate for prestressed concrete except for simple pre-tensioned floor planks or slabs.