In concrete structures there are many different methods that can structure engineer uses in demolition process
1-Top Down — Manual Method
Top down method discussed below is the method that proceeds from the roof to ground in a general trend, there are particular sequences of demolition which may vary, depending on site conditions and structural elements to be demolished.
For reinforced concrete buildings, jack hammers are commonly used to break down the concrete. Oxy-acetylene torch could be used to cut the reinforcements. The structural elements shall be broken down gradually or by alternate methods as described in the following sub-section. The reinforcements shall remain until all the concrete connecting to or supported by the reinforcement is broken away or when its support is no longer required.
Demolition Sequence
(A) All cantilevered structures, canopies, verandahs and features attached to the external walls shall first be demolished prior to demolition of main building and its internal structures on each floor;
(B) When demolishing the roof structure, all lift machine rooms and water tanks at high level shall be demolished in “top down” sequence to the main roof level.
(C) Demolition of the floor slabs shall begin at mid span and work towards the supporting beams;
(D) Floor beams shall be demolished in the order as follows:
(1) cantilevered beams;
(2) secondary beams; then
(3) main beams.
In the case when structural stability of beams are affected, e.g., due to loss of restraints, the affected beams shall be propped prior to loss of support or restraint;
(E) Non-load bearing walls shall be removed prior to demolition of load bearing walls;
(F) Columns and load bearing walls shall be demolished after removal of beams on top; and
(G) If site conditions permit, the first floor slab directly above the ground floor may be demolished by machine sitting on ground level and mounted with demolition accessories.
Demolish of slab (Manual method)
Pre-walking dismantling of column (manual method)
2-Top Down — By Machines
Supports for Machines
The sequence of demolition by machine is typically the same as the top down manual method, except that most of the demolition is done by mechanical plant.
The loading to be imposed on the floors by the mechanical plant shall be checked. If needed, propping shall be installed at floor levels below the working floor to safely support the operation of the mechanical plant.
The mechanical plant shall be lifted onto the roof of the building by the use of mobile crane or other appropriate means
The machine shall descend down to the next floor by means of a ramp. The ramp may be a temporary structure or other appropriate design. The slope of the ramp shall be no steeper that 1.75 to 1 or as recommended by the machine manufacturer.
Temporary Propping System
Propping Requirements on the Operation of Mechanical Plant on Suspended Floor
|
Design imposed load of floor to be demolished |
3 Kpa |
5 Kpa |
7.5 Kpa |
12.5 Kpa |
|
Maximum weight of mechanical plant allowed |
11.6 ton |
11.6 ton |
11.6 ton |
11.6 ton |
|
Minimum no. of consecutive floors required to distribute mechanical plant loading, through propping |
5 |
3 |
2 |
2 |
|
Minimum no. of consecutive floors required to distribute localized loading from temporary ramp, through propping |
5 |
4 |
3 |
2 |
|
Maximum spacing of steel props in each direction |
1.2m |
1.2m |
1.2m |
1.2m |
The crusher attachment breaks the concrete and the reinforcement by the hydraulic thrust through the long boom arm system. The hydraulic crusher can be operated from the ground outside the building. This method is also suitable for dangerous buildings, silos and other industrial facilities. Figure 4.15 illustrates the typical operation of hydraulic crusher with long boom arm. For environmental reason, it should be used wherever practicable because of its quietness.
-Demolish by Hydraulic Crusher with Long Boom Arm
4-Wrecking Ball
The wrecking ball application consists of a crane equipped with a steel ball. The destruction of the building is by the impact energy of the steel ball suspended from the crawler crane. The wrecking ball operates outside the building. This method is suitable for dilapidated buildings, silos and other industrial facilities. However, the operation requires substantial clear space. The application also demands high level skill operators and well-maintained equipment.
5-Implosion
Pre-blast Considerations
If it is intended to blast a building structure, the Registered Specialist Contractor (Demolition) shall carry out a comprehensive Risk Assessment Report and an Environmental Assessment Report on the effect of implosion on the affected neighborhood. With positive results on both the risk assessment and environmental impact assessment and agreed by the relevant approval Authority, through the central processing of the Buildings Department, the Registered Specialist Contractor (Demolition) may begin studying the structure of the building and develop a blasting design
6-Non Explosive Demolition Agent
Non Explosive Demolition Agent (NEDA) is a static demolition agent. When the reaction takes place in a confined drill hole, the NEDA generates an expansive pressure to crack and break concrete and stone.
NEDA may be used on foundation works, pile caps or structures that are fully supported
7-Saw Cutting
Saw cutting is suitable for alteration and additional works where accuracy in the cutting is important and the tolerance to noise and vibration is very limited. It can be used to cut concrete slabs and wall elements into segments. An entire building may be dismantled by saw cutting. Saw cutting generally includes conventional disc saw and chain saw, diamond core stitch drilling and wire saw.
8- Mechanical Demolition
Mechanical demolition generally involves the use of large machinery with attachment to dismantle the building from outside. The common mechanical methods include the use of a
pusher arm, wire rope and clam shell. These methods shall only be applied to isolated buildings on relatively flat ground
Mechanical by pusher arm – pushing in
9-Thermal Lance
Cutting of reinforced concrete by thermal lance involves very high temperature up to 2,000 - 4,000°C. The extremely high heat requires special precautionary measures and care. The use of a thermal lance in cutting reinforced concrete shall not be used unless:
(A) the project demonstrated that there is no other viable alternative;
(B) adequate protective measures are provided to isolate the operation
and to prevent any potential fire spreading out; and
(C) adequate protective measures are provided to prevent the injury of
the workers, and any third party by flame and the molten concrete.
10-Water Jet
Water jetting involves the use of a water jet stream pumped at high pressure to erode the cement matrix and wash out the aggregates. Abrasive compounds may be added for cutting reinforcing steel. The application of the water jetting shall be subject to the following criteria:
(A) City water supply shall be used in water jet cutting. Provision shall be included to dispose the water used in the operation, and to recycle the water for continuous operation through local filtration and sedimentation;
(B) The area behind the structural member to be cut shall be shielded to avoid damage to persons and properties during the cutting; and
(C) In the case when abrasive water jets are used, further precautionary measures shall be provided in accordance with manufacturer recommendations to confine the rebound of the abrasive compounds. All site personnel shall wear adequate safety cover and clothing
Post demolition process
Once the demolition is completed, the site shall be reinstated to eliminate any potential hazard to the public. The following precautionary measures shall be considered:
(A) The site shall be levelled and cleared of any debris. Adequate drainage shall be provided;
(B) If the new development is not immediately commenced, the site shall be completely enclosed to prevent public trespassing;
(C) Supports to adjacent building structures, weather-proofing and stabilization of exposed party walls shall be completed.
(D) Any excavation shall be braced and stabilized
References;
Code of Practice for Demolition of Buildings – Hong Kong
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