SAFE DEMOLITION OF BUILDINGS

Demolition is the tearing-down of buildings and other structures, the opposite of construction. Demolition contrasts with deconstruction, which involves taking a building apart while carefully preserving valuable elements for re-use

For small buildings, such as houses, that are only two or three stories high, demolition is a rather simple process. The building is pulled down either manually or mechanically using large hydraulic equipment: elevated work platforms, cranes, excavators or bulldozers. Larger buildings may require the use of a wrecking ball, a heavy weight on a cable that is swung by a crane into the side of the buildings. Wrecking balls are especially effective against masonry, but are less easily controlled and often less efficient than other methods. Newer methods may use rotational hydraulic shears and silenced rock-breakers attached to excavators to cut or break through wood, steel, and concrete. The use of shears is especially common when flame cutting would be dangerous.

 

     Large buildings, tall chimneys, smokestacks, and increasingly some smaller structures may be destroyed by building implosion using explosives. Imploding a building is very fast — the collapse itself only takes seconds — and an expert can ensure that the building falls into its own footprint, so as not to damage neighboring structures. This is essential for tall structures in dense urban areas.

     Any error can be disastrous, however, and some demolitions have failed, severely damaging neighboring structures. The greatest danger is from flying debris which, when improperly prepared for, can kill onlookers.

Even more dangerous is the partial failure of an attempted implosion. When a building fails to collapse completely the structure may be unstable, tilting at a dangerous angle, and filled with un-detonated but still primed explosives, making it difficult for workers to approach safely.

    A third danger comes from air overpressure that occurs during the implosion. If the sky is clear, the shockwave, a wave of energy and sound, travels upwards and disperses, but if cloud coverage is low, the shockwave can travel outwards, breaking windows or causing other damage to surrounding buildings.

Preparation

It takes several weeks or months to prepare a building for implosion. All items of value, such as copper wiring, are stripped from a building. Some materials must be removed, such as glass that can form deadly projectiles, and insulation that can scatter over a wide area. Non-load bearing partitions and drywall are removed. Selected columns on floors where explosives will be set are drilled and nitroglycerin and TNT are placed in the holes. Smaller columns and walls are wrapped in detonating cord. The goal is to use as little explosive as possible; only a few floors are rigged with explosives, so that it is safer (fewer explosives) and less costly. The areas with explosives are covered in thick geotextile fabric and fencing to absorb flying debris. Far more time-consuming than the demolition itself is the clean-up of the site, as the debris is loaded into trucks and hauled away.

TRADITIONAL DEMOLITION

Before any demolition activities, there are many steps that need to take place — including but not limited to performing asbestos abatement, obtaining necessary permits, submitting necessary notifications, disconnecting utilities, rodent baiting, and development of site-specific safety and work plans.

The typical razing of a building is accomplished as follows:

Hydraulic excavators may be used to topple one- or two-story buildings by an undermining process. The strategy is to undermine the building while controlling the manner and direction in which it falls. The demolition project manager/supervisor will determine where undermining is necessary so that a building is pulled in the desired manner and direction. The walls are typically undermined at a building's base, but this is not always the case if the building design dictates otherwise. Safety and cleanup considerations are also taken into account in determining how the building is undermined and ultimately demolished. Hoe rams are typically used for removing the concrete road deck and piers during bridge demolition, while hydraulic shears are used to remove the bridge's structural steel.

In some cases a crane with a wrecking ball is used to demolish the structure down to a certain manageable height. At that point undermining takes place as described above. However crane mounted demolition balls are rarely used within demolition due to the uncontrollable nature of the swinging ball and the safety implications associated.

High reach demolition excavators are more often used for tall buildings where and explosive demolition is not appropriate or possible.

To control dust, fire hoses are used to maintain a wet demolition. Hoses may be held by workers, secured in fixed location, or attached to lifts to gain elevation.

Loaders or bulldozers may also be used to demolish a building. They are typically equipped with "rakes" (thick pieces of steel that could be an I-beam or tube) that are used to ram building walls. Skid loaders and loaders will also be used to take materials out and sort steel. 

Kajima Deconstruction Technology

The Japanese company Kajima Construction has developed a new method of demolishing buildings which involves using computer-controlled hydraulic jacks to support the bottom floor as the support beams are removed. The floor is lowered and this process is repeated for each floor. This technique is safer and more environmentally friendly, and is useful in areas of high population density.

A new approach to demolition is the deconstruction of a building with the goal of minimizing the amount of materials going to landfills. This "green" approach is applied by removing the materials by type material and segregating them for reuse or recycling. With proper planning this approach has resulted in landfill diversion rates that exceed 90% of an entire building and its contents in some cases. With the rising costs of landfills this method is usually more economical than traditional demolition, and contributes to a healthier environment.^]

The development of plant and equipment has allowed for the easier segregation of waste types on site and the reuse within the construction of the replacement building. On site crushers allow the demolished concrete to be reused as type 1 crushed aggregate either as a piling mat for ground stabilization or as aggregate in the mixing of concrete.

Safety should always be paramount. A site safety officer should be assigned to each project to enforce all safety rules and regulations.

Historical overview

As part of the demolition industry, the history of building implosion is tied to the development of explosives technology. One of the earliest documented attempts at building implosion was the 1773 razing of Holy Trinity Cathedral in Waterford, Ireland with 150 pounds of gunpowder, a huge amount of explosives at the time. The use of low velocity explosive produced a deafening explosion that instantly reduced the building to rubble.

 

The 1900s saw the erection of—and ultimately the need to demolish—the first skyscrapers. This led to other considerations in the explosive demolition of buildings, such as worker and spectator safety and limiting collateral damage. Benefiting from the availability of dynamite, a high-velocity explosive based on a stabilized form of nitroglycerine, and borrowing from techniques used in rock-blasting, such as staggered detonation of several small charges, building demolition edged toward efficient building implosion.

Following World War II, European demolition experts faced with massive reconstruction projects in dense urban areas gathered practical knowledge and experience for bringing down large structures without harming adjacent properties. This led to the emergence of a demolition industry that grew and matured during the latter half of the twentieth century. At the same time, the development of more efficient high-velocity explosives such as RDX and non-electrical firing systems combined to make this a period of time in which the building implosion technique was extensively used.

Meanwhile, public interest in the spectacle of controlled building explosion also grew. The October 1994 demolition of the Sears Merchandise Center in Philadelphia, PA drew a cheering crowd of 50,000, as well as protesters, bands, and street vendors hawking building implosion memorabilia. Evolution in the mastery of controlled demolition led to the world record demolition of the 

In 1997, a building implosion in Canberra, Australia experienced disaster. The main building did not fully disintegrate and had to be manually demolished. Far worse, the explosion was not contained on the site and large pieces of debris were projected towards spectators 500 meters away, in a location considered safe for viewing. A twelve-year old girl was killed instantly, and nine others were injured. Large fragments of masonry and metal were found 650 meters from the demolition site.

NEED OF DEMOLITION

1. Structure has been built with faulty design or construction.

2. Structure is deteriorated because of corrosion 

3. Structure attained its age (i.e., become old and unfit for occupancy)

4. Structure in unsafe and its repair and rehabilitation proves to be costlier

5. Complete removal of structure so as to make way for another structure or none at all.  Example: discarded nuclear power plants

 

METHODS OF DEMOLITION

1. Pneumatic and Hydraulic Breakers 

2. Pressure Bursting 

     (Mechanical Bursting and Chemical Bursting) 

3. Explosives

4. Ball and crane method

5. Diamond Wire Saw Method 

 

PNEUMATIC AND HYDRAULIC BREAKER 

 

These are often used in concrete demolition projects involving the bridge decks, foundation and pavements. Hand held or mounted pneumatic and hydraulic breakers are currently the tools of choice. The amount of work accomplished using this method depends on hammer size, strength of concrete, amount of steel reinforcement used in concrete and working condition. Machine mounted breakers are able to deliver a work done of 14 – 2766 kg-m. 

Advantages of Pneumatic and Hydraulic Breaker 

• A telescoping boom for easy reach and maneuverability.
  
• Remote control operation. 
• Under water demolition capabilities. 

 Disadvantages of Pneumatic and Hydraulic Breaker 

• 
  The machine mounted breakers include generation of noise, dust and vibration.

Fig2  PNEUMATIC AND HYDRAULIC BREAKER

Fig3.HAND RAIL BREAKERS

PRESSURE BURS

It can be used in cases where relatively quiet, dust free, controlled demolition is preferred.
Both mechanical and chemical pressure bursting split the concrete, either with a splitting machine operating on hydraulic pressure provided by a motor in the case of mechanical bursting, or through the insertion of an expansive slurry in to a pre-determined pattern of boreholes incase of chemical bursting. The split concrete is then easily removed, either by hand or by crane

Mechanical and Chemical pressure bursting break up concrete structure with a minimum of noise and flying debris. Both methods work by applying lateral forces against the inside of holes drilled in to the concrete. 

However, rather than shattering the concrete in to bits as dynamite and impact tools would, the lateral forces build up over time to crack the concrete in to smaller sections. 

Advantages of Mechanical Bursting

1. in expensive, quiet, no vibration. 

2. Diamond tipped core machine greatly reduces the noise and vibration during work.

Disadvantages of Mechanical Bursting

1. The initial drilling of holes for splitter dose produce some noise and vibration. 

2. Controlling crack direction and movement of demolished concrete is difficult.

3.Hand held or machine mounted breakers may be needed to expose the reinforcing bars for cutting. 

Advantages of Chemical Bursting

1. It can split concrete in a controlled manner. 

2. Quiet, no vibration. 

3.  Little or no dust.

Disadvantages of Chemical Bursting

1. A temperature sensitive freezing greatly reduces the effectiveness.More costly method. 

3.EXPLOSIVE 

Building implosion is a term in use in the controlled demolition industry. It refers to strategically placing explosive material and timing its detonation so that a structure collapses on itself in a matter of seconds minimizing the physical damage to its immediate surroundings. Despite its terminology, building implosion also applies to the controlled demolition of other structures, such as bridges, smokestacks, towers, and tunnels.

Building implosion (which reduces to seconds a process which could take months or years to achieve by conventional methods) typically occurs in urban areas and often involves large landmark structures.

These are generally used for removing large volume of concrete via insertion of explosive devices in a series of boreholes.

Advantages

Versatile and flexible in terms of work output

Disadvantages

Vibration and air blast may damage the surrounding structure

Heightened safety considerations involved when compared to other demolition methods.

 

 

BALL AND CRANE METHOD 

One of the oldest and most commonly used method for demolition, the ball and crane uses a wrecking ball weighing about 6125 kg to demolish concrete and masonry structure. During the process, the ball is either dropped on to or swung in to the structure that is to be demolished.
The Ball and Crane, however is not suitable for all demolition applications.

Some Limitations

• Only high skilled and experienced crane operators should be used on ball and crane demolition projects.
  
• Smoothness in controlling the swing of ball is important. Since, missing the target may tip or overload the crane and mild swing back may cause the ball to hit the boom. 
• The size of building that can be demolished with this method is limited by the crane size and working room, including the proximity to power lines.
  
• This form of demolition creates a great deal of dust, vibration and noise. 

Fig. 4: Ball and Crane in Action 

DIAMOND WIRE SAW METHOD 

The applications of diamond wire cutting technique are unlimited. This method can be applied regardless of thickness of concrete or amount of reinforcement. Unusual configurations, angled cuts and difficult areas can be easily accomplished. Remote cutting can be performed in hazardous or radioactive areas where direct access to the cut is impractical. 

1. Construction details – Diamond Wire Saw Method 

2. Electroplated beads with compressed steel spring spacers 

3. Impregnated Beads with Compressed steel spring spacers 

4. Impregnated Beads with injection – molded plastic spacing

Operation 

1. A small hole is drilled through the concrete at each corner of the opening or at the end of the cut to be made using precise instruments. Now the wire is passed through one access hole and back through the other. It is then coupled and placed on the driving wheel and rotated at a speed of 910 – 1825 m per min. Wire tension is maintained by rack and pinion arrangement that moves the driving wheel along the wire saw carriage. 

Fig. 8 represents a typical wall cutting

Fig. 7 represents a typical floor cut.

 

SAFETY MEASURES 

• Do not wear loose clothing that can get caught in machinery 
• Pull long hair back 
• Remove any Jewelry that can interfere with safe machinery operation
• Wear safety goggles or glasses with side protection
• Use face mask in dusty application
• Wear ear plugs when site is especially noisy or for extended periods of work
• Heavy work glove should be worn to protect against steady vibration of power tools and heat that can be generated especially in bit. 
• Wear steel toed Shoes or Boots 
• Safety measure should be taken against hand tool hazards
• Care should be taken against power tool hazards
• Care should be taken against machine mounted tool hazards 

CONCLUSION

1. In the present seminar various methods of demolition have been discussed. All the above methods are applicable under certain conditions and it is advisable to call into consultation an experienced demolition contractor at planning stage to ascertain which method is preferable in a particular case.

2. The magnitude of the catastrophe, which could result from demolition of modern buildings without full knowledge of their construction, can be readily imagined. It is important for architects and engineers to realize that a building will not last forever and its eventual demolition should be taken into account while designing and building a new structure. If this is not done the cost of demolishing certain building may easily exceed their construction cost.