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Demolition

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Construction Processes
Wrecking balls pose a risk to safety and are used less in demolition today.
Demolition involves tearing down or imploding a structure to make space for another structure to be built. The process of demolition is commonly thought to involve the use of explosives or a wrecking ball attached to a crane to bring down a building. However, these things are fast becoming relics of the past. In recent years, the demolition industry has changed dramatically as a result of environmental concerns. Part of a demolition contractor’s job is to not only take a structure or building apart, but to avoid dumping demolition debris into landfills. As a result, many demolition contractors have vertically integrated their operations, branching off into complementary streams of activity such as site clearing, earthmoving, highway construction, plant liquidification, and landscaping. The growing importance of recycling and practicing environmental sustainability in demolition has even given rise to new industry buzzwords such as deconstruction and just-in-time (JIT) demolition. Many even prefer to refer to demolition as deconstruction because deconstruction is opposite construction—buildings constructed with materials become deconstructed when the materials are recycled or resold for secondary use.

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[edit] Demolition: An Industry Snapshot

The demolition process is either referred to as the "demolition and wrecking" industry or as the "construction and demolition" (C&D) industry. Demolition is a $6 billion industry.[1] According to the National Demolition Association, the trade organization representing the demolition industry in Canada and the U.S., there are more than 1,000 demolition contractors operating in North America. The industry employs over 19,000 people and generates about $3.5 billion in annual sales. Many demolition firms are small family-owned enterprises.[2]

Demolition is categorized as either building or non-building demolition.[3] Building demolition includes residential and commercial buildings and offices. Non-building demolition includes uninhabitable structures such as bridges, highways, and streets. Traditional demolition involved using wrecking balls or explosives, then sending the materials to a landfill. Today, demolition is very much a controlled operation that is executed safely under the supervision of a demolition expert. The goal of controlled demolition is to minimize the amount of residual debris caused by a demolition project. A push towards environmentally sustainable practices in demolition now has the majority of contractors focused on the recycling and reselling of demolished materials for profit.

[edit] History

Before the 1930s most buildings were demolished using small hand tools. The process of demolishing an average-sized building or home took months. By the early 20th century, larger and sturdier buildings were being built with the use of new construction techniques. This, combined with the clean-up of debris in the aftermath of World War II, led to the development of new demolition techniques designed specifically for the removal of large buildings.

A massive construction boom in the U.S. preceding the war had a direct effect on the demolition and wrecking industry. Troops returned to the home front but settled in outlying rural areas instead of populating urban centers. A baby boom quickly led to a shortage of available housing for returning soldiers and their new families. Older sections of cities were demolished and cleared away to provide more space for apartment buildings and houses.

During the 1930s the Roosevelt administration also pushed for the improvement of living conditions in some of the country’s poorest urban areas. Such areas featured unsanitary conditions and overcrowded housing tenements. By 1937 the U.S. Housing Authority actively began clearing away large tracts of slums replacing them with federal subsidized housing thereby creating a demand for wrecking and demolition contractors to set up shop.[4]

Various demolition methods began to evolve, such as the use of the wrecking ball and the employment of explosives to implode buildings. Both were used in conjunction with more traditional methods such as hand tools. Demolition quickly became big business associated with high overhead. Payroll accounted for one-third of a demolition company’s total operating budget. Supplies and materials were equally costly. Sometimes pre-demolition or post-demolition work was contracted out to other companies for an additional cost. Smaller demolition firms competing with larger firms sometimes couldn’t always afford to purchase equipment. The price of demolition equipment was expensive and there were also maintenance costs to consider. Companies renting demolition and wrecking equipment quickly popped up as a sister industry to demolition. Other changes to the industry included high insurance costs due to the relatively dangerous nature of the work.[5]

[edit] Process

[edit]
Implosion accounts for only one percent of all demolition activity.

[edit] Implosion

Implosion is used in demolition for taking down tall buildings or skyscrapers especially when such buildings are closely surrounded by other buildings. Imploding a building with explosives involves collapsing the building inward down to its footprint. This is achieved through careful planning, using architectural blueprints of the building. A blaster crew strategically chooses and places the explosives throughout the inside of the building to be imploded. The blaster crew may even develop a 3-D model of the building to virtually test the detonation and implosion scenario ahead of time.[6]

The concept behind implosion is that if the support structure of a building is removed, subsequent sections of building above the support structure will fall onto the building below. Explosives are simply used to trigger the demolition and it’s really gravity that causes the building to fall down.

Demolition by implosion accounts for a very small fraction—one percent—of the overall industry.[7] For instance, of the 1,000+ member companies of the National Demolition Association, only seven perform implosions.[8] Most implosions get subcontracted out to a firm specializing is such an activity by a primary demolition contractor. The primary demolition contractor will prepare the building for implosion and then clean up and recycle the demolition debris afterwards.

[edit] Wrecking Ball

Demolition by ball and crane, otherwise known as a wrecking ball, is one of oldest methods of building demolition. At a hefty 13,500 pounds (6,123 kg),  wrecking balls have proven particularly effective for breaking up hard materials like concrete and masonry.[9] Their use today is declining and they have practically been replaced with the use of high reach and long reach excavators. It is possible their use may even be banned one day.

One reason is many of the crane operators specially trained to operate wrecking ball cranes in the past are now retiring and no new operators have been trained to replace them. Also, the shift to more controlled demolition practices has resulted in many cities banning their use. Excavators with specially equipped attachments and reaches greater than 90 feet (27 m) are more cost-effective and can accomplish the same task with more control.

[edit] Deconstruction

According to U.S. Environmental Protection Agency, the U.S. alone generates more than 135 million tons of building-related construction waste and demolition debris every year.[10] NDA estimates only 40 percent of waste created on demolition project sites actually gets recycled.[11] In recent years the majority of demolition material would have ended up in landfills but more and more demolition contractors are taking a “green” approach to demolition with a method called deconstruction. The goal of deconstruction is to salvage, recycle, and resell as many materials as possible from a demolition project, reducing the amount of debris ending up in landfills.

The deconstruction of a building is generally more time consuming than conventional demolition. Salvaged materials need to be removed slowly and carefully so they aren’t damaged. This is accomplished mainly by using smaller hand tools and machinery. In traditional demolition it is common to salvage doors, windows, and cabinets but not the brick or wood of a building. However, deconstruction today involves the salvage and recycling of many more materials, including concrete, asphalt pavement, metals, brick, blocks, and wood.[12] A management strategy known as just-in-time demolition was developed by the industry to facilitate the reuse of building materials.[13]

When bidding on new demolition projects, the environmental track record of the demolition contract is becoming increasingly important. A contractor who can resell or recycle the most demolition materials from a project is more likely to procure a project. Many demolition contractors are working towards a high demolition material recycling rate. For example, NDA reports that many of its members have 75 percent recycling rates while others have as high as 100 percent.[14]

Though more labor intensive, deconstruction is much cheaper than conventional demolition—sometimes as much as 30 to 50 percent.[15] One reason is it doesn’t require the use or rental of heavy equipment for removal. Material can be salvaged and sold for reuse in other projects or the construction of new buildings. Sometimes the recycled material from an old building is saved on-site for reuse in the building replacing it. Rescued materials, even when sold to salvage yards, can equal half the value of new materials. This is why deconstruction is way a more cost-effective method than traditional demolition.

[edit] More Incentives to Recycle

Demolition contractors are also being encouraged to recycle and reuse demolition materials for other reasons. For example, many municipal jurisdictions now impose fines for discarding demolition material into landfills. The cost of dumping in landfills has become more expensive. Some areas also give tax credits for the diversion of waste from landfills. Mobile recycling technology and equipment also permit demolition contractors to recycle materials more easily on-site.[16]

[edit] Equipment Used

Today excavators are the machine of choice on demolition sites.

[edit] Tools of the Trade

A number of tools and equipment are used in demolition. Small tools designed for the removal of material in the interior of a building or in structural renovation include sledgehammers, pick hammers, wrecking bars, shovels, and steel cutting torches. Interior demolition such as breaking up concrete flooring can also be accomplished with a skid steer loader configured with a hydraulic hammer attachment.

In terms of conventional demolition, the use of equipment ranging from cranes to skid steer loaders and hydraulic excavators dominates the industry today. Hydraulic excavators designed with long extended boom arms—called long reach excavators—are the demolition machine of choice. They can perform many functions, eliminating the need for multiple machines. Equipped with special rigging to move structural components from job site to job site, they function similarly to a crane but with less noise and vibration. They also come with a variety of attachments such as grapples, concrete pulverizers, scrap shears, and hydraulic magnets. Crushers and screen plants are also used in the recycling of different types of debris.

[edit] Equipment List


[edit] Equipment Attachment and Tool List

[edit] References

  1. Industry's evolution spurs association change. Daily Commercial News and Construction Record. 27-03-2009.
  2. Wrecking and demolition work. Answers.com. 27-03-2009.
  3. Wrecking and demolition work. Answers.com. 27-03-2009.
  4. Wrecking and demolition work. Answers.com. 27-03-2009.
  5. Wrecking and demolition work. Answers.com. 27-03-2009.
  6. How Building Implosions Work. How Stuff Works. 27-03-2009.
  7. Under control: excavators replace wrecking balls as controlled demolition becomes more popular on job sites. BNET. 27-03-2009.
  8. The Environment Spurs Change in Demolition. Allbusiness. 27-03-2009.
  9. Ball and Crane for Demolishing Masonry and Concrete Structures. Concrete Network. 27-03-2009.
  10. Demolition Industry Promotes Recycling.27-03-2009.
  11. Demolition Industry Promotes Recycling.27-03-2009.
  12. Industry Profile:Demolition. Portable Plants and Equipment. 27-03-2009.
  13. Advancements in the Demolition Industry. Articlesbase Directory. 27-03-2009.
  14. The Environment Spurs Change in Demolition. AllBusiness. 27-03-2009.
  15. Deconstruction - The Green Side of Demolition. The Working Man. 27-03-2009.
  16. The Environment Spurs Change in Demolition. AllBusiness. 27-03-2009.