Science Fair Project Encyclopedia
For most buildings, such as houses, that are only two or three storeys high, demolition is a rather simple process: the building is simply pulled down by excavators or bulldozers (see photo below, left). Larger buildings need the use of a wrecking ball that is swung into the sides of the buildings. Wrecking balls are especially effective against masonry. This process is quite slow, but it puts the operator of the demolition equipment at a safe distance from any falling debris. In recent years this technique has fallen out of favour because of its slowness.
Large buildings, tall chimneys, and increasingly some smaller structures are destroyed by implosion by explosives. Imploding a building is very fast—the collapse itself only takes seconds—and an expert can ensure that a building falls into its own footprint, so as not damage neighbouring structures. This is essential as most tall structures are in dense urban areas. Any error can be disastrous, however, and some demolitions have failed, severely damaging neighbouring structures. The greatest danger is from flying debris which, when improperly prepared for, can kill onlookers. Even more dangerous is when a building fails to collapse completely. This often leaves the structure tilting at a dangerous angle and filled with undetonated explosives, making it impossible for workers to approach. Many of the dangers of a building's demolition can be seen in the destruction of the World Trade Center by terrorists in 2001; its collapse destroyed several neighbouring buildings, and damaged others beyond repair. Flying debris injured and killed those nearby and a huge swath of Manhattan was covered in dust and debris.
A building takes several weeks to be prepared for implosion. All items of value, such as copper wiring, are stripped from a building. Some materials such as glass that can form deadly projectiles, insulation that can cover a wide area, and other materials also must be removed. Selected columns are drilled and nitroglycerin and TNT are placed in the holes. Smaller columns and walls are wrapped in explosive cables. The goal is to use as few explosives as possible, and only a few floors are rigged with explosives. The areas with explosive are covered in thick plastic and fencing to absorb flying debris. Far longer than the demolition itself is the cleaning up of the site where the debris most be loaded into trucks and carted away.
The destruction of large buildings has become increasingly common as the massive housing projects of the 1960s and 1970s are being levelled around the world. The tallest deliberately demolished building was the forty-seven storey Singer Building of New York City, which was built in 1908 and torn down sixty years later to be replaced by One Liberty Plaza .
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