The Singer Building in NYC, demolished in 1968 Source: NYPL

It’s hard to imagine New York without its iconic towers such as the Empire State Building and the Chrysler Building piercing the skyline…but what about when they need to be taken down or replaced? How would one go about it? A recent state commissioned report by Terrapin Bright Green advocated for the demolition of mid-century skyscrapers that cannot be retrofitted to be more environmentally efficient. And the question of how to demolition a skyscraper will become an even hotter topic if the Midtown rezoning is approved. Today we look at techniques, both new and old, for skyscraper demolition.

The tallest demolition project to date took place in New York City with the Singer Building, built in 1908. At 47 stories, the tower was demolished in 1968 because its top tower was rendered economically unviable. Modern businesses were demanding more office space but the tower portion of the building contained only 4,200 square feet per floor compared with the 37,000 square feet per floor of the U.S. Steel Building (now One Liberty Plaza) which replaced it.

There are a number of techniques to demolish skyscrapers:

1. Engineers demolish a building by knocking out the bottom floor and lowering it on computer-controlled hydraulic jacks: Japanese construction firm, Kajima Corporation, adopted this elaborate technique when taking down two office towers in Tokyo in 2008:

2. Dismantling tall buildings from the top down: This is often carried out by a moving scaffold which causes the structure to slowly shrink. Another Japanese corporation, Taisei Corporation, developed this technique. It is using it to tear down the Akasaka Prince Hotel. Work began in early 2011 with two floors being taken down every ten days. The cranes use regenerative braking to generate electricity for the demolition.

3. Implosion: Using explosives to remove a building’s support structure. This causes the building to collapse on itself quickly while causing little damage to the surrounding area. But implosions requires a great deal of preparations–six months to survey the structure and prepare it for the implosion, up to two months to collect debris in the aftermath

One of the most famous implosions was the demolition of the Pruitt-Igoe housing project in St. Louis, Missouri:

Implosions last about eight seconds as was the case with J.L. Hudson department store in Detroit, Michigan:

4. Wrecking Ball: This technique was commonly used during the 1950s and 1960s. With the invention of more modern machinery, the wrecking-ball is being less commonly used, especially due to its likelihood of causing collateral damage.

Demolition of a retirement home in Milwaukee, Wisconsin by wrecking-ball

5. High-reach mechanical excavators: These are designed to reach the higher stories of buildings and then to pull down the structure in a controlled manner. After the 2010 and 2011 earthquakes of New Zealand, the UK imported such a device to Christchurch. Nicknamed “Twinkle Toes,” it was used to pull down damaged high-rises in the city.

In New York, regulations and the make-up of the city do not allow for all of these demolition techniques. For instance, implosions and wrecking-balls are forbidden in the city due to safety and environmental reasons. Although implosions were allowed Governors Island in the demolition of a Coast Guard apartment building, in such a highly densely-populated area as New York City, this simply would not be feasible.

The process of tearing down skyscrapers from top to bottom seems to be the most common in New York. This technique is environmentally friendly as it efficiently separates recyclable materials. There is also a visual advantage here. Due to slow and steady demolition, the vanishing building would look normal for the longest possible time. People wouldn’t witness the demolition in one viewing.

The technique of removing a building’s bottom floor and bringing it down may also work in New York City. There is no need for heavy equipment or workers as the technique is almost exclusively computer-controlled. Nonetheless, Herb Duane, a demolition consultant, told The New York Times that even Kajima’s technique, while cutting-edge, could be problematic in New York as the weight of buildings is greater than those in Tokyo.

As the world’s skyscraper stock gets taller and older, skyscraper demolition will become more commonplace unless new techniques in retrofit are adopted. To conclude, here’s a graphic from the Council on Tall Buildings and Urban Habitat showing the average age of the 10 tallest buildings in select countries:

Read more from our Cities 101 series about how stuff works in the city.