First, the building is stripped as much as possible, removing copper wire and aluminium for recycling, as well as all the windows and insulation to prevent health-and-safety hazards from flying debris. Then explosives are used to shatter the load-bearing structures on some of the floors. The explosives are synchronised using detonating cord – a thin plastic pipe filled with an explosive called pentaerythritol tetranitrate (PETN), which burns at 6.4 kilometres (four miles) per second. The aim is for the upper storeys to fall vertically down, so that the whole building collapses onto its own footprint and surrounding buildings aren’t damaged.
In France, the technique of vérinage is often used instead of explosives. This employs hydraulic rams to push the load-bearing members of one of the middle floors sideways. At a certain point they all give way at the same time and the top half of the building falls straight down onto the bottom half. The end result is very similar to an explosive demolition.
Meanwhile the Japanese Kajima Construction Corporation has developed a system of propping up an entire tower block with hydraulic jacks and demolishing the ground floor using ordinary diggers and bulldozers. The jacks then lower the building down a storey and the process repeats.
Answered by Luis Villazon