Demolition Woman

Because we feel forces most obviously when we're active, it may be surprising to learn that structures such as buildings and bridges withstand tremendous forces simply standing where they are built. A structure's own weight pushes down with nearly unimaginable force, while the ground supporting a structure pushes up simultaneously with equal force.

A structure's parts are constantly being compressed and pulled, but their design and position relative to one another serve to balance the forces on them. The floor of a building, for example, is pulled downward by the force of gravity, while the weight of the floor itself pushes downward on the columns beneath it. The columns push up on the floor in opposition, with a force equal to that of gravity. When these forces are balanced, the structure maintains a state of equilibrium and remains upright.

Understanding the forces at work in a structure is critical for structural engineers, who design structures to withstand myriad forces for decades. Knowing what makes structures stable, however, is also important for the people whose job it is to demolish them.

One of the fastest ways to demolish a building is to use explosives. Contrary to what people might think, demolition crews do not use huge quantities of explosives to blow up buildings. In fact, they use as little dynamite as possible and leave the rest of the work to gravity. This is because setting the explosive charges takes time and costs money. Also, the more explosives a demolition crew uses, the more dangerous the explosion will be.

Demolition crews set explosive charges in strategic locations around a structure. In buildings, they concentrate on the columns that support the most weight, especially those between lower floors. By removing or weakening these crucial supports, the crews dramatically unbalance the forces in the building, causing it to collapse, or "implode", under its own weight.

• What's the difference between an explosion and an implosion?
• Why do demolition teams use as little explosive as possible?
• What are some of the challenges associated with taking something large down, in comparison to building it up?
• What does "load-bearing" mean?
• Build a model apartment house or office building out of blocks, and then try to demonstrate demolition by implosion. What is the smallest force you need to apply? Where would you apply it?