Now that we have a general understanding of gravity, we can now see how experts use it to sucessfully drop a structure. As mentioned before, there are four factors which blasters have to take into account when planning out a job: direction, intensity, placement, and time.
The direction of the blast is one of the most important things which has to be taken into account when raizing a structure, especailly if the structure is located in a place where there is a higher chance for property damage and personal injury. Most of the time, this concerns tall buildings which are located in major cities. After reviewing the design of the structure and its surroundings, the experts first decide
how the building will fall. Contrary to popular belief, most structures are not actually
imploded (collapsed within themselves) but rather designed to fall into an open area. The reason for this is that an implosion is much more complicated and dangerous and only a handful of demolitions experts worldwide are qualified to do this. Also, to ensure that the direction of the blast is correct, sandbags and mesh are wrapped around the support of the structure so that the blast applies more force to the support while on the other hand, containing some of the flying debris. Another way in which experts can control the direction of a falling structure is by placing heavy-duty steel cables which further pull the building in the intended direction.
Intensity and direction are very closely related because often times the intensity of the blast determines which direction the structure falls in. While many times, the blasters would want the entire support blown away, sometimes however, they may desire only part of a support to be blown away. The best way to picture this is when a tree is cut down. When someone cuts down a tree, they usually want it to fall in a specific direction. In order to do this, a wedge-shaped cut is usually made on the side of the tree in the direction in which it is supposed to fall and since the weight to the tree is no longer being supported on that side, it will fall in that direction. The same idea applies to a support. If a wall of a building has to fall in a certain direction, the expert will often times use just enough explosives to blow off part of the support, causing the weight of whatever is being supported to collapse in that direction.
But how does an expert know how much of an explosive to use? While I am not going to talk about the physics of the explosives just yet, I will give you an idea of how the experts get an idea of how much explosives to use. After weeks of determining the composition of the support, they use several calculations to make a guess of what the appropriate amount would be. After an answer is derived, the blasters actually pick a support and rig it with the determined amount of exploives. The explosives are then detonated and they go back to the spot to see what remains of the support. If too much of the support remains, then they go back, revise their calculations, and once again test their new findings on another support.
When structual supports are rigged with explosives, holes are first drilled into the support itself into which the explosives and blasting caps are placed. This is done for a good reason -- when an exlposvie goes off, the shock waves go off in all directions, if the explosive is placed only on the outiside, only 180 degrees of the explosive force would be absorbed by the structure at best. However, if the explosives were now placed inside the support, there would now be more surface area for which the shock from the blast to act, therefore decreasing the amount of explosive needed to be used.
Placement can also be used to make corrections if a support wasn't blown away properly. For example, in stead of placing the explosives on just one part of the structural support, they could be placed all around it to increase shock absorption.
Whatever the case may be, experts are usually very good at placing a minimum amount of explosives to destroy a maximum amount of area. Depending on the structure, on average, around 100-200 pounds of explosives are used to bring down a typical job (however, there are exceptions, such as in the case of the Sears Warehouse in Philadelphia, PA where 12,000 lbs. of explosives were used, the most explosives ever used to drop a single building!).
While at this point, it may be hard to understand how important timing is, what can be said now is that most of the time, explosives are rigged to go off in sections in order to reduce shock waves which can be damaging to nearby property. Experts might also want to let a structure fall in phases in order to ensure that each part of it will fall without another part falling into its way. As you proceed, more about timing will be explained.
Now that we know that the objective of the blaster is to place the minimum amount of explosives in the right area to go off at just the right time in order to fall in the right direction. We can now look at the physics of the explosives themselves and how they work.