Mixing explosives with gravity – the controlled science behind implosions

In just 15 seconds on Saturday, Queen Lane Apartments vanished from the sky. 

A nearly 60-year stay in Philadelphia over in less time than it takes to wash and dry your hands.

Only an implosion – an uncommon, but highly precise form of demolition – could fell a 16-story high-rise so fast, or provide a more visually stunning demise.

Nearby, neighbors cheered as a khaki-colored cloud of dust briefly billowed over a Germantown intersection before revealing a four-story high pile of bricks and concrete.

The short spectacle was the end result of a carefully engineered process built around two main ingredients: explosives and gravity. 

The interplay of that combination enables blasters to rig a building to collapse into itself and almost entirely within its own footprint, to make it look as if a building is melting to the ground.

“We do not blow buildings up,” said Jim Santoro, project manager for Controlled Demolition Inc., the Maryland-based company hired to raze Queen Lane Apartments.

“What we do is use a very, very small quantity of explosives to disrupt a column’s ability to support the building. If we remove those in a sequence, and in a form marching back through the building, we can actually control the timing and the rate of failure and the direction of the fall.”

Gravity does “99.9 percent” of the work.

The prep work 

Before Santoro’s crew gets to work, a building must be completely gutted. Its bones are otherwise left intact. Nothing is pre-weakened before the implosion.

Then, a day or two before the main event, a series of holes are drilled into carefully selected columns throughout the building.

Sticks of dynamite – think those red links you’ve seen in all the old Roadrunner cartoons – are then concealed inside the openings to sever specific columns.

At Queen Lane, less than a pound of explosives were placed at each of the roughly 426 locations, or charges, throughout the tower.

That total was dictated by, of course, science.

“You use a certain quantity of explosives in order to disrupt a certain quantity of concrete. We can go through and estimate how much we’re going to use based on our experience and the rebar configuration in the building and the hardness of the concrete, we’ll adjust that load to make it either a bit heavier or a bit lighter,” said Santoro.

Most of the dynamite is loaded on the lower levels, especially the first floor. That’s where the greatest amount of drop happens during an implosion.

Once the foundation goes, gravity takes over and the rest follows.

The lower levels are also detonated first so that there’s a debris pile to cushion the tower’s top floors when they fall toward the ground.

Or put more simply, the bottom goes down first. The top rides down over it.

During the implosion, dynamite is detonated at split-second intervals thanks to a series of pre-programmed delays.

Muffled booms –about a third to a half a second apart – sound as columns break apart and the building races to the ground.

“The timing of the detonation is to allow for the building to start to fail and drop and allow the beam connection between columns to start tugging on the column behind it so that when it goes off it keeps pulling the building in a continuous motion in the direction you want it to,” said Santoro.

Minimizing impact 

But there’s another part of the equation that Santoro has to account for: the impact of all this debris hitting the ground.

“It’s coming down as hundreds of thousands or millions of small pieces of debris, each striking the ground at a different moment,” he said.

That means building implosions don’t cause much vibration. In most cases, not even enough to knock a secured picture off a wall.

The process is so precise, that only neighbors within a roughly one-block radius of Queen Lane had to evacuate their homes the day of the implosion.

The evolution of implosions 

It’s all the product of another pair of ingredients: time and technology. The science behind building implosions really started developing in earnest in the decades leading up to World War II, and really took off after it.

Blasters started refining the technique while taking down bombed out buildings in towns across Germany and Europe – buildings that had to be removed without compromising other structures.

Brent Blanchard, director of operations for Protec Documentation Services, Inc, a South Jersey-based vibration consulting company, said back then, there were two reasons for using explosives to knock down buildings.

“One was you’re blowing up a structure that you can’t reach otherwise, that there’s really no other way to do it. And the other is to bring down a large number of structures in a timely manner,” said Blanchard, perhaps one of the only people who has a working knowledge of structural demolition history.

“It’s just more expeditious to perform building implosions than it is to run around and piecemeal demolish hundreds of structures after WW II,” he added. 

Those building implosions in the 1940s and 1950s were effective, but a bit clumsier than what’s seen today.

“You didn’t have explosives placed in the same precise places that you do now. You also had different construction materials back then. You’re blasting columns that were made of brick. There wasn’t reinforced concrete back then. There were no steel beams in many cases back then. So you were more pulverizing and obliterating columns than you were trying to defeat a few columns to get a controlled collapse,” said Blanchard.

Over the next few decades, though, building implosions became more refined as commercial companies got into the blasting biz.

The military made that private industry possible by showing city governments that building implosions could be done safely in tight quarters.

That comfort level has grown still thanks to advances in technology.

“You started having a lot more control over what you’re detonating and over how the structure reacts. Now, there’s non-electric technologies. Now there’s wireless technologies. So now you don’t have tohave wires running through the building. It’s all remote control,” said Blanchard.

Other demolition options 

Still, these days, an implosion is not the only way to take down a tall building.

Using what’s known as a High Reach for example, a type of crane equipped with a pulverizing arm, a building can be safely deconstructed floor by floor.

That, said Blanchard, means there are fewer implosions being done today than even a decade ago.

But don’t expect to see the building implosion industry become rubble anytime soon.

It’ll always be the fastest – and most fun – gun in the West.

The demolition video featured at the top of this page was provided by Controlled Demolition, Inc.