Early Sunday mornings in Vancouver, British Columbia, are normally quiet and peaceful. However, the morning of November 6, 1994 was going to be different.
The Pacific Palisades Hotel was a twenty-year old, reinforced concrete building. This Hotel was about to come down a heck of a lot quicker than it had been built. The Pacific Palisades Hotel took one year to design, one year and millions of dollars to build and would take 10 seconds to bring down.
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Planning
Bringing down a building basically involves removing vertical supports--the columns--in a controlled, sequential way that then uses gravity to collapse the structure. You can control the direction of fall by taking out supports that force the building to fall where you want it to go…kind of like taking one leg away from a three-legged stool. The weight of the structure and gravity do the rest.
While the theory is simple, planning a successful drop is quite complicated. The shape and composition of the columns has to be studied and tested. Core samples are taken, the original construction drawings studied, and at least one test "shot" will be fired to verify the calculations.
Although less than 200 lbs. of dynamite and detonating ("det") cord will be required for the Pacific Palisades building, those charges have to be placed with great precision to be effective. The basic idea is to weaken the columns on one side of the building's lower floors, starting at the bottom and working upward over a period of about ten seconds. Each charge will cut through the concrete of a column and the weight of the structure above will start the collapse.
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Part of the art of implosion demolition involves slowing the event down in many small, calculated blasts instead of one huge explosion. That is accomplished with time-delay blasting caps that will initiate the dynamite and det cord over a period of several seconds.
The explosives will shatter the concrete around the reinforcing rods--and since the concrete provides nearly all the strength of the column, that part of the building quickly begins to fall. If enough columns are shattered the building will collapse. This much is easy. The art of demolition, is knowing which part of the building to take out at each moment, over a period of ten seconds or so; errors in this kind of calculation can be rather embarrassing, especially if the structure comes down where it isn't supposed to.
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Explosives
Explosives come in all sorts of forms--gels, granules, powders, cord, liquids, plastics (in blocks and sheets), and old reliable, stick dynamite. All have properties designed for specific conditions. Huge quantities are used every year, often in urban areas and often without anybody even noticing the detonations.
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Dynamite is a mixture of nitroglycerin, a liquid and a binder. It was the first practical high explosive and revolutionized mining and construction by making the blasting process safer and more efficient. As everybody knows, dynamite is sold in sticks, typically 1.25 inches by 8 inches. As few people know. Dynamite is rather insensitive and difficult to initiate.
Dynamite won't detonate unless "initiated" with a priming charge, normally from a blasting cap. While some blasting is still done with time fuse and suitable nonelectric caps, virtually all construction and building demolition blasting today uses only electrical caps, fired by wire from a remote location. That means that you can wire 100 charges into a big firing circuit and fire them all at the same instant with a single push of a button. Timed detonating caps allow you to press that same single button and stagger your single explosions by ten seconds or more.
Rather than firing all the charges at once, they design the shot to evolve over a period of ten or fifteen seconds. That's possible because blasting caps are now available with built-in and extremely accurate tiny fuses that permit delays of ten or more seconds. For a tall building like Pacific Palisades, the charges on the lowest floors and in the basement fire first, chopping the base from under part of the building and leaving part intact to act as a kind of hinge. The weight of the structure will begin to pull the building down in a controlled direction. The remaining charges fire at preset intervals of about one second, fracturing the structure's internal supports, weakening it from the inside out. Then, as it falls, the once strong structure's own weight tears it apart leaving nothing but a pile of pulverized concrete and reinforcing rod.
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Test ShotEarly in the design sequence, a column is found, normally in the basement, for a test shot. Based on a core sample and available information about the nature of the material in the column, locations will be marked for placement of explosives. The drill crew bore the holes to specifications, usually dead center and almost all the way through. On a job like Pacific Palisades, four holes might be drilled into the test column, each deep enough for four 8 inch sticks of dynamite.
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Then, after receiving authority from the city for the shot. The holes are loaded and fired. Deep inside the building, the blast's noise and "fly rock" are fully enclosed; people nearby probably don't even hear the detonation. Then the crew reenters the structure to inspect the damage. The column should be completely shattered, although the rebar will still be intact; if the column isn't demolished, more holes and more explosives are required.
So, how much dynamite does it take to drop a twenty-two-story building? Not much, if it is placed correctly. The design for the Pacific Palisades building uses a bit less than 200 lbs. Plus a small amount of 'det' cord. Actually, gauging the amount of explosive to be used is key to a successful drop. Dynamite is cheap, about $1 per stick, so the cost of the material isn't a factor. The trick is using enough to be sure that the building comes down exactly where it is supposed to without excess flying debris or breaking windows with the noise of the shot.
There is a real art to the business and some do it better than others. If the dust clears from a shot and the building is still standing, it's more than embarrassing--the standing building is now a disaster waiting to happen, weakened by the blast and threatening to fall at any time.
Once the test shot confirms or refines the understanding of the structure, the exact location of each bore hole is marked. Then the drillers come along, bore the holes, then mark each with a length of red-painted rebar. Other people will wrap exposed columns with a special fabric used in construction, then enclose all with chain link fencing, leaving the red rebar exposed to mark the bore holes.
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| Priming and LoadingFor most people, whose notion of dynamite is that it is extremely sensitive and dangerous, the priming process is full of surprises. It begins, for example, by taking a stick of dynamite and poking a hole in it with a sharp tool. Into the hole goes the blasting cap, a bright aluminum tube with a long pair of wires attached. The wires are extended, then a quick pair of half-hitches are tied around the stick, then inserted in the bore hole. Finally, using a wooden tamping rod, the stick is pushed into position at the bottom of the hole. Two other sticks follow the first, each slit with a pocket knife before insertion. The exposed ends of the wires are twisted together as a safety measure until the firing circuit is connected later.
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But first, to finish off the bore hole, an applicator is inserted in the hole releasing a bit of foam. The material quickly expands, then hardens, forming a seal that will concentrate the force of the explosion. Without it, the charge would squirt some of its energy out the hole, drastically reducing the effect of the explosives.
Once all the charges are loaded, the wires are spliced together into a big electrical circuit. A continuity test is applied and then everything is done, except the waiting.
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A button is pressed that connects the firing circuit, sending voltage to all the electrical blasting caps in all the holes throughout the structure.
Down on the ground level, in the old lobby, the charges fire instantaneously with a sharp, hard, startling, BANG that echoes from the buildings surrounding the site as small puffs of dust squirt from under the fabric and chain link fencing.
Two seconds later, another BANG, this one much milder, as the columns on the second and third stories fire. Another set of small dust clouds reveal the location of the blasts, but nothing else happens. After another brief interval, another set of charges fire. No chunks of concrete fly through space, no dramatic eruptions of material but the front of the structure begins to slide toward the ground.
BANG, another set of charges fire. Now the elevator house on top of the building starts to lean. The basic structure remains essentially vertical, but the front of the building is shattered. It tears itself apart, progressively, just as intended.
The rear of the structure, without any explosives and reinforced by massive cables, provides a hinge for the collapse, anchoring the back of the building and forcing the decaying building to fall into what had once been a handsome and elegant entry.
The roof disappears into a massive cloud of dust, right where it was supposed to go. The long, rippling roar of the dying building echoes for a few seconds against the surviving buildings of Vancouver's skyline.
That roar is replaced by another, this time from the audience who hoot and cheer and yell. It was, indeed, a fine performance.
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Zero-plus-two seconds:
About two seconds after the button was pushed, the big charges at ground level have all fired, notching the structure and beginning the failure sequence. It will certainly fall now, but nobody (except a few hundred blood-thirsty spectators, perhaps) wants it to fall over intact. A second set of charges has just detonated along the right side of the structure, along with those in some interior columns. While the building is still apparently intact, the lower floors already show evidence of structural failure.
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Zero-plus-three seconds:
Only one second later, the whole right side of the building collapsed and the elevator support structure began to lean crazily, while the back wall remained intact.
The falling elements of the structure will pull the back wall away from the adjacent building, only twenty feet from the back wall.
Charges continue to fire inside the building even through the firing circuit has probably been cut in dozens of places.
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Zero-plus-four seconds:
Shattered, but still held somewhat together by rebar and inertia, the Pacific Palisades starts toward the ground.
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Zero-plus-five seconds:
With the front of the structure tugging at the back wall, the destruction of the building is about half complete.
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