What Are Controlled Demolitions?
Controlled demolitions are demolitions of structures engineered to achieve certain objectives. Demands of safety and economy are paramount in commercial demolitions, whose objectives include:
- Avoiding damage to surrounding structures
- Minimizing production of dust and other disturbances
- Facilitating debris removal
Usually those demands are best met by bringing a building down into its own footprint. In some cases controlled demolitions are designed to lay buildings down on their sides. The control in controlled demolition lies in the ability to control the pattern and timing of the destruction to make the building fall as desired. The most common type of controlled demolition of large urban structures is called implosion, but very different types of controlled demolitions are possible.
A classic controlled demolition implosion not only brings a building down into its footprint, it causes the periphery of the building to fall inward, towards the building's central vertical axis. Although controlled demolition implosions are not implosions in the literal sense of the word, since the do not use pressure differences to push a building's exterior inward, they achieve a similar result by destroying structural components in a particular order. Given the structural designs typical of most large buildings, breaking that structure from inside to outside and from bottom to top will tend to implode a building.
Typical building implosions employ numerous small charges, totaling a modest quantity of explosives. Since small cutter charges are quite effective at slicing through steel members when properly installed facing them, it doesn't take a large amount of explosvies to implode a skyscraper. It does, however, require a large number of charges, since, not only is it necessary to sever all of a building's support columns, it is necessary to do so with precise timing. The timing of the charges is critical to a successful implosion, since an asymmetries in the charge detonation will tend to make a building lean to the side being destroyed first.
Controlled demolition experts boast about the technical complexity and precision required to pull off a successful implosion.
An implosion severs the columns at least at the ground level, and usually on multiple levels higher up in the building, particularly if the building is tall.
Because columns are often rigged on multiple levels, the number of charges in a typical demolition far outnumbers the number of columns.
The following table summarizes statistics about the deployment of explosives in a number of controlled demolitions documented by PhillyBlast.com. Although PhillyBlast.com does not document the number of columns in the buildings, it is clear that the number of charges vastly outnumbers them.
|Philadelphia Naval Hospital||15 stories||6/9/2001||600||200 lbs.|
|Flag House Courts||11 stories||2/10/2001||1050||200 lbs.|
|Broadway Homes||22 stories||8/19/2000||900||200 lbs.|
|Hollander Ridge Tower||20 stories||7/8/200||1,166||56||403 lbs.|
|Hayes Homes||12 stories||12/11/1999||350||120 lbs.|
|Reading Grain Elevator||235 feet||2/28/1999||300||200 lbs.|
|PennDOT Building||12 stories||8/1/1998||1,000||240||300 lbs.|
|Jack Frost Sugar Refinery||8 stories||11/2/1997||4,000||506||700 lbs.|
It is doubtful that the explosive detonations in a typical demolitions, being small, numerous, and staggered over time, would generate a seismic signals with much magnitude, but the secondary effects can. Ground coupled explosives can trigger earthquakes in regions with high crustal strain, such as Los Vegas, NV.
A number of unsuccessful controlled demolitions attest to the fact that large buildings do not easily collapse, and that careful planning and execution is required to bring them down with explosives, despite the ability of explosives to cut through columns. In some cases buildings that were left standing by unsuccessful controlled demolitions had to be destroyed with wrecking balls.
On June 29, 1997, the 10-floor Jack Frost Sugary Refinery withstood multiple attempts to destroy it with explosives. The factory building was destroyed six months later in a successful controlled demolition. 7
|The Zip Feeds Mill fell about two stories and then came to a halt, in response to a botched controlled demolition that did not adequately account and plan for the building's strength.|
Another example is the failure of the Zip Feeds Mill Tower to collapse, even after it fell several stories.
2. Interview with Stacey Loizeaux, pbs.org,
3. Implosion to turn Garfield Heights highrise to rubble, Pittsburgh Tribute-Review, [cached]
4. The Controlled Progressive Collapses of The Twin Towers, SueTheTerrorists.net, [cached]
5. Phillyblast Was Here, phillyblast.com, [cached]
6. If The Big One Hits Here, Will We Be Ready?, reviewjournal.com, 4/11/99 [cached]
7. Phillyblast Was Here ...,