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I came across the following Youtube video which shows the controlled demolition of a chimney.

https://www.youtube.com/watch?v=Sb62_QesDTw

The construction of the chimney appears to be reinforced concrete, with the reinforcing bars in both the axial and circumferential directions.

From the video, around the 0:24s mark, you can see some explosions at the base of the chimney, to the right of the chimney, which then collapses to the right. Can someone please explain how the location of the charges determines the direction of the fall?

How does the type/construction of the building, the explosives used (detcord, cutting charges etc.), charge placement (placed in drilled holes, superficial to structure, etc.), prevailing wind (yikes!), pre-existing damage to the building, affect the outcome?

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  • $\begingroup$ Check out other videos to show how errors affect the outcome. $\endgroup$ – Solar Mike Apr 8 at 10:31
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This is similar to felling a tree:

enter image description here

The idea is that the lack of support at the right corner (in the image) will create a rotational moment that will direct the tree toward one side.

The key point in the chimney case is the timing between the explosions, because although using explosives for demolishing suggests that the building or structure will be "blown up", this is not actually the case. Controlled demolition uses small explosions of charges that are strategically placed within a structure. The detonation is progressive thus encouraging the collapse by weakening or removing critical supports. Therefore the explosives on the lower floors initiate a controlled collapse and the building fails under its own weight, succumbing to gravity (much like a domino).

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  • $\begingroup$ Unless the wind blows at the right (wrong) moment... and yes had it happen. :) $\endgroup$ – Solar Mike Apr 8 at 14:21
  • $\begingroup$ Thanks. I was thinking about it, and the concrete is probably what is destroyed (strength -> 0) by the explosion, not the reinforcing bars. So the right side can not support any compressive load anymore, and therefore the chimney collapses to the right. If linear cutting charges were used, that can sever the tensile reinforcement, will charges on the right in fact set off a collapse to the left instead? Or the simultaneous loss in compressive strength in the right will still cause a collapse to the right? $\endgroup$ – L.Z. Apr 8 at 18:04
  • $\begingroup$ Although I am not an expert by any means, severing the steel reinforcement in the concrete is not crucial. The reason is that they don't perform well in buckling. Of course that can be different between cases. $\endgroup$ – NMech Apr 8 at 18:35

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