Free Fall Felling Without Underpinning

Benar textile factory chimney Demolition.

Stack In free fall; Click to view video

Felling a stack

Demolition of a 50-meter chimney in the last part of the former Benar textile factory in 2018.

"Due to arsenic accumulation, Aquatest Praha hired could not use explosives and chose a less common method. After pulling out the large wedge, only one strut was holding the chimney, after which the chimney slid to the ground within a few seconds. Everything was statically calculated and absolutely safe. “

As you can see in the video, the demolition of this 164’ chimney was done by one man with a jackhammer. The man in the video appears to be experienced, he confidently parks his car behind the chimney on one side and the compressor on the other and lets the chimney fall between them. The method he is using is to undermine the chimney until it falls over under its own weight, very similar to the way a lumberjack fells a tree. The difference between a lumber-jack job and this job is that the steeple-jack essentially cuts a notch in the base in the chimney until it falls over. There is no need for a back cut, the chimney simply falls over under its own weight.

There have been a lot of comments in the video that this demolition was done incorrectly, that the chimney should have been shored up with lumber and then burned. But there is no guarantee that this will always work either. Both of these methods have their risks. This is because an industrial chimney may have rotten spots in the chimney column or annular space where water has accumulated with sulfuric acid and rotted out the bricks above the side where the chimney will hinge on. Neither method is completely without risk

The only way a chimney could be guaranteed to be dismantle safely is to take it down piecemeal until an excavator can reach it. If there is nothing for the chimney to fall on in any direction, the top third or more should be removed piece-meal one brick at a time. Or if it is concrete, one slab at a time. And according to standard practice the bricks should go down the inside of the chimney, not down the outside. This should be the industrial standard. In this way the brick or cement that the chimney hinges on can better fully support the weight as it tips over. After the top part of the chimney is removed then the base of the chimney can be removed on one side while being shored up with timbers, that can then be burned. In this way the size of the hinge is essentially increased thus providing greater stability allowing the chimney to lean under a stronger structural base until it can fall in the desired direction.

When loggers cut a tree they can sometimes leave a section of wood in the back that acts as a trigger, which can be cut when the tree is ready to be dropped. In this case there is a trigger as well, and it is in the front of the chimney in the middle of the notch. It is a little difficult to make it out in the photo, but if you look carefully you will see that this is the case. The problem with this trigger however is that 1.) it is not in the back of the chimney but in the front and 2.) it does nothing to insure the chimney will fall in the proper direction. The side of the chimney can still collapse at any time near the end of this demolition process.

By Donald Perry 02/06/2019 Uncategorized

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