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I Chimney vs building demolition physics

  1. Feb 6, 2017 #1
    I have seen a bunch of chimney and building demolition videos on youtube and what I typically see is that whenever they bring down a building with explosives especially in urban areas they make the structural elements cut in such a way as for the building to fall in its own footprint and the building usually fall into their own footprint happily , even on 9/11 the structure of the Wtc fell through itself due to the large mass of the upper floors and the specific design of the building.
    (i'm not saying twin towers were blown up as they were not , just making this clear due to the large tin foil hat community around these days)



    Whenever I see a reinforced concrete chimney being blown up I almost always see the chimney fall to the side in other words they blow it up at one side of the base and let the gravity top the chimney over and then it falls but almost never have I seen a reinforced concrete chimney being brought down vertically in its own footprint. I wonder why is that ?



    I would like to make my own attempt at explanation which was the idea that came to my mind.

    A building is usually rather heavy with respect to the structural strength of its load bearing core or elements while a chimney being almost always circular in shape and built from reinforced concrete is very light with respect to its own structural strength and because of this if it were to lose a part of its base and fall vertically it would slow down after a while and come to a stop because its strength is higher than its own gravity powered acceleration force to crush itself so it doesn’t do that.

    Here is a link to a videos where I think it can be observed. Am I correct or no ?









    P.S. I wonder whether the wtc twin towers would still be standing in they had some tubular and tich reinforced concrete cores as most of the newer higher skyscrapers have instead of a lightweight structural steel core and outer walls.
     
    Last edited: Feb 6, 2017
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  3. Feb 6, 2017 #2

    russ_watters

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    One is steel, the other masonary?

    A concrete chimney is a continuous masonary structure that doesn't react much to little explosions. A steel frame building has a certain number of specific, much more concentrated supports (and some in tension!) that can be cut with a small amount of explosives.

    If you look around (and I can later), there is a somewhat funny video of a ~10 story masonary apartment building that someone tried to demolish using explosive implosion and it fails: the bottom floor got blown-out and the entire structure just droped 10 feet and remained intact.
     
  4. Feb 7, 2017 #3

    A.T.

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    Well, just repeat it 10 times.
     
  5. Feb 7, 2017 #4

    sophiecentaur

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    It could be to do with the height / base ratio. A chimney would probably be too unstable to rely on it collapsing downwards so they choose a safe direction for it to drop. A building can be relied on to fall in on itself and that is the cheapest and most compact solution, when possible.
     
  6. Feb 8, 2017 #5
    I can't find at the moment but sophie I have seen few cases were a chimney is blown at the base it falls a few meters and then it stops and only after some 5 and more seconds slowly tops over , but I believe if it weren't to topple over due to its height and thin structure it would be left standing much like some of the failed building demolitions like one in the link I posted , so that makes me think that they tend to make chimneys of reinforced concrete type fall sideways if they can not just because its easier but also because it costs less explosives because I assume their strength vs weight ratio is such that they cannot crush themselves in a vertical collapse like a building can.




    Well the way I have seen atleast in my part of the world but also in videos , almost all taller chimneys are built with a reinforced concrete base up to a certain point , then the rest is either a metal pipe mounted on top continuing or a continuing built using masonry. In both the RT video of the large chimney in Poland or the CDI video you can see that they blow the very base at one side the chimney falls for a few meters and then stops and slowly topples over since they have usually bored out one side of the chimney base.

    Yet with most highrises and skyscrapers if not all of them they usually take out nothing or only a few things , they simply blow out the support columns and the building brought down by gravitational acceleration destroys itself nicely almost like a lego , I wonder why does a building can dismantle itself almost perfectly while a chimney of this type cannot even crush a few meters of itself under its own weight, as I said my own answer was that it has to do almost entirely with the weight versus strength ratio of a structure is that correct ? I assume the reinforced concrete circular tube structure of a chimney is very rigid yet lightweight enough.

    I assume a fully masonry chimney which are usually lower would be able to fall vertically due to the larger mass of bricks vs not as rigid as reinforced concrete which if I remember correctly was the reason why you could only building with bricks no higher than approximately 15 floors or some 60/80 meters.



    It is especially interesting with metal only buildings like most of the 60,70s skyscrapers and highrises like the twin towers , wtc 7, and now the Iran's Plasco building, apart from heat it seems that a metal structure is more vulnerable to a complete collapse than a reinforced concrete one. The twin towers basically dismantled themselves almost perfectly to individual girders and columns.



    I'll add some videos of the recent plasco building collapse , as well as some brick chimneys vs reinforced concrete ones.

    I mostly ask this because I have noticed the trend in the last 10/20 years to build very highrises and especially supertall skyscrapers using the reinforced concrete core method with lightweight outer columns and often steel decks all attached to a monolith reinforced concrete core stretching all the way from base to the top , such a core resembles a chimney in many aspects so I was thinking how it would behave under a catastrophic event like a bomb explosion or other terror attack.





    this chimney being brick seems to crush itself when falling , only the top fell aside due to initial falling asymmetry.



    at the end of this video one can see the base is still mostly intact just sideways.





    this one is interesting , not sure if it fell exactly the way they wanted as it hit trees but the base again is mostly intact just sideways , seems like the closer to the top the higher the speed of acceleration so more damage happens there. But that's falling sideways.



    I assume this is the video you are referring to Russ.

    Actually quite remarkable because to the best of my knowledge the structurally weakest of all high rise building are the 20th century popular high rise flats made from large reinforced panels with weld joints.

    Given that the panels are strong but the weld joints were usually even below design quality.

    Next ones are probably steel structures due to their low heat resistance and probably among the strongest are reinforced concrete cored structures were the whole building is a continuous solid concrete slab. I was surprised watching a video where they told that while building the Moscow FED tower they interconnected the concrete column rebar not with welding but with making the rebar ends like that of a screw and screwing the rebar at each floor together to the one in the next.

    http://www.hengshuitaotong.com/uploadfile/201510/20151028155126587.jpg

    this I believe was used for connecting the rebar rods.







    Here is the recent Plasco building collapse in Iran.



    I apologize for such long writing there are just many aspects to my question.

    Maybe someone is a structural engineer would love to hear some comments, seems like a large fire can cause a progressive and complete structural failure in certain types of steel framework buildings , this latest Iran's case is very similar to the wtc7 collapse.
     
  7. Feb 8, 2017 #6

    A.T.

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    Lego is actually not like that at all.
     
  8. Feb 8, 2017 #7

    sophiecentaur

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    As happened on the Twin Towers.
    = (in)stability
     
  9. Feb 8, 2017 #8

    russ_watters

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    It isn't -- the one I was referring to I think was just one floor of a smaller building. That one is even more amazing as it looks like about half the building collapsed and then it stopped. I'll look for it...

    Not seeing what I remembered, but here's one that takes out the 5th floor of a 15 floor building and it fails to collapse (at 6min):

     
  10. Feb 8, 2017 #9

    CWatters

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    Reinforced concrete and brick behave quite differently. You can crumble/shatter a brick wall into brick sized lumps much more easily than reinforced concrete. I believe this is why a brick building is more likely to collapse into its own footprint that a concrete chimney.
     
  11. Feb 8, 2017 #10

    Baluncore

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    It all comes down to maximising control over the footprint. A low building can be made to fall into itself. A tall structure must start to drop by having part of the lower structure removed, then rely on momentum to crush the remaining floors from the bottom up.

    Falling a chimney is like felling a tree in a particular direction. There is a horizontal cut, part way through the tree that defines the axis of a hinge line. A wedge shaped scarf is cut on the other side that allows the tree to fall that way. The scarf wedge angle sets the angle of fall before the remaining hinge material will be torn by the closing of the scarf.

    Steel and reinforced concrete do not crumble like bricks. At some point one side will crush before the other and the tower will then fall sideways.

    If you blasted away a sufficient part at the bottom of a brick chimney, the chimney would start to drop and then crush itself by momentum. But it would take only a minor variation in the position of the maximum crush rate at the base of a chimney to take control and then fall the remaining upper part of the chimney sideways. The engineer's insurance company requires complete control of the destruction, so it is safest to leave any chimneys till last, then fall them onto lines that have been prepared.
     
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