Unlike a tree, a sky scraper just crumbles drectly on top of this. Even the world trade center fell straight down.
Why does it not lean into other buildings and fall sideways like a tree?
Buildings are mostly hollow. Trees can not collapse upon themselves.
Plus, a tree has long fibers running down its length that act as a single piece. A building is formed of millions of discrete pieces that are fairly weakly bonded to one another.
Like, if you have a house of cards, knocking out a wall near the bottom simply removes the support from all the upper layers. The wall isn’t a single part that continues up to the roof. Its scope is limited to the one floor it is on. Tipping it over doesn’t tip over anything else in the structure. But now that the support has been removed, everything is going to cave in.
But, the WTC towers collapsed from the top.
If they were compromised at the bottom, the fall might not have been as vertical.
When you cut down a tree, you cut it from one direction at the base. That weakens the support for the tree on one side, and when the support is sufficiently weakened, it will fall to the side of the cut.
When a tree falls due to high wind, it’s the same thing: the force pushing the tree to fall is coming from one direction, so the tree falls in the direction pushed by the wind.
As beowulff says, if you were to start to destroy a building from one side, then it would fall towards the loss of support. But that would be very dangerous, to have a building fall over, especially a tall building in a built-up area. So when a building is destroyed intentionally, by implosion, the explosives are planted in a way that the building loses support simultaneously on all sides of the building. With uniform loss of support, the building will pancake down.
Similarly, for the WTC, the support was lost at the upper levels essentially uniformly, and therefore the upper floors started falling straight down. The overload of weight from the initial collapsed floors was distributed uniformly, so the next floor down collapsed uniformly, and so on all the way down.
They’re designed to fall like that; a lot of structural studies are done specifically to design them that way. Even so, when there is a scheduled demolition a lot more structural studies are done specifically to ensure the building falls in rather than out.
Now if you want the mathematical details, that’s beyond me.
Just realised that in the example I gave of cutting a tree down, I was thinking of an axe cut, which removes the support from one side.
Saw cuts don’t remove the support from one side, and do allow for a tree to be pulled over towards the other side from the cut, by ropes, but again, the cut being asymetrical leads to the tree falling in one direction.
I think the question has been answered pretty well. I just wanted to add skyscrapers have a much higher width to hieght ratio than a tree. The WTC buildings hieght to width ratio was about 6to1. An eastern white pines ratio is 20to1.
Even if trees were hollow like buildings they would have less area within themselves to fall into.
Skyscrapers don’t have the strength to support their weight while leaning horizontally, they would just fall apart. They’re built to withstand horizontal forces like wind, earthquakes, or the momentary shock of an airplane crash.
I can imagine a building sliding to one side, but it wouldn’t topple like a tree. More like pushing over a giant pile of stuff. Has anything like that ever happened?
AFAIK it’s like when Superman or other super strength heroes hold up an airplane or ocean cruiser. Cute, but you’d just end up with a giant hole where Superman is because the superstructure can’t support its weight like that.
A toppling tree is subject to a lot of forces you don’t really appreciate, and it is only the inherent strength of the tree that stops you seeing these, plus the way a tree is felled also hides some of this.
Perhaps one the the best things to observe is an industrial chimney being felled. These are not always dropped inside their own base, and if there is room they will be felled by toppling them. But they won’t fall over a a lump like a tree, they will fracture as they fall. The reason is that any falling object likes to fall as accelerated by gravity - this means it accelerates at 9.8ms[sup]-2[/sup]. But a long object falling in an arc can’t accelerate like this, the top is being pulled down faster than this by the lower parts, and eventually the stresses build up to the point where the chimney fractures.
This chimney in Poland was reinforced concrete, so had a lot of tensile strength and didn’t fracture until quite a way down.
This one in Sydneyneatly fractures at its mid point.
You will see simple brick chimneys fracture in multiple places.
The other think with trees that you don’t often appreciate is that they want to rotate around their centre of rotational inertia, which is not at ground level. A tree falling is held in place at the ground by the remaining wood attaching it to the root system, and so rotates around the base. A tree falling where this attachment fails will kick back as its centre of rotation changes. This is why you never stand behind a tree being felled.
A concrete shell cannot support itself without eternal steel skeleton.
In Japan, an earthquake proof building DID just fall other.
Not always true. Masonry (brickwork) typically lacks tensile strength, but a modern skyscraper with welded steel columns can have pretty good structural integrity.
Also, this building fell over and mostly maintained its integrity, not really collapsing until it started laying down on the ground.
These were all relatively short, fat buildings though. Apart from the WTC towers, I don’t think we’ve seen a truly monstrous skyscraper suffer an inadvertent collapse, so it’s hard to say for certain what they would do if you compromised the foundation on one side.
Got a cite for that? Older homes in Japan sometimes collapse during earthquakes, but I’m not aware of any skyscrapers that have done so. In fact, it seems to me that they performed pretty well in one of the biggest quakes ever.
Didn’t you guys play with blocks when you were kids? Building a huge tower and toppling it would result in most of the blocks falling near the base of the tower as gravity took over once it began to topple. Most structures seem to be a solid structure but they are mainly held together by their weight bearing down vertically. If you constructed a building horizontally on the ground and tried to raise it to a vertical position from one end it would break in the center.
Expert demolitionist can pretty much make a building fall in any direction they want.
you can see video of chimney demolition gone wrong and they come down like a tree (sort of until part way).
Axe or chainsaw you basically use the same concept. You cut the “directional notch” first to point the tree to fall in the direction you want. Then cut the other side a bit above the notch and generally the tree falls down in the direction you want.
I spent two summers doing just that with both axe and chainsaw cutting trails long long ago in my younger days.
Trees fall in the direction of their center of balance unless you apply some other force to them. Straight tall trees can be made to fall in the direction of a notch, if you don’t trim the branch from spreading trees they’ll fall in the direction of the heaviest part of the tree.
When it comes down to it, gravity pulls down (by the definition of “down”). If some part of a building/tree fell to the side there must have been some significant force other than gravity working on it. For trees, the internal stress forces are comparable in size to gravity so they can “push” the higher parts of the tree to the side as they fall. Buildings are made much more sparsely of heavier materials so the ratio of internal stress forces to gravity is smaller on average.
I was thinking of a hand-held lumberjack saw, which makes a straight cut without a notch, but agree a chain-saw notch cut works the same as an axe.
Think of how much horizontal momentum you would have to impart to the upper storeys to get it to topple sideways. Tall buildings just are not strong under tension (why would you bother to make it so?) to resist breaking while trying to impart that momentum. Trees bend rather than break, but tall buildings cannot.
In the 1993 attempt they tried to take down one vertical pillar. I read that had they suceeded (the truck bomb was just a bit too far away from it) the building would have fallen. As it happened I was just a few blocks away (walking south of Broadway at about Chambers street) and I tried to imagine what lower Manhattan would have looked like with the tower lying across it. But eventually I realized that it would have broken into several pieces and mostly fallen straight down.