Was there a design flaw or would any building of that height have failed?
In: Engineering
Check out some videos of explosive demolition for old buildings. A few tiny explosions crack some beams in a particular order and the whole building collapses into its footprint. Lifting those heavy beams way up in the air stores a lot of energy in them. If something were to weaken the support for that beam, all that energy accelerates the beam to the ground breaking stuff along the way. Once the building is moving down, hitting the Earth just crumples it up.
There was no design flaw.
The planes that hit the towers took out a good few of the beams supporting the part of the tower above the crash site. The remaining supports were perfectly strong enough to keep the building above supported. The towers stayed up for a good while after the crash.
The damage caused by the crash also damaged the fire proofing on the building’s structure around the crash site.
However there was a lot of jet fuel on board those aircraft which promptly caught fire in the crash. All that burning jet fuel caused high temperatures. These high temperatures combined with the damaged fire proofing caused the steel structure to heat up a lot. Steel gets weaker when it gets hot. The temperature at which it starts to weaken is a lot lower than the melting temperature.
Eventually the steel structure at the crash site was weakened enough that the remaining supports could not hold up the weight of the building above the crash site. They failed and the top of the building started to fall.
Now it fell on the part of the building below the crash site. Buildings are not designed to withstand the forces caused by the top half being dropped onto the lower half. This caused the entire building to collapse.
If there was no fire the towers would have stayed up at least for time to evacuate.
Structures are designed to withstand only so much load. Typically it’s the weight of the building, plus the natural forces it will experience, plus a little extra for safety margin. The more force you try to prepare for the more money you have to spend on constructing the support. Why spend money trying to reinforce a building against the force of another building falling on top of it?
Well on 9/11 that’s exactly what happened. The fires buckled the support beams causing the dozen or so floors to fall and crash into the building beneath it. It wasn’t just the weight of the material, but the force of thousands of tons of steel and glass and concrete falling only dozen feet. That caused other beams to buckle and give way, floor by floor as each floor collapsed into the other, causing the building to fall within its own shadow (more or less).
No design flaw. The steel columns of the floors where the planes hit got heated by the first. They did not melt, but heat softens metal even if the metal isn’t a liquid yet. Those softened columns then failed, and the weight of all the floors above came crashing down and that momentum was enough to cause the columns beneath to fail as well.
The floor was designed to withstand fire for hours, but the problem was the the impact damaged or completely blew off the fire proofing in the columns, so you just had the steel columns getting heated directly.
And again because some one will say “oh jet fuel can’t melt steel beams!!” They are right, it can’t. But you don’t have to melt something to make it soft and bendable. Go watch a video of a blacksmith making horseshoes or knives. Those don’t have to melt to become bendy.
You need to understand only three things to understand why.
1. Steel is made by heating it up and shaping it. The buildings were on fire and the steel structures weakened by head and the loads on them.
2 Gravity wins. It is ever present and is always in the direction downwards.
3. The buildings were designed well and aircraft commission was one of the things considered.
No matter how well built or designed a structure that is hot with a fuel bomb is likely to collapse. The attack was planned with build collapse in mind.
Any building of any significant height (anything taller than something like 4-6 floors) fails when a significant portion of the building comes down like a hydraulic hammer ram on the rest of the building.
Basically the supports at one level breaks, then everything above it comes down like a hammer on the rest of the building, using its weight and momentum to break floor after floor.
This is the fundamental principle of any form of high-rise demolition, although in construction they tend to go for overkill to make sure that the building comes straight down (to minimize collateral damage).
Gently set a rock on your phone. No damage.
Drop a rock on your phone from a second-story window. _Smash._
Now consider a situation where, three quarters of the way up a building, several floors holding the rest of it up suddenly cease to exist. _Smash._
In the real incident, the collapse didn’t happen immediately after the crash. The crash didn’t cause the stricken floors to _completely_ stop holding up the rest of the building. But as fires burned, the heat weakened the remaining supports. At some critical point, the supports became so weak that they suddenly gave way. _Then_ the buildings collapsed.
[This](https://www.youtube.com/watch?v=1NkBfLBov5Q) video goes into detail what happened.
TLDW: Unlike many other buildings the outer structure of the WTC was load-bearing. It had a very stiff center and the floors rested on steel beams between that and the load-bearing outer steel girders. When the plane hit the fire proofing of the flooring got demolished. That allowed the fire to heat up the floor steel beams much faster. Steel gets weaker the hotter it is and the floors started to sag. Eventually some floors broke off and crashed on the still strong floors below. That also made them sag but since they were still strong they didn’t bend as much but pull the outer shell inwards, creating a bulge. In the video they demonstrate that with a can of coke which can carry a lot of weight but as soon as they artificially create a bulge it collapses. That happened also at the WTC. Once the bulge started to collapsed inwards the same thing happened over and over again at each floor, just with tons and tons of more weight.
It was not a design flaw as they considered an impact in the planning stage. However, at the time of the planning the largest plane they could expect was a fraction in size and weight of the planes that eventually hit. They could not have planned for such a difference.
Things move because of a change in momentum. Remove a single floor, and add the mass of all the other floors above it, then collide it with the remaining building. That’s something like 20,000 metric tons crashing at a speed of 4.9m/s (roughly 11mph).
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