The Collapse of World Trade Center | The Complete Physics

the 911 terrorist attack the huge impact force of the airplane did not cause the Twin Towers to have a tilted fall like this instead the towers stood strong for more than an hour however to the world's surprise the towers collapsed like a house of cards a vertical fall the main reason for this strange collapse was the tower's structural design the perimeter tube design the concrete floors of the World Trade Center are supported by these trusses during the airplane crash the jet fuel burn increased the temperature and caused the trusses to Sag can you tell what the effect of a heavy and sagging floor on the building wait wait we're going too fast to properly understand the physics behind the collapse first we need to understand the geometry and design details of this building the perimeter tube design comes to life when you rearrange evenly distributed columns in a building to a tube in tube Arrangement these office spaces were incredibly elegant convenient and spacious with no irritating columns inside the office areas this was the sole reason why the engineers chose this unique perimeter tube design for both skyscrapers to truly appreciate this Choice let's quickly compare the design of these offices with those found in the iconic Empire State Building as you can see the gigantic columns here eat a lot of useful office floor area if you look closely you'll notice that a single external column does not run throughout the tower's height instead it's a collection of many prefabricated panels a single panel consists of three columns reaching three floors in height these panels are then held together by spandrel plates the engineers connected these panels in an ascending order of height repeatedly rather than taking a straightforward approach this method ensured a good stiffness to the external columns these cute but gigantic cranes called kangaroo cranes played a major role in speeding up the construction work of the World Trade Center by viewing this animation can you tell how this innovat of crane climbs up by itself when the Piston is activated the crane increases its height this interesting support structure which can Flex only towards one side helps the crane rest at this position at this stage the bottom portion of the Piston is retracted once again the one- side Flex support mechanism Springs into action now that the crane is properly rested it's time to attach these frames to the base in order to stabilize it the way this crane climbs looks cool right if it wants to increase its height it must simply repeat the same series of motions when it came time to assemble the frame lock neighboring kangaroo cranes provided assistance the World Trade Center towers were one of the first in the United States to use these Innovative cranes did you notice what this worker is doing here this simple technique is in fact the reason why the towers after being struck held up for more than an hour before they collapsed this is a spray foam insulation process when sprayed on a metal surface this special liquid creates a foamlike texture in the case of fire this foam can prevent the heat from reaching the steel for an hour or two as per its ratings this useful technique protects the building until First Responders are able to make it to the scene and control the fire the Twin Towers trusses and columns were covered with this spray foam insulation now it's time for us to explore the second major component of the design the core although the core occupies just 1/4 of the area this core Bears the majority of the gravity load of the tower each column of the core was welded to the one below it and each had a height of around 30 to 50 m later on ibeam girds were attached at every floor if you zoom in and analyze the fall of the World Trade Center Tower it's evident that what transpired was basically a sequential collapse of all of its floors let's learn something about the floor connection of the World Trade Center to better understand the collapse connecting the external columns and core at a high span of 60 ft was only possible by a truss Arrangement the beams would have sagged to facilitate the construction process they pre-fabricated parts of the floor they then clubbed some of the trusses and secured them together by placing a steel plate over them the trusses are connected with the columns in a rather unique fashion these truss connections were made possible via these tiny and crucial steel seats the main culprit of the collapse and least mentioned in the reports offered by the government let's take a CL closer look at it the truss's upper cord is rested on this inverse L-shaped steel piece this seat is then welded to the spandrel plate via these tiny standoff plates keep these connection details in mind as they played a crucial role during the collapse of the towers as we'll learn later interestingly the knuckles of the truss were made to pop out of the steel plate so that later when concrete is poured the rebars make the connection to the steel system stronger please note that in this floor design design the steel truss is exposed with the cast in place concrete the floor of the Twin Towers are ready ready for the lavish open floor office spaces to be sold out like hot cakes at the very top of each Tower set a fourth important structural element the Hat truss this huge truss structure served as a bridge between the core and the external columns the Hat truss also provided support for an antenna at top the tower now that we've covered the entire structure of the World Trade Center towers let's get into the physics behind their intriguing House of Cards kind of collapse as the hijacked jetliner crashed into the North Tower it ruptured the Maze of perimeter columns for up to 6 to8 floors this crash also damaged about 60% of the core columns the elevators and stairwells were damaged too trapping around 1,600 people above the tower this impactful hit of the Boeing jetliners could have led to a Tower collapse like this but it didn't thanks to the strong Foundation of this building and also due to thousands of these tiny visco elastic dampers in the building these rubber-based dampers absorbed some impact force in fact these dampers provided the towers with Incredible flexibility such that on a windy day they could sway up to 12 in moreover the towers stood strong for more than an hour after impact even after a giant gaping hole was left in its walls of columns this was possible because the remaining columns both the perimeter and core columns did a great job of carrying the weight of the building above the crash area so if the remaining columns did such a good job you might be curious about what exactly led to the collapse of the tower the Catalyst of the collapse was the huge increase in temperature following the crash a large amount of jet fuel burned vigorously on those 6 to8 floors causing a spike in temperature if the trusses lacked the protective thermal coating we discussed earlier in the video the tower would likely have collapsed and as little as 10 to 15 minutes however thanks to the thermal insulation spray this was delayed for over an hour burning fuel cause the heat to reach a whopping 1,000° C although steel does not melt at 1,000° C it does surely soften and would have caused the truss to Sag as shown can you imagine what the effect of a sagging concrete floor would be on the perimeter tubes I have two Floor Designs in front of me the first one is having a straight floor and second one is having a floor with a sank when you apply a force on the first design on the straight floor this is in fact gravity load there is no issue for the walls they're standing strong right but if I apply the same force on the second design on the S FL design you can see the walls are ding inward that's a huge deflection the same thing happened in the WTC perimeter design as well and this Dent is so clear in some of the collapse videos this means that the weight of the sagging concrete floor would Dent the perimeter columns inside this is due to the force which is acting inward remember the perimeter tubes are also weakened due to the temperature increase now let's see what the effect of a dent is on a perimeter tube design this simple can is equivalent to a perimeter tube design surprisingly such a small can is able to handle weight of three big ups but if you add a small dent on the can it just collapsed and if you Che the details oh my God it was a perfect vertical collapse you can see that the same thing happened in WTC perimeter design as well similar to the experiment when an axial Force passes through a perimeter tube designed with a dent it just collapses at the region of the dent this is a clear case of buckling you've probably all experienced or at least seen buckling buckling occurs when a compressive load acts axially on a slender beam because of the dent buckling happened at a much lower load can the strong central core save the top floors from falling down unfortunately no it can't we've already seen how the floors are weakly connected to the Core moreover due to the high temperature these connections were further weakened during the collapse of the floors due to the dent the core's support would simply break with almost no connection to the core the result is clear the collapse of the entire top floors downwards the mass of the top 15 floors was equivalent to the Titanic coming in at a mind-blowing 38,000 tons in addition the Hat truss and the Boeing were also falling down the Sudden Impact for but his huge weight over the floor right below it trashed the lower floors the perimeter tubes also almost immediately buckled since the impact force was huge and compressive in nature this caused an unfortunate chain reaction and one by one the floors kept crashing down the weight also kept increasing as the building collapsed when viewing footage of the tragedy you must have surely seen the shredding of the perimeter columns during the collapse this was due to buckling the North Tower came down in 13 seconds and the South Tower took just 11 seconds did you know this exact scenario was tested before the towers were constructed in the plane crash test this Tower design stood well enough however the engineers did not consider two conditions one was the fire from the gallons of jet fuel and the second was the size of the plane at the time of the test the largest plane the engineers considered was the Boeing 707 which was half the size of the one that actually hit The One World Trade Center Tower which was erected in The Honorable position of the twins consists of measures to improve rescue strategies and the building's thermal design in the One World Center the engineers built a thick wall of reinforced concrete surrounding the egress stairs this thick concrete wall acts as the core of the building for obvious reasons they moved away from the steel Core Design during an airplane crash the concrete core will stay strong regarding the floors this time they didn't expose any of the steel structure opting to cover it completely with concrete this prevents any chance of floor steel structure sagging due to a rise in temperature the government also Incorporated many new regulations for building safety with these new safety measures in place the new one World Trade Center is the tallest building in the Western Hemisphere now that you understand the physics of the collapse well here is an interesting question for you after being struck the North Tower stood strong for 1 hour and 42 minutes before collapsing however the South Tower collapsed in just 56 minutes what could be the reason for this time difference the reason is the weight of the floors above the crash region the South Tower crash happened 14 floors lower than the North Tower this means the South Tower had to carry a much higher load during the fuel burn time this would obviously lead to easy buckling this is the reason why even if the North Tower was hit first it collapsed second generally during a fire accident the stairs play a major role in evacuating the occupants however in this case the stairways could only save a few people this was mainly because most of the stairs became obstructed by gypsum debris also all the stairs were located in the central core area according to the reports the South Tower's a staircase was the only one which was usable many people compare the 1945 Empire State Building crash with the 9911 attack the empire State survived but the World Trade Center did not the reality is that these two events are simply incomparable when considering the size of the airplane involved in the crash the airplane involved in the Empire State Building incident was a tiny bomber aircraft whereas the World Trade Center was struck by a gigantic passenger aircraft the care Engineers gave while constructing the foundation of the Twin Towers was incredible you can see a strange concrete wall within the soil and around the building this is known as a slurry wall and its main duty is to prevent water intrusion from the nearby Hudson River look at the way they've strengthened the concrete walls using tieback cables the engineers were able to locate a strong bedrock at a depth of seven Stories the concrete foundation of this building progressed from there the engineers called this New Foundation Technology a bathtub interestingly the bathtub Foundation also accommodated two tunnels this strong Foundation was the main reason why the impact force of such a gigantic airplane couldn't topple the building the Twin Towers may not have collapsed if the terrorist had hit the building say five floors above the actual hit location or if instead of a perimeter tube design a normal steel skeleton column design was used it may also have been avoided if the engineers had used a strong joint at the floor core connection even the terrorists behind the attack likely could not have imagined all the physics that happened after the plane crash or even anticipated such a Monumental building collapse sometimes physics is cruel you can see the huge amount of debris which was formed after the collapse of tin Towers surprisingly when they cleared the debris you can see a column which is standing strong and brave this is a column from the central core of the building I know even after all the scientific explanation about the collapse I will be able to read in the comment section what about wtc7 Tower it got collapsed even it was Untouched by the airplane there is a scientific explanation for that as well but not in this video this video is only 15 minutes long just give me one week of time we will cover that portion as well now about this Brave column it is currently shifted to a museum a museum dedicated to WTC Memorial and this column is still standing strong there but if you like to see this channel to stay strong please support us on patreon we are not doing well financially hoping for your support take care bye-bye

Channel: Lesics

Views: 10,963,887

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Id: 1NkBfLBov5Q

Length: 15min 29sec (929 seconds)

Published: Mon Oct 02 2023