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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 feet was only possible by a truss arrangement .

The beams would have sagged .

To facilitate the construction process , they prefabricated 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 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 , 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 hotcakes .

At the very top of each tower set a 4th 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 atop 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 to 8 floors .

This crash also damaged about 60% of the core columns .

The elevators and stairwells were damaged too , trapping around 1600 people above the tower .

This impactful hit of the lone 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 viscoelastic 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 inches .

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 to 8 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 in as little as 10 to 15 minutes .

However , thanks to the thermal insulation spray , this was delayed for over an hour .

Burning fuel caused the heat to reach a whopping 1,000 degrees Celsius .

Although steel does not melt at 1,000 degrees Celsius , 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 2 floor designs in front of me .

The first one is having a straight floor and second one is having a floor with a sag .

When I 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 sharp floor design , you can see the walls are denting inward .

That's a huge deflection .

The same thing happened in WTC pavement tube 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 3 big UPS .

But if you add a small dent on the can , it just collapsed .

And if you look at the details , oh my god .

It was a perfect vertical collapse .

You can see that .

The same thing happened in w two c perimeter design as well .

Similar to the experiment , when an axial force passes through a perimeter tube design 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 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 force with this 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 1 by 1 , 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 2 conditions .

1 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 1 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 9 11 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 tie back cables .

The engineers were able to locate a strong bedrock at a depth of 7 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 2 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 terrorists had hit the building , say , 5 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 terrorist 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