Twenty two years later and people still ask what happened on 9/11.
On September 11, 2001, American Flight 11 was hijacked and flown into the North Tower of The World Trade Center in New York City. (Hijacked: unlawfully seize [an aircraft, ship, or vehicle] in transit and force it to go to a different destination or use it for one’s own purposes.)
At 9:03 am, United Airlines Flight 175, was hijacked and flown into the south tower.
But before I tell you more, let’s look at the build design to better understand the impact these crashes had on the towers.
In December of 1970 the World Trade Center construction was started and was made for the sole purpose as the name suggests, world trade.
The original World Trade Center contained 7 different buildings:
- One World Trade (North Tower)
- Two World Trade (South Tower)
- World Trade Three (Marriott Hotel)
- World Trade Four (Low Rise Office Building)
- World Trade Five (Bank)
- World Trade Six (U.S Custom house)
- World Trade Seven (office building)
Both of the towers were held by steel bars as seen in the photo. Each bar went up three stories and were held together by spandrel plates. Each of the bars were placed on top of each other.
The image shows why the towers didn’t collapse on impact. This was called the bathtub method. The construction workers found some hard bedrock underneath the concrete. And they made a slurry wall to prevent floods from the nearby Hudson River. And the steel bars went from the ground to the top of the Twin Towers. And the slurry wall was held together by tie-back cables. And tunnels were built onto the slurry wall to add strength.
Now let’s talk about the steel core, which helped hold the two towers up. 
As the images show, the towers had an inner steel core instead of the classic skeletal design. The designers did this because they didn’t want tubes interrupting the office spaces. The inner core design is very similar to the perimeter tube design. Each of the bars went up about three stories and were held together by tubes placed through them.
Now I must mention one of the least mentioned facts by the government on the reason for the collapse: the floors, and how poorly they were connected to the core and the perimeter tubes, as seen in the image.
There are very tiny steel seats connected to the outer bars.
Here is a zoomed in image of the seats. These seats did some good; because of these seats and some dampers, the floors and buildings were able to sway over 12 inches in heavy winds and hurricanes.
But these tiny seats were expected to hold floor trusses, steel plates and concrete.
Finally in 1973 The Twin Towers and The World Trade Center were finished. And the office spaces sold out like hot cakes. An addition to the towers were that bathrooms, elevators and staircases were built into the steel core.
Now, let’s go back to the information of 9/11
After the planes crashed the jets exploded and made a fire, which after thirty minutes, made the floors sag. This caused the exterior tubes to bend and caused buckling.
(buckling: bend and give way under a weight or force.) All that weight caused all the floors to collapse, which caused an awful echoing effect. This was due to the top floors coming down weighing almost 38,000 tons (the equivalent to the Titanic) which the buildings could not hold as the top floors were falling down so fast.
And half of the steel inner core bars were knocked out when the plane crashed causing the building to become weaker . The buckling and the sagging happened because the fire reached over 1,000 degrees. It might not melt steel but it will soften and bend making the floors and outer cores weak.
Sadly the South Tower collapsed first.
Almost 15 minutes later the North Tower collapsed.
Some people asked and still do, “Why did the South Tower collapse first even though it was second to be hit?”
This was because the South Tower was hit near the middle of the tower adding more weight to the broken core and weak steel bars.
And the small steel seats bent and the connections were further weakened on both towers.
Today the World Trade Center has been replaced with 7 new buildings and 9/11 Memorial Pools, each representing where the Twin Towers used to sit.
Today the One World Trade has been modified but has the same sort of design as the original World Trade Center.
The new World Trade Center core has been replaced with concrete and can burn for over 3 hours without collapsing. It has three different staircases, one for first responders and evacuation and another for regular use. And the floors are held directly to the core and exterior glass.
The End
Made by Jacob P
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