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The Collapse of the World Trade Center Towers

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Old 24th September 2001, 06:54
hipo hipo is offline
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Since I am a Civil Engineer, many of my friends keep asking me questions about the WTC building collapse. I thought it would be interesting to post some facts about this matter. This is taken from the Americans Society of Civil Engineers public information department.

The Collapse of the World Trade Center Towers
Frequently Asked Questions

1. Why didn't the towers collapse upon impact?

The towers did not collapse upon impact because they were originally designed to withstand enormous loads. The impact of the aircraft did not take the buildings down. In fact one tower stood for about an hour after it was hit and the other stood for an hour and forty-five minutes after impact.

When engineers design a building they calculate the loads and forces to which a building will be subjected over its lifetime. Typically, these include the effects of hurricanes, blizzards, floods, or earthquakes. These events are far from common. For a hurricane, the loads used by the structural engineering profession are those from a hurricane that has only a two percent chance of occurring in a year.

For New York City this means structural engineers design buildings that can withstand winds gusting up to 100 mph, possibly coming from multiple directions, and lasting for hours as the hurricane passes. This approach to structural design produces tremendously strong buildings. Nevertheless, structural engineers are constantly striving to improve their ability to predict the loads their structures must withstand.

The forces from the impact of the airliners alone did not collapse the towers because those forces, as extreme as they were, did not exceed the capacity of the overall structural system.

2. Why did the towers collapse?

We do not yet know the answer to this and, in reality, we might never know with total precision. However the following appears to be what occurred:

The weight of a typical high-rise building is supported vertically by its columns. These columns commonly extend for the entire height of the building. The weight of each floor is transferred to the columns by a complex network of beams and slabs connecting to and spanning between the columns. Structural engineers design the beams, the columns, the slabs, and their connections to resist the anticipated loads.

When the airplanes were flown into the World Trade Center towers a number of columns were severely damaged. The damage from the impact, though significant, weakened the structure but did not cause it to collapse. Rather, the intense heat of the resulting fire fed by great quantities of jet fuel further weakened the already damaged structural system. This is what is believed to have led to the complete collapse of the crucial structural elements in the impact area. The failure of these elements caused the portion of the building above to drop, touching off a progressive failure as the entire structure collapsed onto itself.

3. How vulnerable is a typical high-rise building to collapse?

The typical high-rise building has a very low degree of vulnerability to collapse.

The disaster at the World Trade Center was unprecedented in the devastating combination of forces produced by the impact of the airliner crashes and the burning jet fuel. Yet, the towers were able to stand long enough for tens of thousands of people to escape. Unfortunately, it appears that several thousand people did not escape the subsequent collapse of the towers.

As it usually does in disaster situations, ASCE is dispatching teams of engineering experts to the World Trade Center and to the Pentagon in Washington to conduct studies of the sites. These studies may further our knowledge and help to enhance structural design practices in the future.

4. Can engineers design buildings that are terrorist-resistant?

Yes, but they would likely resemble fortresses and people probably would not use them and might not be able to afford them.

Engineers can, and do, use many methods to enhance the security of buildings and other structures. These approaches include structural elements such as reinforced frames and perimeters, the use of Kevlar-curtains or bullet-proof glass, or designs that eliminate or restrict vehicle access or parking, minimize windows, or secure entrances.

In designing buildings to withstand threats a balance must be reached between safety and freedom and the effect of each upon our nation's infrastructure. Events such as those of September 11 may well challenge America's commonly held feelings about that balance.


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Old 24th September 2001, 11:04
El_Jibaro El_Jibaro is offline
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Talking From a fellow engineer (Aerospace by job description and Mechanical by education):

[b][i]The owner of the WTC doesn't want to build them again that big, but I think that is a mistake in two fronts:

- It would be a perpetual symbol of terrorism victory over America.

- A building this tall can be the best place to defend the rest of New York City.

I think that to protect the next edition of the World Trade Center Twin Towers two things should be done:

1 - Double the number of columns, from the 4 corner columns it had before to 8, the new ones being inside the building.

2 - Put an Aegis Radar and Missile Battery on top of one of the towers. An Aegis Radar and Missile Battery can handle 100 simulataneous air targets, and it would provide complete Air Cover Protection to Manhattan and outliying boroughs. With some modifications it can even provide protection from nuclear missile threat from "rogue nations".
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Old 24th September 2001, 11:47
hipo hipo is offline
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Actually, part of the US corps of Engineers' initiative is to study the possibility of rebuilding them. I guess it's too early to say something definite. I am trying to be part of one of the teams that will go there to study the collapse.
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Old 27th September 2001, 09:37
PRtaina PRtaina is offline
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Lightbulb

This is interesting. I have a question. Though I doubt it do you think any explosives were involved?
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Old 28th September 2001, 08:07
hipo hipo is offline
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Not directly. I believe the plane was used as a form of explosive. I think part of their plan was to hijack a plane with a lot of fuel. Notice that the planes they hijacked were going to California, being them the ones that require a larger amout of fuel. Explosives would have been difficult to pass trough to the plane, but the plane itself was used as an explosive.

These people carefully planned this attack. They definitely knew what they were doing very well. They probably knew the towers won't collapse unless they are put under extreme heat.
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Old 28th September 2001, 08:13
PRtaina PRtaina is offline
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Smile

Yeah it's sad to see how they blinded and brainwashed to this. They definetly had a lot of planning involved. I was reading about the first time they had bombed the WTC in 93 that the guy who was behind it all said that next time the buildings wouldn't be standing.

I understand about the fuel and the plane being used as an explosive itself i just thought that maybe it might have had explosives because people kept asking my opinion. Though from the beginning they said that the collapse was initiated by the impact and heat of the fuel. Everytime I even write about this I still find it so hard to believe that it happened. It is slowly sinking in my brain.

Thanks hipo for the insight on an engineers perspective.
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Old 28th September 2001, 08:39
hipo hipo is offline
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Unhappy

Even though I didn't loose any loved one, to me this has been traumatic. I used to go to Borders bookstore there at the WTC every Saturday to read books and have coffee. I used to stand in front of tower one and look up and think "Will I ever have the chance as an Engineer to work in a design of something like this?". I guess I do now. Yesterday I finally had the courage to go around the area and it was depressive to see the destruction. You can still smell the burned steel and bodies from 10 blocks away from where the buildings were. I cried, I just needed to see it for myself.
The US Corps of Engineers are getting volunteers to study the zone, the collapse, and the feasibility of rebuilding the towers. Some people think the towers shouldn't be built again. Me, not as an Engineer, but as a human being, I really think we should build the towers back and leave a space for a memorial park to remember the poor victims. I guess I just don't want those cowards to win. I want those towers back, I really do.
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