What Caused The Buildings To Collapse?

I dont know but there have been reports of a possible bomb in one of the buildings. I heard them say that the buildings are built to withstand a 707.

 

Well they hit in the middle of the buildings, and fire then weakened the metal supporting the tops of thos buildings. As soon as the tops lost support and came crashing down on the rest, gravity+physics took over.

DaDakota

 

The obvious, although uneducated answer would be, structural failure. The power of the explosion combined with a plane landing in it basically caused the support structure on the top to weaken. This eventually caused the top to crash down on the rest of the floors which started a domino effect downwards.

 

Large buildings like this don't have the same types of structural supports of shorter buildings. They have to be able to sway in the wind in order to remain upright. As a result, when the outer supports are weakened in this way, the building cannot support the weight of the stories above.

 

Do you think this will lead to a new design structure to withstand things like this?

 

I watched the first building fall, so I'll tell you what I saw:

There was already a big chunk of the building missing on the southern side of the Tower (which was where I was -- south and a little east of the South Tower). The fire in the building was, of course, already raging by the time I arrived on the scene. As I watched the building burn, I noticed that the fire got worse and worse. Across the area where the big chunk was missing, smoke was pouring out of windows previously not on fire or broken. The fire raged from east to west (from my vantage point). As the fire built up steam, all the windows in a row across the building began to pour out thick, black smoke (meaning they were also smashed). Soon, I saw actual flames spread across the building. About a few minutes later (after everything in that area was on fire), the top of the building (meaning everything from the very top to the area I observed) buckled and crashed down on top of other floors, starting a domino effect. After the initial implosion of the top 30 most floors or so (I'm estimating), I decided it was time to leave and made a mad dash down Broadway away from the debris and dust cloud making its way toward the stunned crowd.

 

They don't build tall buildings to withstand this type of explosion. There is no design out there that could protect a building from it.

 

The intense heat from fires burning weaken the structure and the weight from above (minus structural support lost in the initial damage) collapses down on the remaining structure providing a stress that the building was never designed for.

 

Of course. Who designs a building with the possibility of an airliner crashing into it in mind?

 

Several skyscrapers are designed with this in mind, especially monster skyscrapers like the World Trade Center. After reading up on this for the past hour or two, the WTC was actually designed to withstand the impact of passenger planes. The problem is, it was designed to withstand the impact of planes common in the 60's. Here are a few facts about the "disaster resistance" of the WTC I got off the Internet :

It was designed to withstand... :

1) ... the effects of a bomb so that a progressive collapse would not take place.

2) ... the effects of 150 mph winds

3) ... the loss of perimeter columns by sabotage

4) ... the impact of a Boeing 707.

From looking at Boeing's site, a Boeing 767 is about 50% larger (depending on configuration) than a 707, not to mention it probably carries thousands of more gallons of fuel.

 

It could supposedly handle a two degree variance ( by wind) so it could handle a lot, but something that large just knocked it's socks off from top to bottom. It had to fall. What is amazing is that the building swallowed the plane. It should have fallen much faster than it did.

 

I saw footage of people jumping out of windows.
Did you see any of that or where you not that close?!
I'll bet the smoke and heat was too much to handle.

 

Even without these special considerations, buildings are built to withstand tremendous lateral loading. Wind loading is the limiting factor, and the buildings are built to limit the deflection, or sway, of the building so that the people on the top floors don't get motion sickness in high wind. The force of strong wind on the side of a 110 story building is tremendous, even more than the force of a large jet flying at high speed into it. You'll notice that the building survived the hit from the aircraft quite well. It is true that steel is "weakened" by fire, which is why you see all structural steel covered with fire protection of some sort. This may have been a contributing factor to the initial failure. After the initial failure, however, the building seems to have pancaked and progressively collapsed. This surprised me too. I'm an engineer, but not a structural engineer, so I can't shed much light on why this may have happened. I suspect there will be more talk about it in coming days.

 

I'm glad I didn't see anyone jumping. Otherwise, I probably would have vomited. Other people I ran into around Battery Park had horror stories to tell about people jumping. I'm lucky I didn't, because that image would probably haunt me forever.

The smoke was hard to handle. I feel like I still have ash in my system. Guess I need to drink some more water.

 

One thing I dont know if it was posted yet or not but all of these planes were big planes flying across country and there for were loaded with flamable jet fuel. And they crashed them fairly early.

 

One further point. This building has been targeted before. Were they targeting it because they knew its design made it susceptible to this kind of failure? I'm sure all this will be discussed in the future, but for now perhaps we should be thinking more about the victims and their families.

 

Notice that the planes hit in such a way as to provide the same effect as a 'notch' when cutting down a tree. By doing this they introduced torque to the remaining side of the frame. I'd be willing to bet if they'd hit them streight on, they'd still be standing.

 

Grizzled,

First of all, become a Rockets fan.

Secondly, I think the building is targeted simply because it is one of, if not the most noticeable building(s) in the capital of US commerce. The World Trade Center and the area around it is the heart of Amercian economy. The twin towers are also well-recognized by many and contained thousands of lives to be destroyed. As much as I hate to put it like this, it was the perfect target. The nearby Statue of Liberty would be a more symbolic destruction, but it wouldn't cause such a huge loss of life. Also, if you can manage to topple one of the Twin Towers, the area is so congested, its toppling could topple the other Twin Tower and possibly destroy the surrounding area.

In New York, there are 3 constructions that stand out as monuments in America : The World Trade Center, The Empire State Building, and the Statue of Liberty.

 

I also find it intresting in a morbid sort of way that the second hit, which was lower on the structure collapsed first. I assume that the additional weight added more torque.