bcheary Loc: Jacksonville, FL

Quora.com

How did a plane crash on the upper floors of the World Trade Center result in the entire building collapsing?

That’s a legitimate question, and kudos for not just automatically declaring “ergo conspiracy”.

In fact, the damage caused by the aircraft impacting the towers — even at the high rate of speed they were traveling — was not fatal to either tower. These structures were famously designed and constructed to flex in strong gale-force winds or earthquakes, and even absorb the impact of the largest commercial aircraft at the time which was a Boeing 707 four-engine passenger jet. You can’t really tell from available video or film footage, but the initial impact caused the upper section of the structures to displace 10–20 feet from the center of gravity, and then swayed back like a pendulum. People who were up there and survived said it felt like they were going to capsize. So up to this point the buildings performed beautifully, testament to the soundness of the engineering and the redundancies in redistributing stress loads. So, what happened to cause total failure?

The fires is what happened. Ah, come on! Jet fuel doesn’t burn hot enough to melt steel. And how could a bunch of paper and office furniture burn at such high temperatures?

One needs to understand that heat causes steel to expand and warp, and lose structural strength. Think of a Japanese sword maker. He’s heating that very hard steel over a simple carbon fire. The steel hasn’t melted, but it has become softened and malleable to the extent he can fold it and hammer it, over and over.

Normally, if a fire starts in an electrical closet, it will burn everything in that immediate area and then progress to other areas. Overhead sprinkler systems and fire fighters will usually get things under control before it gets out of hand. But on 9/11, those planes were fueled for a cross-continental flight. So at the moment of impact there was a deflagration of 10,000 gallons of jet fuel which ignited large fires on 2 or 3 floors simultaneously. This is way beyond the pale in terms of manageability by fire fighters even under the best of circumstances. But in this case we have the water pipes which fed the sprinkler system severed. We have an elevator system rendered inoperative. And we have thousands of people who need to be evacuated. This was a very grave situation.

Okay, this is way up in the upper portions of the structure. How could those fires have affected lower floors? What caused them to fail?

This is so. The fires had virtually no impact on the lower 50 stories. The steel that was affected by fire was the trusses and columns in the impact zone. For Truthers who question the severity of the fires, please explain why so many people jumped to their deaths from the area above the fires. Police helicopter footage shows the perimeter columns on the floors of the impact zone bowing outward in the minutes before the collapse. The steel that was holding up all the floors above this zone was getting ready to buckle. And buckle it did. This resulted in a 25-story building falling 12 feet. That is a dynamic load the floor below cannot stand up to. Now you have a 26-story building falling onto the next floor down…and so on.

Think of it as tower of cards; you destroy the entire section of the 51st floor; the sections over them fall and add more pressure damaging the full structure and another floor goes down. It’s a chain reaction; the first floors are the ones that take longer to crumble but it accelerates as it gets more pressure and momentum from the fall.

A quick aside: there is a crucial difference in engineering and physics between a “static” load and a “dynamic” load. For example, when cars used to have robust bumpers, Car A could get behind Car B and push it forward and even push it onto the freeway and gradually speed up to 90 mph. For the most part, neither car will sustain significant damage to their bumpers. This is a static load. There is significant force applied but in a smooth way.

In a slightly different scenario Car B is stationary and Car A is travelling at 90 mph. Car A decides he’s going to push Car B again at 90 mph. But this time the moment Car A makes contact with Car B there is a horrific sound of smashing metal and the gas tank ruptures and catches fire. This is a dynamic load. Even though Car A can push Car B up to 90mph, the second scenario involves sudden acceleration.

So with the WTC towers when everything is fastened together properly, the lower floors support the weight of the upper floors. But when those upper floors undergo an acceleration, now you’re looking at a force the building was not designed to handle.

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therwol Loc: USA

The beams holding up the upper floors didn't have to melt. Some were already displaced by the impact. The heat of the massive fires warped the rest, and they ultimately gave way. The impact of the upper floors falling onto what was under them caused the entire buildings to pancake to the ground. The buildings were designed to survive an impact by any plane in the air in the 1970s, i.e. Boeing 707. The planes that impacted the buildings were much larger and carried more fuel.

bcheary Loc: Jacksonville, FL

Mr. SONY Loc: LI, NY

Excellent.
I've been trying to educate those Truther's about the unique design of those towers and why they collapsed.
During the construction of those towers during the late 60's, some people in the surrounding neighborhood
became alarmed when they saw how tall those towers were becoming.
They questioned what would happen if a tower fell over onto the nearby building.
Manura Yamasaki assured them his buildings would fall down within their own footprint.
Pretty much what happened didn't it?

bcheary Loc: Jacksonville, FL

yssirk123 Loc: New Jersey

I was in an AT&T building on 9-11 and watched the 2nd tower come down from 13 blocks away. It seemed surreal as there was just the briefest pause between each of the lower floors giving way.

Bultaco Loc: Aiken, SC

but why was this brought up now?

kerry12 Loc: Harrisburg, Pa.

I also read that originally the steel was coated with asbestos. It was removed from all of the inner steel structure so as to mitigate the dangers of asbestos. The outer steel structure could not be reached so it remained. This resulted in the inner structure being more susceptible to the heat from the fire. Don't know if that's true, but sounds reasonable.

fourlocks Loc: Londonderry, NH

The beams were coated with a spray-foam fire retardant. Unfortunately, the initial blast from the crashing plane and exploding fuel stripped the retardant away exposing the bare metal to the extreme heat. As Therwol noted, the steel didn't have to melt, it just had to become soft enough that it no longer supported the building's weight. As I recall there was a good NOVA on the subject that explained exactly how the buildings collapsed.

sippyjug104 Loc: Missouri

The World Trade Center buildings were structurally designed to resemble a "birdcage" with the main supporting members located along the perimeter with post-tension concrete floor slabs. The design was considered to be an innovative way to create the most usable floor space. Floor space is rented in terms of the 'net usable space' so all of the areas that interior columns take up are a loss of potential revenue and interior designers hate designing around them.

The elevator system was another unique feature of the building and....believe it or not....the designers were given an exemption on the number of stairwells required reducing the normal number required by building code in half.

I highly doubt that the terrorists intended to bring the buildings down and they were probably as surprised as we were to see it happen.

bcheary Loc: Jacksonville, FL

Wow! I couldn't imagine what was going through your head at that time! I watched it on TV and was appalled at the sight of those towers crumbling down and people jumping out of windows. A sad day for all.

bcheary Loc: Jacksonville, FL

I never read or heard anything about that but who knows.

bcheary Loc: Jacksonville, FL

Interesting.

therwol Loc: USA

The OP brought up the subject, which I find quite interesting. My post is the "Cliff Notes" version of the story, but I did make one mistake. I said that the buildings were designed to survive the impact of the planes flying in the 70s. It was designed to survive the impact of the planes flying in 1966 when the buildings were designed. By 1970, Pan Am was starting to fly the 747, a plane even larger than the ones that took down the towers. The military was flying the Lockheed C5a, a plane even larger than the 747.

Mr. SONY Loc: LI, NY

As Therwol noted, the steel didn't have to melt, it just had to become soft enough that it no longer supported the building's weight.

Steel doesn't have to melt to fail.
About a year before 9-11 there was an episode on the science channel about the most modern steel plant in the country.
Very informative about the production of steel.
The engineer who gave the reporter the tour told him and ultimately the viewers that steel only needs to be heated
to 800 degrees to lose half its load bearing strength.

A show about blacksmithing debunked the theory about steel melts when over 1000 degrees.
The fires in the towers were estimated to be about 850 degrees.
In his blast furnace he heated a piece of steel till it was glowing bright red.
He put the glowing piece of steel on his anvil and hit it with a hammer.
It was as hard as a piece of room temperature steel.
Then with a finger he bent it over.
Steel doesn't have to heated to its melting point to fail.
I miss being able to walk on the roof of the south tower.