Did you know a third tower fell on 9/11?

I never know this until today.
apparently there is another building called "building 7" that was collapsed on the day.
Very curious..

rethink911.org/

youtu.be/hZEvA8BCoBw

Yup, due to low water pressure the fires couldn't be put out, and eventually the building's iron structure gave way due to the heat.

Dimitri

Right off the bat. It's Ed Asner. A "the President done it" believer. He's sooo left wing he even hates himself.

They seem to forget that in controlled demolition they take out PIECES of the structure to make the rest collapse. The steel doesn't have to melt, it just has to soften from the heat. Heck, how many of us have bent steel at home? I know I have, and it wasn't hard.

Even Popular Mechanics debunked some of this stuff.

www.popularmechanics.c...ws/1227842

A big chunk of the front of WTC7 was taken out by debris from one of the other towers.
This damaged some of the cantilevered structure over the Con-Ed substation & the fire did the rest.
No sounds of explosions when ANY of the towers fell either & I've been at one demolition of a 20 floor block; the demolition charges were LOUD even half a mile away.

I wasnt there so I don't know for sure.. but the sound on the news does have booms of explosion..

Geland, when you have tons of steel and concrete falling on each other it sounds like explosions somewhat.

Due to the concrete literally shattering into dust as it falls from the large hunks that it was.

Dimitri

I made knives, I know it take a hell of heat to make it red hot, and take much more to melt it.
I am sure if I burn everything I got at home together, with kerosene as accelerant, I may not able to melt 1 kg of steel.

In house/ flat/unit/factory fires, even a total destruction type, the fridges, kettles etc usually still hold their shape.

To get the high heat, with common fuel ( timber/ carpet/ polyester etc) you would need a hell lot of air and a good containment to concentrate the fire.

Just saying..

It is surprising just how hot a simple fire can get. I have heat treated steel in a simple wood fire in a Weber kettle BBQ. The heat of the metal exceeded 800 degrees Centigrade (the temperature that steel becomes non magnetic). I don't have a thermometer that can measure the exact temperatures, but I regularly heat steel with a small propane torch to make it easier to bend, and it is surprising how easily it reaches the temperature that makes it easy to bend.

Fire is a strange beast...it can perform amazing feats of physics with small amounts of fuel.

gelandangan wrote:

I made knives, I know it take a hell of heat to make it red hot, and take much more to melt it.

You don't need to melt it to make it fail though. The steel in buildings is already in the 70-80% of yield point due to the building's own weight.

You only need to get it to around 400-600*F to get it to fail at that one joint. And once joints start to go, it's almost like a domino effect as the rest of the structure cannot support the entire finished weight of the building on it's own. Ie the heat from your BBQ is well over and above what you need to make a building's joint fail.

The fires took a while to knock out enough joining members to let WTC7 finally fall. But once that one last joint that needed to be heated, causing the rest of the structure to be over it's yield point got hot and failed, the rest of the building fell.



This chart assumes that the building was over built by 66% (which is generous), and that failure point would still reach itself by 550*F.

Fire suppression systems (when they work, unlike the issues the area was having due to low water pressures in WTC7), are more so to protect the building then to protect the occupants. It was well known before 9/11 that a uncontrolled fire could cause building failure, due to the very thin safety margins built into buildings and how steel can react due to heat.

Dimitri

Agreed that all you have to do is weakening the columns and the building will collapse.

But, the building is a tall multi storeys building, and the fire is in lower 1/3 of the floors.
Surely the columns on the upper 1/3 does not get heated as far?
For one or two floors up from the fire, yes I can understand that it will heat up that far.
But all the way to 40 th floor?


I learn about building pancake failures.
They are called as so when the floors of the building stacked up like pancakes on a plate.
Here, the floors are pulverized.

I learn about building failures.
For example, recently there is a building failed from its foundation in China.



I believe this has been posted in our Joke section too. a tall building laying on its side due to foundation failure.
what do you see? A BUILDING LIES on its side!! NOT pulverized mess that even the concrete slabs become fine dust.
A building falling down on its side would have nothing supporting it from the side but air, it would have absorbs 100% of the kinetic energy due to a long free fall.
A building falling down on itself have the floors below to absorbs the impacts surely it would at least have a few floors intact and shows up as pancake layers.. instead of the totally mashed up mess in WTC?

So WHY one building survives INTACT, and the other PULVERIZED?

Surely the concrete in USA to build WTC 7 would be better or higher quality of the one in China?
Why does the one in China survives the fall?

gelandangan wrote:

Surely the concrete in USA to build WTC 7 would be better or higher quality of the one in China?

The building in China did a Leaning Tower of Pisa type of a fall, the entire building's structure was sound, it was the dirt under neath it that let it tip to one side then fall.

Your also forgetting kinetic energy, the building at rest, doesn't have to worry about the acceleration of gravity. A joint that isn't moving doesn't have to withstand as much force as one that is falling into the ground.

When the building is static, and not moving, all you worry about is it's weight (mass * gravity)

The total energy available is (mass * gravity * height) once the object falls.

WTC1 & WTC2 each at 500,000 tons each, or 500,000,000kg.

Their potential energy, at about 1/3rd it's height (to compensate for the fact the lower floors are more heavily built then upper floors), ends up being factoring gravity and height of 415m (WTC2 with it's slightly shorter size).

2,035,575,000,000J of energy, or 2 TJ of energy waiting to be tapped.

That is the same potential energy, of a 486 MT nuclear bomb. The biggest nuclear bomb created was a mere 58MT.

Each tower had a 125,000kg plane hit them, both at about 223.5m/s. Giving a Kenetic energy of 3,122,015,625 or 3.1GJ of energy. That on it's own is the energy of nearly .75MT of TNT ramming the buildings.

Everyone is so heavily focused on WTC7's demise, which was probably a combination of things, but if we look at the site map ...



And look at the photograph from above after the fact ...



Let's talk about the building's that were damaged during the attacks beyond WTC1, WTC2 and WTC7 that people like talking about ...

WTC3



WTC4



WTC5



WTC6



Deusche Bank Building across from WTC2 & WTC4



90 West Street



The big thing, as to why other WTC buildings didn't fall completely but WTC7 did due to the joints failing is the lower floors construction, and the way the entire structure had to go around the electrical substation in the first 4 floors, making the building's structure pretty flimsy already.

Due to the arched layout for most of the building's footprint on one side ...



Dimitri

Quote::

Your also forgetting kinetic energy, the building at rest, doesn't have to worry about the acceleration of gravity. A joint that isn't moving doesn't have to withstand as much force as one that is falling into the ground. When the building is static, and not moving, all you worry about is it's weight (mass * gravity) The total energy available is (mass * gravity * height) once the object falls. WTC1 & WTC2 each at 500,000 tons each, or 500,000,000kg. Their potential energy, at about 1/3rd it's height (to compensate for the fact the lower floors are more heavily built then upper floors), ends up being factoring gravity and height of 415m (WTC2 with it's slightly shorter size).

I am well aware of the kinetic energy built up. At the same time I am also aware of the crumple zone theory.
The kinetic energy calculation presented conveniently dismiss the crumple zone effect that absorbs tremendous energy as the floors falls stacking up one on top of the other. Just as crumple zones in a modern vehicle, they are designed to crumple and absorbs impact energy due to the sudden change of velocity.

Due to crumple zone effect, the floor level fall velocity would be arrested by the impact to the level below it.
And thus it should :
1. spent energy on bending and twisting the colums and beams
2. spent energy into the inertia of the floors below.
3. spent energy the air pistoning effect of the trapped air between levels.
These energy spent would slow the falling velocity, and thus much lower the kinetic energy

I am a skeptic

Geland,

Your also forgetting, like most relatively modern urban buildings in the developed world, they are designed to be taken out by controlled demolitions before they are even built.

Eventually it will have to come down, and they want it falling as quickly and safely as possible straight down.

WTC1, WTC2, and WTC7 did exactly that once the building started to collapse, as they should have.

Even with the "crumple zone" theory, when you have several dozen nuclear bombs worth of energy to expand, the buildings will come down in short order. Even if you only had 1% of the Kinetic energy that ended up actually crushing the Twin Towers, your still talking about a bigger energy release then both nuclear bombs dropped on Japan.

However, as my earlier point in this post, buildings are designed to collapse on themselves, you don't want a building trying to hold itself together when you do eventually demolish it as it can pose a safety hazard as the building will try to fall by tipping itself in one direction or the other instead of straight down. A crumple zone design of a building would probably violate building code due to this.

Dimitri

Ok, many modern building demolitions are done by controlled explosion (implosion), but to do so, the engineers would have to spend MONTHS of preparation to blow up strategic locations on the building.

As far as I know there are no single commercial/office building on earth that are designed with weakened spots to ease their eventual demolition. Every engineer/architect/project developer want their building to last as long as possible, the last thing they want to happen is that their building is demolished whether by damage or obsolescence. If they are eventually need to be demolished, the job would fall to the demolition expert to find and weaken the building

On the other hand they are designed to withstand and prolong failure as long as possible in the event of catastrophic incident. In fact the WTC is actually designed to withstand the impact of a 707, the largest plane at the time.

And Yep, I agree that the power that demolish the building is indeed very large.
But, I do not agree that it come simply from the falling mass of the building itself.