WTC failure mode analysis

[Mac the Knife]

http://www.civil.usyd.edu.au/wtc.htm has a set of technical architectural analyses of the structural failure modes of the WTC consequent on the attack.

I was astonished by the apparently rapid collapse of such huge structures but now understand it better.

Deepest condolences to all who lost their lives and loved ones in this dreadful act of terror.

[sky9]

Yes Mac, I wouldn't be at all surprised if when things have quietened down, a lot of legal action will be directed at the designers and constructors of what was basically a "tower of cards"
I am sure that a block built to normal methods would have stood up better to the punishment and a lot of deaths would have been prevented.

[pax anglia]

Gentlemen....
On Monday 17/9 The Times had a couple of articles relevant to this topic,the second of which concerned last week's catastrophe.
Go to www.thetimes.co.uk/section/0,,74,00.html. (http://www.thetimes.co.uk/section/0,,74,00.html.)
Should this fail go to the home page,click on Features,then Science.
My initial reaction to the collapse of both buildings in such a short time was one of surprise.However,on today's edition of Jimmy Young's programme he interviewed a Professor of Structural Engineering (my apologies if I haven't got his Academic discipline quite correct)and he said it was a tribute to the structural strength of the buildings that they did not collapse immediately.Consequently many more people survived than might have done.
On a personal note,I have given much time over the past week to try and comprehend what motivated these people to do what they did.I am no nearer to an answer.
My condolences to all concerned.

[DC Meatloaf]

sky9, you must not have read the linked article.

From the BBC...
The design of the World Trade Center saved thousands of lives by standing for well over an hour after the planes crashed into its twin towers, say structural engineers.

"It was the fire that killed the buildings - nothing on Earth could survive those temperatures with that amount of fuel burning," structural engineer Chris Wise

And from Engineering and News Record...

In the aftermath of the Sept. 11 terrorist attacks in New York City and Washington, D.C., which brought down the twin 110-story towers of the World Trade Center and damaged the Pentagon, designers and contractors say they are skeptical that signature structures can be hardened against extreme acts of barbarism.

"Only the containment building at a nuclear powerplant" is designed to withstand such an impact and explosion, says Robert S. Vecchio, principal of metallurgical engineer Lucius Pitkin Inc., referring to the hijacked Boeing 767 airplanes, heavy with fuel, that slammed into each WTC tower.

[sky9]

Mac

I did read the article;
I quote "The floor construction is of prefabricated trussed steel, only 33 inches in depth, that spans the full 60 feet to the core, and also acts as a diaphragm to stiffen the outside wall against lateral buckling forces from wind-load pressures."

The floor is subject to compression on the top and tension on the bottom of the truss. When subjected to heat the steel lost its strength, especially in tension and collapsed onto the floor below, coming away from the side walls. Add to that the weight of the floors above the fire and the floors were severely overloaded.

I am not an engineer however I would have thought that the lightweight trussed beams were considerably weaker when heated than a equivalent, comparable RSJ.

When you see the north tower collapse it is plain that the floors gave way before the outside walls.

Read the article again Mac, I agree entirely with the consensus view of those writing it.

[Genghis the Engineer]

This weeks issue of Professional Engineering contains a very interesting analysis, based mainly upon work by a Prof. Ed Galea from the University of Greenwich. There are some interesting points in it, since I'd get in trouble if I posted it, here are the bare bones of it: -

- After the 1993 bomb, 52% of people took over 1 hour to exit the building, some took up to 3 hours.
- Emergency services would have been aware of the risk of collapse, but had no reason to expect it to happen so soon and without warning.
- The building structure consists of steel cores with concrete cladding. The cladding would have been destroyed by the impact, and the fire would have roughly halved the strength of the steel.
- The building would probably have withstood the impact alone, it's the fire that almost certainly caused the collapse. The article quotes 24,000 gallons of fuel.

Speaking as an Engineer, I'm unsurprised that the failure mode was rapid once it started, the structure was based upon a stiff outer shell, not a central core. This (like carefully standing on a coke can then tapping it) will fail rapidly and in a fairly symmetric manner. Equally, the heat must have very rapidly reduced the strength of the steel, and more to the point it's stiffness - the Euler buckling load would have come right down very fast.

However, you can hardly blame anybody for not sitting there and working all this out at the time.

G

[fobotcso]

Given that the impacts and fires were not survivable, it was probably better that the Towers slumped inwards and down rather than toppled.

If the Towers had toppled through failure of their outer walls before their cores, the damage to neighbouring structures would have been enormous. Loss of life would have shifted from the occupants of WTC to occupants of other buildings.

Informed letter from Mark Whitby, Civil Engineer to The Times, today (27 Sep 2001) sets this out clearly. Whether this was luck or was planned in design we have yet to be told. But he implies that this was the least worst outcome. (I can't use the word "better" again.)

[pigboat]

Saw an interview with a structural engineer some days after the NYC atrocity. He explained the collapse in terms even I could understand.
The towers survived the initial collisions with no problems, except from the 'local damage' caused by the aircraft, even though they had never been designed for that type of impact. Once the method became available to release the kinetic energy stored in the structure - in this case a fuel fed fire to weaken the steel - gravity would do the rest.
At least that's how I understood it.

[Evo7]

Pigboat

Potential, not kinetic, energy.

[pigboat]

Oops! :o Could sworn he said kinetic, but now that you mention it....

['I' in the sky]

sky9

'..I wouldn't be surprised if a lot of legal action were directed at the designers and constructors..'

Quite an irritating remark really especially when put into context with the rest of the blame culture said to be heading towards taking legal action against airlines and airports who allowed the bas***ds on board.

Please don't anybody lose sight of the fact that that the ONLY people responsible for that death toll were the losers that hijacked the planes.