'' Among my friends, the environmental and the mechanical engineer are prepared to consider the claim that fires could not make a building collapse, while the structural and metallurgical engineers immediately know that of course fires can bring buildings down''
You are being devious Oystein. Your engineering friends mentioned here of course do not know that fire can bring down a steel framed hi-rise building (which is the only kind of building that matters for this conversation). For the simple reason that no hi-rise steel framed building has ever collapsed from fire in the recorded history of the Planet Earth. (Apart from at the WTC on 9/11 of course)
How absurdly wrong you are, Bill.
I know you're trolling, but anyway. Let me educate you:
Every structural and fire protection engineer knows, of course, that fire is a hazard to steel constructions.
Why else would they spray fireproofing in steel, huh? Can you explain that?
It is obvious that failure of steel members can lead to collapse.
The Kader toy factory is the usual example for this: A low.rise steel frame building that collapsed completely due to fire (and gravity).
Can you please acknowledge, Bill, that you are informed now that low-rise steel buildings can collapse completel due to fire? If so, please repeat that in your own words!
Now - what if you build a low-rise building, only a lot higher? Is there some strange law that prevents steel structures from failing once they exceed, say, 30 meters, or 10 floors, or wherever "hi-rise" begins in your book? If so, then please explain!
If not, then please think about this for a second: If low-rise buildings can collapse from fires, so can hi-rise buildings.
In fact, it is much
harder to construct hi-rise buildings than low-rise buildings such that they
don't collapse.
Now on to my engineering friends - here is a little true story I have told a few times already:
One friend, who was also my neighbour at the time, and I had a chance to climb into the spire of our church tower, which is pretty high as churches go, something like 87m, IIRC. More than 80 at any rate. The tower is traditional brickwork up to maybe 55m, on top of that is a copper-clad helmet, which has had a steel-frame since the late 19th century.
You climb up inside the helmet on open stairs that allow you to see the naked structure. My friend, who is a structural engineer, noticed quickly that several of the long steel elements were naked, with no fire protection on them. This, he explained, is a major hazard: Steel like that is prone to heat up VERY quickly to a point where their strength decreases below actual loads. He said that he's expect our church's tower cap to collapse complely within 15 or 20 minutes in case of an ordinary fire inside the tower. He went to explain that even old traditional wood beams would last considerably longer than that. And he explained to me that this is why the World Trade Center towers collapsed they way they did: Because
no structural steel can survive intense fires for very long, unless the fire is speedily fought, and fire protection intact.
Of course we know that the fire protection in the towers was NOT intact, and that the fires were not fought.
My structural engineering friend knew very well, that WTC were fire-induced collapses, and that was far from a mystery or surprise to him. Quite the opposite is true: It is what any structural engineer worth his title finds obvious, expected and natural.